Merge remote-tracking branch 'upstream/main' into fix-enum-variant-inconsistency

94 files changed, 3670 insertions, 0 deletions

diff --git a/solutions/00_intro/intro1.rs b/solutions/00_intro/intro1.rs
new file mode 100644
index 0000000..4fe8454
--- /dev/null
+++ b/solutions/00_intro/intro1.rs
@@ -0,0 +1,5 @@
+fn main() {
+    // Congratulations, you finished the first exercise 🎉
+    // As an introduction to Rustlings, the first exercise only required
+    // entering `n` in the terminal to go to the next exercise.
+}
diff --git a/solutions/00_intro/intro2.rs b/solutions/00_intro/intro2.rs
new file mode 100644
index 0000000..b8e031a
--- /dev/null
+++ b/solutions/00_intro/intro2.rs
@@ -0,0 +1,4 @@
+fn main() {
+    // `println!` instead of `printline!`.
+    println!("Hello world!");
+}
diff --git a/solutions/01_variables/variables1.rs b/solutions/01_variables/variables1.rs
new file mode 100644
index 0000000..58d046b
--- /dev/null
+++ b/solutions/01_variables/variables1.rs
@@ -0,0 +1,6 @@
+fn main() {
+    // Declaring variables requires the `let` keyword.
+    let x = 5;
+
+    println!("x has the value {x}");
+}
diff --git a/solutions/01_variables/variables2.rs b/solutions/01_variables/variables2.rs
new file mode 100644
index 0000000..50b8d1b
--- /dev/null
+++ b/solutions/01_variables/variables2.rs
@@ -0,0 +1,16 @@
+fn main() {
+    // The easiest way to fix the compiler error is to initialize the
+    // variable `x`. By setting its value to an integer, Rust infers its type
+    // as `i32` which is the default type for integers.
+    let x = 42;
+
+    // But we can enforce a type different from the default `i32` by adding
+    // a type annotation:
+    // let x: u8 = 42;
+
+    if x == 10 {
+        println!("x is ten!");
+    } else {
+        println!("x is not ten!");
+    }
+}
diff --git a/solutions/01_variables/variables3.rs b/solutions/01_variables/variables3.rs
new file mode 100644
index 0000000..15f6557
--- /dev/null
+++ b/solutions/01_variables/variables3.rs
@@ -0,0 +1,15 @@
+#![allow(clippy::needless_late_init)]
+
+fn main() {
+    // Reading uninitialized variables isn't allowed in Rust!
+    // Therefore, we need to assign a value first.
+    let x: i32 = 42;
+
+    println!("Number {x}");
+
+    // It is possible to declare a variable and initialize it later.
+    // But it can't be used before initialization.
+    let y: i32;
+    y = 42;
+    println!("Number {y}");
+}
diff --git a/solutions/01_variables/variables4.rs b/solutions/01_variables/variables4.rs
new file mode 100644
index 0000000..7de6bcb
--- /dev/null
+++ b/solutions/01_variables/variables4.rs
@@ -0,0 +1,9 @@
+fn main() {
+    // In Rust, variables are immutable by default.
+    // Adding the `mut` keyword after `let` makes the declared variable mutable.
+    let mut x = 3;
+    println!("Number {x}");
+
+    x = 5;
+    println!("Number {x}");
+}
diff --git a/solutions/01_variables/variables5.rs b/solutions/01_variables/variables5.rs
new file mode 100644
index 0000000..9057754
--- /dev/null
+++ b/solutions/01_variables/variables5.rs
@@ -0,0 +1,9 @@
+fn main() {
+    let number = "T-H-R-E-E";
+    println!("Spell a number: {}", number);
+
+    // Using variable shadowing
+    // https://doc.rust-lang.org/book/ch03-01-variables-and-mutability.html#shadowing
+    let number = 3;
+    println!("Number plus two is: {}", number + 2);
+}
diff --git a/solutions/01_variables/variables6.rs b/solutions/01_variables/variables6.rs
new file mode 100644
index 0000000..25b7a1e
--- /dev/null
+++ b/solutions/01_variables/variables6.rs
@@ -0,0 +1,6 @@
+// The type of constants must always be annotated.
+const NUMBER: u64 = 3;
+
+fn main() {
+    println!("Number: {NUMBER}");
+}
diff --git a/solutions/02_functions/functions1.rs b/solutions/02_functions/functions1.rs
new file mode 100644
index 0000000..dc52744
--- /dev/null
+++ b/solutions/02_functions/functions1.rs
@@ -0,0 +1,8 @@
+// Some function with the name `call_me` without arguments or a return value.
+fn call_me() {
+    println!("Hello world!");
+}
+
+fn main() {
+    call_me();
+}
diff --git a/solutions/02_functions/functions2.rs b/solutions/02_functions/functions2.rs
new file mode 100644
index 0000000..f14ffa3
--- /dev/null
+++ b/solutions/02_functions/functions2.rs
@@ -0,0 +1,11 @@
+// The type of function arguments must be annotated.
+// Added the type annotation `u64`.
+fn call_me(num: u64) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}
+
+fn main() {
+    call_me(3);
+}
diff --git a/solutions/02_functions/functions3.rs b/solutions/02_functions/functions3.rs
new file mode 100644
index 0000000..c581c42
--- /dev/null
+++ b/solutions/02_functions/functions3.rs
@@ -0,0 +1,10 @@
+fn call_me(num: u32) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}
+
+fn main() {
+    // `call_me` expects an argument.
+    call_me(5);
+}
diff --git a/solutions/02_functions/functions4.rs b/solutions/02_functions/functions4.rs
new file mode 100644
index 0000000..f823de2
--- /dev/null
+++ b/solutions/02_functions/functions4.rs
@@ -0,0 +1,17 @@
+fn is_even(num: i64) -> bool {
+    num % 2 == 0
+}
+
+// The return type must always be annotated.
+fn sale_price(price: i64) -> i64 {
+    if is_even(price) {
+        price - 10
+    } else {
+        price - 3
+    }
+}
+
+fn main() {
+    let original_price = 51;
+    println!("Your sale price is {}", sale_price(original_price));
+}
diff --git a/solutions/02_functions/functions5.rs b/solutions/02_functions/functions5.rs
new file mode 100644
index 0000000..677f327
--- /dev/null
+++ b/solutions/02_functions/functions5.rs
@@ -0,0 +1,9 @@
+fn square(num: i32) -> i32 {
+    // Removed the semicolon `;` at the end of the line below to implicitly return the result.
+    num * num
+}
+
+fn main() {
+    let answer = square(3);
+    println!("The square of 3 is {answer}");
+}
diff --git a/solutions/03_if/if1.rs b/solutions/03_if/if1.rs
new file mode 100644
index 0000000..079c671
--- /dev/null
+++ b/solutions/03_if/if1.rs
@@ -0,0 +1,32 @@
+fn bigger(a: i32, b: i32) -> i32 {
+    if a > b {
+        a
+    } else {
+        b
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+// Don't mind this for now :)
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn ten_is_bigger_than_eight() {
+        assert_eq!(10, bigger(10, 8));
+    }
+
+    #[test]
+    fn fortytwo_is_bigger_than_thirtytwo() {
+        assert_eq!(42, bigger(32, 42));
+    }
+
+    #[test]
+    fn equal_numbers() {
+        assert_eq!(42, bigger(42, 42));
+    }
+}
diff --git a/solutions/03_if/if2.rs b/solutions/03_if/if2.rs
new file mode 100644
index 0000000..440bba0
--- /dev/null
+++ b/solutions/03_if/if2.rs
@@ -0,0 +1,33 @@
+fn foo_if_fizz(fizzish: &str) -> &str {
+    if fizzish == "fizz" {
+        "foo"
+    } else if fizzish == "fuzz" {
+        "bar"
+    } else {
+        "baz"
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn foo_for_fizz() {
+        assert_eq!(foo_if_fizz("fizz"), "foo");
+    }
+
+    #[test]
+    fn bar_for_fuzz() {
+        assert_eq!(foo_if_fizz("fuzz"), "bar");
+    }
+
+    #[test]
+    fn default_to_baz() {
+        assert_eq!(foo_if_fizz("literally anything"), "baz");
+    }
+}
diff --git a/solutions/03_if/if3.rs b/solutions/03_if/if3.rs
new file mode 100644
index 0000000..571644d
--- /dev/null
+++ b/solutions/03_if/if3.rs
@@ -0,0 +1,53 @@
+fn animal_habitat(animal: &str) -> &str {
+    let identifier = if animal == "crab" {
+        1
+    } else if animal == "gopher" {
+        2
+    } else if animal == "snake" {
+        3
+    } else {
+        // Any unused identifier.
+        4
+    };
+
+    // Instead of such an identifier, you would use an enum in Rust.
+    // But we didn't get into enums yet.
+    if identifier == 1 {
+        "Beach"
+    } else if identifier == 2 {
+        "Burrow"
+    } else if identifier == 3 {
+        "Desert"
+    } else {
+        "Unknown"
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn gopher_lives_in_burrow() {
+        assert_eq!(animal_habitat("gopher"), "Burrow")
+    }
+
+    #[test]
+    fn snake_lives_in_desert() {
+        assert_eq!(animal_habitat("snake"), "Desert")
+    }
+
+    #[test]
+    fn crab_lives_on_beach() {
+        assert_eq!(animal_habitat("crab"), "Beach")
+    }
+
+    #[test]
+    fn unknown_animal() {
+        assert_eq!(animal_habitat("dinosaur"), "Unknown")
+    }
+}
diff --git a/solutions/04_primitive_types/primitive_types1.rs b/solutions/04_primitive_types/primitive_types1.rs
new file mode 100644
index 0000000..fac6ec0
--- /dev/null
+++ b/solutions/04_primitive_types/primitive_types1.rs
@@ -0,0 +1,11 @@
+fn main() {
+    let is_morning = true;
+    if is_morning {
+        println!("Good morning!");
+    }
+
+    let is_evening = !is_morning;
+    if is_evening {
+        println!("Good evening!");
+    }
+}
diff --git a/solutions/04_primitive_types/primitive_types2.rs b/solutions/04_primitive_types/primitive_types2.rs
new file mode 100644
index 0000000..eecc680
--- /dev/null
+++ b/solutions/04_primitive_types/primitive_types2.rs
@@ -0,0 +1,21 @@
+fn main() {
+    let my_first_initial = 'C';
+    if my_first_initial.is_alphabetic() {
+        println!("Alphabetical!");
+    } else if my_first_initial.is_numeric() {
+        println!("Numerical!");
+    } else {
+        println!("Neither alphabetic nor numeric!");
+    }
+
+    // Example with an emoji.
+    let your_character = '🦀';
+
+    if your_character.is_alphabetic() {
+        println!("Alphabetical!");
+    } else if your_character.is_numeric() {
+        println!("Numerical!");
+    } else {
+        println!("Neither alphabetic nor numeric!");
+    }
+}
diff --git a/solutions/04_primitive_types/primitive_types3.rs b/solutions/04_primitive_types/primitive_types3.rs
new file mode 100644
index 0000000..8dd109f
--- /dev/null
+++ b/solutions/04_primitive_types/primitive_types3.rs
@@ -0,0 +1,11 @@
+fn main() {
+    // An array with 100 elements of the value 42.
+    let a = [42; 100];
+
+    if a.len() >= 100 {
+        println!("Wow, that's a big array!");
+    } else {
+        println!("Meh, I eat arrays like that for breakfast.");
+        panic!("Array not big enough, more elements needed");
+    }
+}
diff --git a/solutions/04_primitive_types/primitive_types4.rs b/solutions/04_primitive_types/primitive_types4.rs
new file mode 100644
index 0000000..4807e66
--- /dev/null
+++ b/solutions/04_primitive_types/primitive_types4.rs
@@ -0,0 +1,23 @@
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn slice_out_of_array() {
+        let a = [1, 2, 3, 4, 5];
+        //       0  1  2  3  4  <- indices
+        //          -------
+        //             |
+        //             +--- slice
+
+        // Note that the upper index 4 is excluded.
+        let nice_slice = &a[1..4];
+        assert_eq!([2, 3, 4], nice_slice);
+
+        // The upper index can be included by using the syntax `..=` (with `=` sign)
+        let nice_slice = &a[1..=3];
+        assert_eq!([2, 3, 4], nice_slice);
+    }
+}
diff --git a/solutions/04_primitive_types/primitive_types5.rs b/solutions/04_primitive_types/primitive_types5.rs
new file mode 100644
index 0000000..46d7ae8
--- /dev/null
+++ b/solutions/04_primitive_types/primitive_types5.rs
@@ -0,0 +1,8 @@
+fn main() {
+    let cat = ("Furry McFurson", 3.5);
+
+    // Destructuring the tuple.
+    let (name, age) = cat;
+
+    println!("{name} is {age} years old");
+}
diff --git a/solutions/04_primitive_types/primitive_types6.rs b/solutions/04_primitive_types/primitive_types6.rs
new file mode 100644
index 0000000..9b7c277
--- /dev/null
+++ b/solutions/04_primitive_types/primitive_types6.rs
@@ -0,0 +1,16 @@
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn indexing_tuple() {
+        let numbers = (1, 2, 3);
+
+        // Tuple indexing syntax.
+        let second = numbers.1;
+
+        assert_eq!(second, 2, "This is not the 2nd number in the tuple!");
+    }
+}
diff --git a/solutions/05_vecs/vecs1.rs b/solutions/05_vecs/vecs1.rs
new file mode 100644
index 0000000..55b5676
--- /dev/null
+++ b/solutions/05_vecs/vecs1.rs
@@ -0,0 +1,23 @@
+fn array_and_vec() -> ([i32; 4], Vec<i32>) {
+    let a = [10, 20, 30, 40]; // Array
+
+    // Used the `vec!` macro.
+    let v = vec![10, 20, 30, 40];
+
+    (a, v)
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_array_and_vec_similarity() {
+        let (a, v) = array_and_vec();
+        assert_eq!(a, *v);
+    }
+}
diff --git a/solutions/05_vecs/vecs2.rs b/solutions/05_vecs/vecs2.rs
new file mode 100644
index 0000000..87f7625
--- /dev/null
+++ b/solutions/05_vecs/vecs2.rs
@@ -0,0 +1,55 @@
+fn vec_loop(input: &[i32]) -> Vec<i32> {
+    let mut output = Vec::new();
+
+    for element in input {
+        output.push(2 * element);
+    }
+
+    output
+}
+
+fn vec_map_example(input: &[i32]) -> Vec<i32> {
+    // An example of collecting a vector after mapping.
+    // We map each element of the `input` slice to its value plus 1.
+    // If the input is `[1, 2, 3]`, the output is `[2, 3, 4]`.
+    input.iter().map(|element| element + 1).collect()
+}
+
+fn vec_map(input: &[i32]) -> Vec<i32> {
+    // We will dive deeper into iterators, but for now, this is all what you
+    // had to do!
+    // Advanced note: This method is more efficient because it automatically
+    // preallocates enough capacity. This can be done manually in `vec_loop`
+    // using `Vec::with_capacity(input.len())` instead of `Vec::new()`.
+    input.iter().map(|element| 2 * element).collect()
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_vec_loop() {
+        let input = [2, 4, 6, 8, 10];
+        let ans = vec_loop(&input);
+        assert_eq!(ans, [4, 8, 12, 16, 20]);
+    }
+
+    #[test]
+    fn test_vec_map_example() {
+        let input = [1, 2, 3];
+        let ans = vec_map_example(&input);
+        assert_eq!(ans, [2, 3, 4]);
+    }
+
+    #[test]
+    fn test_vec_map() {
+        let input = [2, 4, 6, 8, 10];
+        let ans = vec_map(&input);
+        assert_eq!(ans, [4, 8, 12, 16, 20]);
+    }
+}
diff --git a/solutions/06_move_semantics/move_semantics1.rs b/solutions/06_move_semantics/move_semantics1.rs
new file mode 100644
index 0000000..ac34e7a
--- /dev/null
+++ b/solutions/06_move_semantics/move_semantics1.rs
@@ -0,0 +1,25 @@
+fn fill_vec(vec: Vec<i32>) -> Vec<i32> {
+    let mut vec = vec;
+    //  ^^^ added
+
+    vec.push(88);
+
+    vec
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn move_semantics1() {
+        let vec0 = vec![22, 44, 66];
+        let vec1 = fill_vec(vec0);
+        // `vec0` can't be accessed anymore because it is moved to `fill_vec`.
+        assert_eq!(vec1, vec![22, 44, 66, 88]);
+    }
+}
diff --git a/solutions/06_move_semantics/move_semantics2.rs b/solutions/06_move_semantics/move_semantics2.rs
new file mode 100644
index 0000000..7bcd33a
--- /dev/null
+++ b/solutions/06_move_semantics/move_semantics2.rs
@@ -0,0 +1,28 @@
+fn fill_vec(vec: Vec<i32>) -> Vec<i32> {
+    let mut vec = vec;
+
+    vec.push(88);
+
+    vec
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn move_semantics2() {
+        let vec0 = vec![22, 44, 66];
+
+        // Cloning `vec0` so that the clone is moved into `fill_vec`, not `vec0`
+        // itself.
+        let vec1 = fill_vec(vec0.clone());
+
+        assert_eq!(vec0, [22, 44, 66]);
+        assert_eq!(vec1, [22, 44, 66, 88]);
+    }
+}
diff --git a/solutions/06_move_semantics/move_semantics3.rs b/solutions/06_move_semantics/move_semantics3.rs
new file mode 100644
index 0000000..7ba4006
--- /dev/null
+++ b/solutions/06_move_semantics/move_semantics3.rs
@@ -0,0 +1,22 @@
+fn fill_vec(mut vec: Vec<i32>) -> Vec<i32> {
+    //      ^^^ added
+    vec.push(88);
+
+    vec
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn move_semantics3() {
+        let vec0 = vec![22, 44, 66];
+        let vec1 = fill_vec(vec0);
+        assert_eq!(vec1, [22, 44, 66, 88]);
+    }
+}
diff --git a/solutions/06_move_semantics/move_semantics4.rs b/solutions/06_move_semantics/move_semantics4.rs
new file mode 100644
index 0000000..b7919ac
--- /dev/null
+++ b/solutions/06_move_semantics/move_semantics4.rs
@@ -0,0 +1,21 @@
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    // TODO: Fix the compiler errors only by reordering the lines in the test.
+    // Don't add, change or remove any line.
+    #[test]
+    fn move_semantics5() {
+        let mut x = 100;
+        let y = &mut x;
+        // `y` used here.
+        *y += 100;
+        // The mutable reference `y` is not used anymore,
+        // therefore a new reference can be created.
+        let z = &mut x;
+        *z += 1000;
+        assert_eq!(x, 1200);
+    }
+}
diff --git a/solutions/06_move_semantics/move_semantics5.rs b/solutions/06_move_semantics/move_semantics5.rs
new file mode 100644
index 0000000..678ec97
--- /dev/null
+++ b/solutions/06_move_semantics/move_semantics5.rs
@@ -0,0 +1,23 @@
+#![allow(clippy::ptr_arg)]
+
+fn main() {
+    let data = "Rust is great!".to_string();
+
+    get_char(&data);
+
+    string_uppercase(data);
+}
+
+// Borrows instead of taking ownership.
+// It is recommended to use `&str` instead of `&String` here. But this is
+// enough for now because we didn't handle strings yet.
+fn get_char(data: &String) -> char {
+    data.chars().last().unwrap()
+}
+
+// Takes ownership instead of borrowing.
+fn string_uppercase(mut data: String) {
+    data = data.to_uppercase();
+
+    println!("{data}");
+}
diff --git a/solutions/07_structs/structs1.rs b/solutions/07_structs/structs1.rs
new file mode 100644
index 0000000..98fafcc
--- /dev/null
+++ b/solutions/07_structs/structs1.rs
@@ -0,0 +1,49 @@
+struct ColorRegularStruct {
+    red: u8,
+    green: u8,
+    blue: u8,
+}
+
+struct ColorTupleStruct(u8, u8, u8);
+
+#[derive(Debug)]
+struct UnitStruct;
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn regular_structs() {
+        let green = ColorRegularStruct {
+            red: 0,
+            green: 255,
+            blue: 0,
+        };
+
+        assert_eq!(green.red, 0);
+        assert_eq!(green.green, 255);
+        assert_eq!(green.blue, 0);
+    }
+
+    #[test]
+    fn tuple_structs() {
+        let green = ColorTupleStruct(0, 255, 0);
+
+        assert_eq!(green.0, 0);
+        assert_eq!(green.1, 255);
+        assert_eq!(green.2, 0);
+    }
+
+    #[test]
+    fn unit_structs() {
+        let unit_struct = UnitStruct;
+        let message = format!("{unit_struct:?}s are fun!");
+
+        assert_eq!(message, "UnitStructs are fun!");
+    }
+}
diff --git a/solutions/07_structs/structs2.rs b/solutions/07_structs/structs2.rs
new file mode 100644
index 0000000..589dd93
--- /dev/null
+++ b/solutions/07_structs/structs2.rs
@@ -0,0 +1,51 @@
+#[derive(Debug)]
+struct Order {
+    name: String,
+    year: u32,
+    made_by_phone: bool,
+    made_by_mobile: bool,
+    made_by_email: bool,
+    item_number: u32,
+    count: u32,
+}
+
+fn create_order_template() -> Order {
+    Order {
+        name: String::from("Bob"),
+        year: 2019,
+        made_by_phone: false,
+        made_by_mobile: false,
+        made_by_email: true,
+        item_number: 123,
+        count: 0,
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn your_order() {
+        let order_template = create_order_template();
+
+        let your_order = Order {
+            name: String::from("Hacker in Rust"),
+            count: 1,
+            // Struct update syntax
+            ..order_template
+        };
+
+        assert_eq!(your_order.name, "Hacker in Rust");
+        assert_eq!(your_order.year, order_template.year);
+        assert_eq!(your_order.made_by_phone, order_template.made_by_phone);
+        assert_eq!(your_order.made_by_mobile, order_template.made_by_mobile);
+        assert_eq!(your_order.made_by_email, order_template.made_by_email);
+        assert_eq!(your_order.item_number, order_template.item_number);
+        assert_eq!(your_order.count, 1);
+    }
+}
diff --git a/solutions/07_structs/structs3.rs b/solutions/07_structs/structs3.rs
new file mode 100644
index 0000000..3f878cc
--- /dev/null
+++ b/solutions/07_structs/structs3.rs
@@ -0,0 +1,83 @@
+#[derive(Debug)]
+struct Package {
+    sender_country: String,
+    recipient_country: String,
+    weight_in_grams: u32,
+}
+
+impl Package {
+    fn new(sender_country: String, recipient_country: String, weight_in_grams: u32) -> Self {
+        if weight_in_grams < 10 {
+            // This isn't how you should handle errors in Rust, but we will
+            // learn about error handling later.
+            panic!("Can't ship a package with weight below 10 grams");
+        }
+
+        Self {
+            sender_country,
+            recipient_country,
+            weight_in_grams,
+        }
+    }
+
+    fn is_international(&self) -> bool {
+        //                     ^^^^^^^ added
+        self.sender_country != self.recipient_country
+    }
+
+    fn get_fees(&self, cents_per_gram: u32) -> u32 {
+        //                                  ^^^^^^ added
+        self.weight_in_grams * cents_per_gram
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[should_panic]
+    fn fail_creating_weightless_package() {
+        let sender_country = String::from("Spain");
+        let recipient_country = String::from("Austria");
+
+        Package::new(sender_country, recipient_country, 5);
+    }
+
+    #[test]
+    fn create_international_package() {
+        let sender_country = String::from("Spain");
+        let recipient_country = String::from("Russia");
+
+        let package = Package::new(sender_country, recipient_country, 1200);
+
+        assert!(package.is_international());
+    }
+
+    #[test]
+    fn create_local_package() {
+        let sender_country = String::from("Canada");
+        let recipient_country = sender_country.clone();
+
+        let package = Package::new(sender_country, recipient_country, 1200);
+
+        assert!(!package.is_international());
+    }
+
+    #[test]
+    fn calculate_transport_fees() {
+        let sender_country = String::from("Spain");
+        let recipient_country = String::from("Spain");
+
+        let cents_per_gram = 3;
+
+        let package = Package::new(sender_country, recipient_country, 1500);
+
+        assert_eq!(package.get_fees(cents_per_gram), 4500);
+        assert_eq!(package.get_fees(cents_per_gram * 2), 9000);
+    }
+}
diff --git a/solutions/08_enums/enums1.rs b/solutions/08_enums/enums1.rs
new file mode 100644
index 0000000..9724883
--- /dev/null
+++ b/solutions/08_enums/enums1.rs
@@ -0,0 +1,14 @@
+#[derive(Debug)]
+enum Message {
+    Quit,
+    Echo,
+    Move,
+    ChangeColor,
+}
+
+fn main() {
+    println!("{:?}", Message::Quit);
+    println!("{:?}", Message::Echo);
+    println!("{:?}", Message::Move);
+    println!("{:?}", Message::ChangeColor);
+}
diff --git a/solutions/08_enums/enums2.rs b/solutions/08_enums/enums2.rs
new file mode 100644
index 0000000..b19394c
--- /dev/null
+++ b/solutions/08_enums/enums2.rs
@@ -0,0 +1,27 @@
+#[allow(dead_code)]
+#[derive(Debug)]
+enum Message {
+    Move { x: i64, y: i64 },
+    Echo(String),
+    ChangeColor(u8, u8, u8),
+    Quit,
+}
+
+impl Message {
+    fn call(&self) {
+        println!("{self:?}");
+    }
+}
+
+fn main() {
+    let messages = [
+        Message::Move { x: 10, y: 30 },
+        Message::Echo(String::from("hello world")),
+        Message::ChangeColor(200, 255, 255),
+        Message::Quit,
+    ];
+
+    for message in &messages {
+        message.call();
+    }
+}
diff --git a/solutions/08_enums/enums3.rs b/solutions/08_enums/enums3.rs
new file mode 100644
index 0000000..8baa25c
--- /dev/null
+++ b/solutions/08_enums/enums3.rs
@@ -0,0 +1,75 @@
+enum Message {
+    ChangeColor(u8, u8, u8),
+    Echo(String),
+    Move(Point),
+    Quit,
+}
+
+struct Point {
+    x: u8,
+    y: u8,
+}
+
+struct State {
+    color: (u8, u8, u8),
+    position: Point,
+    quit: bool,
+    message: String,
+}
+
+impl State {
+    fn change_color(&mut self, color: (u8, u8, u8)) {
+        self.color = color;
+    }
+
+    fn quit(&mut self) {
+        self.quit = true;
+    }
+
+    fn echo(&mut self, s: String) {
+        self.message = s;
+    }
+
+    fn move_position(&mut self, point: Point) {
+        self.position = point;
+    }
+
+    fn process(&mut self, message: Message) {
+        match message {
+            Message::ChangeColor(r, g, b) => self.change_color((r, g, b)),
+            Message::Echo(s) => self.echo(s),
+            Message::Move(point) => self.move_position(point),
+            Message::Quit => self.quit(),
+        }
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_match_message_call() {
+        let mut state = State {
+            quit: false,
+            position: Point { x: 0, y: 0 },
+            color: (0, 0, 0),
+            message: String::from("hello world"),
+        };
+
+        state.process(Message::ChangeColor(255, 0, 255));
+        state.process(Message::Echo(String::from("Hello world!")));
+        state.process(Message::Move(Point { x: 10, y: 15 }));
+        state.process(Message::Quit);
+
+        assert_eq!(state.color, (255, 0, 255));
+        assert_eq!(state.position.x, 10);
+        assert_eq!(state.position.y, 15);
+        assert!(state.quit);
+        assert_eq!(state.message, "Hello world!");
+    }
+}
diff --git a/solutions/09_strings/strings1.rs b/solutions/09_strings/strings1.rs
new file mode 100644
index 0000000..f7ba811
--- /dev/null
+++ b/solutions/09_strings/strings1.rs
@@ -0,0 +1,9 @@
+fn current_favorite_color() -> String {
+    // Equivalent to `String::from("blue")`
+    "blue".to_string()
+}
+
+fn main() {
+    let answer = current_favorite_color();
+    println!("My current favorite color is {answer}");
+}
diff --git a/solutions/09_strings/strings2.rs b/solutions/09_strings/strings2.rs
new file mode 100644
index 0000000..7de311f
--- /dev/null
+++ b/solutions/09_strings/strings2.rs
@@ -0,0 +1,15 @@
+fn is_a_color_word(attempt: &str) -> bool {
+    attempt == "green" || attempt == "blue" || attempt == "red"
+}
+
+fn main() {
+    let word = String::from("green");
+
+    if is_a_color_word(&word) {
+        //             ^ added to have `&String` which is automatically
+        //               coerced to `&str` by the compiler.
+        println!("That is a color word I know!");
+    } else {
+        println!("That is not a color word I know.");
+    }
+}
diff --git a/solutions/09_strings/strings3.rs b/solutions/09_strings/strings3.rs
new file mode 100644
index 0000000..a478e62
--- /dev/null
+++ b/solutions/09_strings/strings3.rs
@@ -0,0 +1,48 @@
+fn trim_me(input: &str) -> &str {
+    input.trim()
+}
+
+fn compose_me(input: &str) -> String {
+    // The macro `format!` has the same syntax as `println!`, but it returns a
+    // string instead of printing it to the terminal.
+    // Equivalent to `input.to_string() + " world!"`
+    format!("{input} world!")
+}
+
+fn replace_me(input: &str) -> String {
+    input.replace("cars", "balloons")
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn trim_a_string() {
+        assert_eq!(trim_me("Hello!     "), "Hello!");
+        assert_eq!(trim_me("  What's up!"), "What's up!");
+        assert_eq!(trim_me("   Hola!  "), "Hola!");
+    }
+
+    #[test]
+    fn compose_a_string() {
+        assert_eq!(compose_me("Hello"), "Hello world!");
+        assert_eq!(compose_me("Goodbye"), "Goodbye world!");
+    }
+
+    #[test]
+    fn replace_a_string() {
+        assert_eq!(
+            replace_me("I think cars are cool"),
+            "I think balloons are cool",
+        );
+        assert_eq!(
+            replace_me("I love to look at cars"),
+            "I love to look at balloons",
+        );
+    }
+}
diff --git a/solutions/09_strings/strings4.rs b/solutions/09_strings/strings4.rs
new file mode 100644
index 0000000..9dc6917
--- /dev/null
+++ b/solutions/09_strings/strings4.rs
@@ -0,0 +1,38 @@
+fn string_slice(arg: &str) {
+    println!("{arg}");
+}
+fn string(arg: String) {
+    println!("{arg}");
+}
+
+fn main() {
+    string_slice("blue");
+
+    string("red".to_string());
+
+    string(String::from("hi"));
+
+    string("rust is fun!".to_owned());
+
+    // Here, both answers work.
+    // `.into()` converts a type into an expected type.
+    // If it is called where `String` is expected, it will convert `&str` to `String`.
+    // But if is called where `&str` is expected, then `&str` is kept `&str` since no
+    // conversion is needed.
+    string("nice weather".into());
+    string_slice("nice weather".into());
+    //                         ^^^^^^^ the compiler recommends removing the `.into()`
+    //                                 call because it is a useless conversion.
+
+    string(format!("Interpolation {}", "Station"));
+
+    // WARNING: This is byte indexing, not character indexing.
+    // Character indexing can be done using `s.chars().nth(INDEX)`.
+    string_slice(&String::from("abc")[0..1]);
+
+    string_slice("  hello there ".trim());
+
+    string("Happy Monday!".replace("Mon", "Tues"));
+
+    string("mY sHiFt KeY iS sTiCkY".to_lowercase());
+}
diff --git a/solutions/10_modules/modules1.rs b/solutions/10_modules/modules1.rs
new file mode 100644
index 0000000..873b412
--- /dev/null
+++ b/solutions/10_modules/modules1.rs
@@ -0,0 +1,15 @@
+mod sausage_factory {
+    fn get_secret_recipe() -> String {
+        String::from("Ginger")
+    }
+
+    // Added `pub` before `fn` to make the function accessible outside the module.
+    pub fn make_sausage() {
+        get_secret_recipe();
+        println!("sausage!");
+    }
+}
+
+fn main() {
+    sausage_factory::make_sausage();
+}
diff --git a/solutions/10_modules/modules2.rs b/solutions/10_modules/modules2.rs
new file mode 100644
index 0000000..298d76e
--- /dev/null
+++ b/solutions/10_modules/modules2.rs
@@ -0,0 +1,24 @@
+#[allow(dead_code)]
+mod delicious_snacks {
+    // Added `pub` and used the expected alias after `as`.
+    pub use self::fruits::PEAR as fruit;
+    pub use self::veggies::CUCUMBER as veggie;
+
+    mod fruits {
+        pub const PEAR: &str = "Pear";
+        pub const APPLE: &str = "Apple";
+    }
+
+    mod veggies {
+        pub const CUCUMBER: &str = "Cucumber";
+        pub const CARROT: &str = "Carrot";
+    }
+}
+
+fn main() {
+    println!(
+        "favorite snacks: {} and {}",
+        delicious_snacks::fruit,
+        delicious_snacks::veggie,
+    );
+}
diff --git a/solutions/10_modules/modules3.rs b/solutions/10_modules/modules3.rs
new file mode 100644
index 0000000..99ff5a7
--- /dev/null
+++ b/solutions/10_modules/modules3.rs
@@ -0,0 +1,8 @@
+use std::time::{SystemTime, UNIX_EPOCH};
+
+fn main() {
+    match SystemTime::now().duration_since(UNIX_EPOCH) {
+        Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
+        Err(_) => panic!("SystemTime before UNIX EPOCH!"),
+    }
+}
diff --git a/solutions/11_hashmaps/hashmaps1.rs b/solutions/11_hashmaps/hashmaps1.rs
new file mode 100644
index 0000000..3a787c4
--- /dev/null
+++ b/solutions/11_hashmaps/hashmaps1.rs
@@ -0,0 +1,42 @@
+// A basket of fruits in the form of a hash map needs to be defined. The key
+// represents the name of the fruit and the value represents how many of that
+// particular fruit is in the basket. You have to put at least 3 different
+// types of fruits (e.g apple, banana, mango) in the basket and the total count
+// of all the fruits should be at least 5.
+
+use std::collections::HashMap;
+
+fn fruit_basket() -> HashMap<String, u32> {
+    // Declare the hash map.
+    let mut basket = HashMap::new();
+
+    // Two bananas are already given for you :)
+    basket.insert(String::from("banana"), 2);
+
+    // Put more fruits in your basket.
+    basket.insert(String::from("apple"), 3);
+    basket.insert(String::from("mango"), 1);
+
+    basket
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn at_least_three_types_of_fruits() {
+        let basket = fruit_basket();
+        assert!(basket.len() >= 3);
+    }
+
+    #[test]
+    fn at_least_five_fruits() {
+        let basket = fruit_basket();
+        assert!(basket.values().sum::<u32>() >= 5);
+    }
+}
diff --git a/solutions/11_hashmaps/hashmaps2.rs b/solutions/11_hashmaps/hashmaps2.rs
new file mode 100644
index 0000000..a5e6ef9
--- /dev/null
+++ b/solutions/11_hashmaps/hashmaps2.rs
@@ -0,0 +1,95 @@
+// We're collecting different fruits to bake a delicious fruit cake. For this,
+// we have a basket, which we'll represent in the form of a hash map. The key
+// represents the name of each fruit we collect and the value represents how
+// many of that particular fruit we have collected. Three types of fruits -
+// Apple (4), Mango (2) and Lychee (5) are already in the basket hash map. You
+// must add fruit to the basket so that there is at least one of each kind and
+// more than 11 in total - we have a lot of mouths to feed. You are not allowed
+// to insert any more of these fruits!
+
+use std::collections::HashMap;
+
+#[derive(Hash, PartialEq, Eq, Debug)]
+enum Fruit {
+    Apple,
+    Banana,
+    Mango,
+    Lychee,
+    Pineapple,
+}
+
+fn fruit_basket(basket: &mut HashMap<Fruit, u32>) {
+    let fruit_kinds = [
+        Fruit::Apple,
+        Fruit::Banana,
+        Fruit::Mango,
+        Fruit::Lychee,
+        Fruit::Pineapple,
+    ];
+
+    for fruit in fruit_kinds {
+        // If fruit doesn't exist, insert it with some value.
+        basket.entry(fruit).or_insert(5);
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // Don't modify this function!
+    fn get_fruit_basket() -> HashMap<Fruit, u32> {
+        let content = [(Fruit::Apple, 4), (Fruit::Mango, 2), (Fruit::Lychee, 5)];
+        HashMap::from_iter(content)
+    }
+
+    #[test]
+    fn test_given_fruits_are_not_modified() {
+        let mut basket = get_fruit_basket();
+        fruit_basket(&mut basket);
+        assert_eq!(*basket.get(&Fruit::Apple).unwrap(), 4);
+        assert_eq!(*basket.get(&Fruit::Mango).unwrap(), 2);
+        assert_eq!(*basket.get(&Fruit::Lychee).unwrap(), 5);
+    }
+
+    #[test]
+    fn at_least_five_types_of_fruits() {
+        let mut basket = get_fruit_basket();
+        fruit_basket(&mut basket);
+        let count_fruit_kinds = basket.len();
+        assert!(count_fruit_kinds >= 5);
+    }
+
+    #[test]
+    fn greater_than_eleven_fruits() {
+        let mut basket = get_fruit_basket();
+        fruit_basket(&mut basket);
+        let count = basket.values().sum::<u32>();
+        assert!(count > 11);
+    }
+
+    #[test]
+    fn all_fruit_types_in_basket() {
+        let fruit_kinds = [
+            Fruit::Apple,
+            Fruit::Banana,
+            Fruit::Mango,
+            Fruit::Lychee,
+            Fruit::Pineapple,
+        ];
+
+        let mut basket = get_fruit_basket();
+        fruit_basket(&mut basket);
+
+        for fruit_kind in fruit_kinds {
+            let Some(amount) = basket.get(&fruit_kind) else {
+                panic!("Fruit kind {fruit_kind:?} was not found in basket");
+            };
+            assert!(*amount > 0);
+        }
+    }
+}
diff --git a/solutions/11_hashmaps/hashmaps3.rs b/solutions/11_hashmaps/hashmaps3.rs
new file mode 100644
index 0000000..54f480b
--- /dev/null
+++ b/solutions/11_hashmaps/hashmaps3.rs
@@ -0,0 +1,83 @@
+// A list of scores (one per line) of a soccer match is given. Each line is of
+// the form "<team_1_name>,<team_2_name>,<team_1_goals>,<team_2_goals>"
+// Example: "England,France,4,2" (England scored 4 goals, France 2).
+//
+// You have to build a scores table containing the name of the team, the total
+// number of goals the team scored, and the total number of goals the team
+// conceded.
+
+use std::collections::HashMap;
+
+// A structure to store the goal details of a team.
+#[derive(Default)]
+struct Team {
+    goals_scored: u8,
+    goals_conceded: u8,
+}
+
+fn build_scores_table(results: &str) -> HashMap<&str, Team> {
+    // The name of the team is the key and its associated struct is the value.
+    let mut scores = HashMap::new();
+
+    for line in results.lines() {
+        let mut split_iterator = line.split(',');
+        // NOTE: We use `unwrap` because we didn't deal with error handling yet.
+        let team_1_name = split_iterator.next().unwrap();
+        let team_2_name = split_iterator.next().unwrap();
+        let team_1_score: u8 = split_iterator.next().unwrap().parse().unwrap();
+        let team_2_score: u8 = split_iterator.next().unwrap().parse().unwrap();
+
+        // Insert the default with zeros if a team doesn't exist yet.
+        let team_1 = scores.entry(team_1_name).or_insert_with(Team::default);
+        // Update the values.
+        team_1.goals_scored += team_1_score;
+        team_1.goals_conceded += team_2_score;
+
+        // Similarely for the second team.
+        let team_2 = scores.entry(team_2_name).or_insert_with(Team::default);
+        team_2.goals_scored += team_2_score;
+        team_2.goals_conceded += team_1_score;
+    }
+
+    scores
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const RESULTS: &str = "England,France,4,2
+France,Italy,3,1
+Poland,Spain,2,0
+Germany,England,2,1
+England,Spain,1,0";
+
+    #[test]
+    fn build_scores() {
+        let scores = build_scores_table(RESULTS);
+
+        assert!(["England", "France", "Germany", "Italy", "Poland", "Spain"]
+            .into_iter()
+            .all(|team_name| scores.contains_key(team_name)));
+    }
+
+    #[test]
+    fn validate_team_score_1() {
+        let scores = build_scores_table(RESULTS);
+        let team = scores.get("England").unwrap();
+        assert_eq!(team.goals_scored, 6);
+        assert_eq!(team.goals_conceded, 4);
+    }
+
+    #[test]
+    fn validate_team_score_2() {
+        let scores = build_scores_table(RESULTS);
+        let team = scores.get("Spain").unwrap();
+        assert_eq!(team.goals_scored, 0);
+        assert_eq!(team.goals_conceded, 3);
+    }
+}
diff --git a/solutions/12_options/options1.rs b/solutions/12_options/options1.rs
new file mode 100644
index 0000000..4d615dd
--- /dev/null
+++ b/solutions/12_options/options1.rs
@@ -0,0 +1,39 @@
+// This function returns how much icecream there is left in the fridge.
+// If it's before 22:00 (24-hour system), then 5 scoops are left. At 22:00,
+// someone eats it all, so no icecream is left (value 0). Return `None` if
+// `hour_of_day` is higher than 23.
+fn maybe_icecream(hour_of_day: u16) -> Option<u16> {
+    match hour_of_day {
+        0..=21 => Some(5),
+        22..=23 => Some(0),
+        _ => None,
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn raw_value() {
+        // Using `unwrap` is fine in a test.
+        let icecreams = maybe_icecream(12).unwrap();
+
+        assert_eq!(icecreams, 5);
+    }
+
+    #[test]
+    fn check_icecream() {
+        assert_eq!(maybe_icecream(0), Some(5));
+        assert_eq!(maybe_icecream(9), Some(5));
+        assert_eq!(maybe_icecream(18), Some(5));
+        assert_eq!(maybe_icecream(22), Some(0));
+        assert_eq!(maybe_icecream(23), Some(0));
+        assert_eq!(maybe_icecream(24), None);
+        assert_eq!(maybe_icecream(25), None);
+    }
+}
diff --git a/solutions/12_options/options2.rs b/solutions/12_options/options2.rs
new file mode 100644
index 0000000..0f24665
--- /dev/null
+++ b/solutions/12_options/options2.rs
@@ -0,0 +1,37 @@
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn simple_option() {
+        let target = "rustlings";
+        let optional_target = Some(target);
+
+        // if-let
+        if let Some(word) = optional_target {
+            assert_eq!(word, target);
+        }
+    }
+
+    #[test]
+    fn layered_option() {
+        let range = 10;
+        let mut optional_integers: Vec<Option<i8>> = vec![None];
+
+        for i in 1..=range {
+            optional_integers.push(Some(i));
+        }
+
+        let mut cursor = range;
+
+        // while-let with nested pattern matching
+        while let Some(Some(integer)) = optional_integers.pop() {
+            assert_eq!(integer, cursor);
+            cursor -= 1;
+        }
+
+        assert_eq!(cursor, 0);
+    }
+}
diff --git a/solutions/12_options/options3.rs b/solutions/12_options/options3.rs
new file mode 100644
index 0000000..0081eeb
--- /dev/null
+++ b/solutions/12_options/options3.rs
@@ -0,0 +1,26 @@
+#[derive(Debug)]
+struct Point {
+    x: i32,
+    y: i32,
+}
+
+fn main() {
+    let optional_point = Some(Point { x: 100, y: 200 });
+
+    // Solution 1: Matching over the `Option` (not `&Option`) but without moving
+    // out of the `Some` variant.
+    match optional_point {
+        Some(ref p) => println!("Co-ordinates are {},{}", p.x, p.y),
+        //   ^^^ added
+        _ => panic!("No match!"),
+    }
+
+    // Solution 2: Matching over a reference (`&Option`) by added `&` before
+    // `optional_point`.
+    match &optional_point {
+        Some(p) => println!("Co-ordinates are {},{}", p.x, p.y),
+        _ => panic!("No match!"),
+    }
+
+    println!("{optional_point:?}");
+}
diff --git a/solutions/13_error_handling/errors1.rs b/solutions/13_error_handling/errors1.rs
new file mode 100644
index 0000000..f552ca7
--- /dev/null
+++ b/solutions/13_error_handling/errors1.rs
@@ -0,0 +1,37 @@
+fn generate_nametag_text(name: String) -> Result<String, String> {
+    //                                    ^^^^^^         ^^^^^^
+    if name.is_empty() {
+        // `Err(String)` instead of `None`.
+        Err("Empty names aren't allowed".to_string())
+    } else {
+        // `Ok` instead of `Some`.
+        Ok(format!("Hi! My name is {name}"))
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn generates_nametag_text_for_a_nonempty_name() {
+        assert_eq!(
+            generate_nametag_text("Beyoncé".to_string()).as_deref(),
+            Ok("Hi! My name is Beyoncé"),
+        );
+    }
+
+    #[test]
+    fn explains_why_generating_nametag_text_fails() {
+        assert_eq!(
+            generate_nametag_text(String::new())
+                .as_ref()
+                .map_err(|e| e.as_str()),
+            Err("Empty names aren't allowed"),
+        );
+    }
+}
diff --git a/solutions/13_error_handling/errors2.rs b/solutions/13_error_handling/errors2.rs
new file mode 100644
index 0000000..0597c8c
--- /dev/null
+++ b/solutions/13_error_handling/errors2.rs
@@ -0,0 +1,58 @@
+// Say we're writing a game where you can buy items with tokens. All items cost
+// 5 tokens, and whenever you purchase items there is a processing fee of 1
+// token. A player of the game will type in how many items they want to buy, and
+// the `total_cost` function will calculate the total cost of the items. Since
+// the player typed in the quantity, we get it as a string. They might have
+// typed anything, not just numbers!
+//
+// Right now, this function isn't handling the error case at all. What we want
+// to do is: If we call the `total_cost` function on a string that is not a
+// number, that function will return a `ParseIntError`. In that case, we want to
+// immediately return that error from our function and not try to multiply and
+// add.
+//
+// There are at least two ways to implement this that are both correct. But one
+// is a lot shorter!
+
+use std::num::ParseIntError;
+
+#[allow(unused_variables)]
+fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
+    let processing_fee = 1;
+    let cost_per_item = 5;
+
+    // Added `?` to propagate the error.
+    let qty = item_quantity.parse::<i32>()?;
+    //                                    ^ added
+
+    // Equivalent to this verbose version:
+    let qty = match item_quantity.parse::<i32>() {
+        Ok(v) => v,
+        Err(e) => return Err(e),
+    };
+
+    Ok(qty * cost_per_item + processing_fee)
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::num::IntErrorKind;
+
+    #[test]
+    fn item_quantity_is_a_valid_number() {
+        assert_eq!(total_cost("34"), Ok(171));
+    }
+
+    #[test]
+    fn item_quantity_is_an_invalid_number() {
+        assert_eq!(
+            total_cost("beep boop").unwrap_err().kind(),
+            &IntErrorKind::InvalidDigit,
+        );
+    }
+}
diff --git a/solutions/13_error_handling/errors3.rs b/solutions/13_error_handling/errors3.rs
new file mode 100644
index 0000000..63f4aba
--- /dev/null
+++ b/solutions/13_error_handling/errors3.rs
@@ -0,0 +1,32 @@
+// This is a program that is trying to use a completed version of the
+// `total_cost` function from the previous exercise. It's not working though!
+// Why not? What should we do to fix it?
+
+use std::num::ParseIntError;
+
+// Don't change this function.
+fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
+    let processing_fee = 1;
+    let cost_per_item = 5;
+    let qty = item_quantity.parse::<i32>()?;
+
+    Ok(qty * cost_per_item + processing_fee)
+}
+
+fn main() -> Result<(), ParseIntError> {
+    //    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ added
+    let mut tokens = 100;
+    let pretend_user_input = "8";
+
+    let cost = total_cost(pretend_user_input)?;
+
+    if cost > tokens {
+        println!("You can't afford that many!");
+    } else {
+        tokens -= cost;
+        println!("You now have {tokens} tokens.");
+    }
+
+    // Added this line to return the `Ok` variant of the expected `Result`.
+    Ok(())
+}
diff --git a/solutions/13_error_handling/errors4.rs b/solutions/13_error_handling/errors4.rs
new file mode 100644
index 0000000..f4d39bf
--- /dev/null
+++ b/solutions/13_error_handling/errors4.rs
@@ -0,0 +1,44 @@
+#![allow(clippy::comparison_chain)]
+
+#[derive(PartialEq, Debug)]
+enum CreationError {
+    Negative,
+    Zero,
+}
+
+#[derive(PartialEq, Debug)]
+struct PositiveNonzeroInteger(u64);
+
+impl PositiveNonzeroInteger {
+    fn new(value: i64) -> Result<Self, CreationError> {
+        if value == 0 {
+            Err(CreationError::Zero)
+        } else if value < 0 {
+            Err(CreationError::Negative)
+        } else {
+            Ok(Self(value as u64))
+        }
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_creation() {
+        assert_eq!(
+            PositiveNonzeroInteger::new(10),
+            Ok(PositiveNonzeroInteger(10)),
+        );
+        assert_eq!(
+            PositiveNonzeroInteger::new(-10),
+            Err(CreationError::Negative),
+        );
+        assert_eq!(PositiveNonzeroInteger::new(0), Err(CreationError::Zero));
+    }
+}
diff --git a/solutions/13_error_handling/errors5.rs b/solutions/13_error_handling/errors5.rs
new file mode 100644
index 0000000..c1424ee
--- /dev/null
+++ b/solutions/13_error_handling/errors5.rs
@@ -0,0 +1,54 @@
+// This exercise is an altered version of the `errors4` exercise. It uses some
+// concepts that we won't get to until later in the course, like `Box` and the
+// `From` trait. It's not important to understand them in detail right now, but
+// you can read ahead if you like. For now, think of the `Box<dyn ???>` type as
+// an "I want anything that does ???" type.
+//
+// In short, this particular use case for boxes is for when you want to own a
+// value and you care only that it is a type which implements a particular
+// trait. To do so, The `Box` is declared as of type `Box<dyn Trait>` where
+// `Trait` is the trait the compiler looks for on any value used in that
+// context. For this exercise, that context is the potential errors which
+// can be returned in a `Result`.
+
+use std::error::Error;
+use std::fmt;
+
+#[derive(PartialEq, Debug)]
+enum CreationError {
+    Negative,
+    Zero,
+}
+
+// This is required so that `CreationError` can implement `Error`.
+impl fmt::Display for CreationError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let description = match *self {
+            CreationError::Negative => "number is negative",
+            CreationError::Zero => "number is zero",
+        };
+        f.write_str(description)
+    }
+}
+
+impl Error for CreationError {}
+
+#[derive(PartialEq, Debug)]
+struct PositiveNonzeroInteger(u64);
+
+impl PositiveNonzeroInteger {
+    fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
+        match value {
+            x if x < 0 => Err(CreationError::Negative),
+            0 => Err(CreationError::Zero),
+            x => Ok(PositiveNonzeroInteger(x as u64)),
+        }
+    }
+}
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let pretend_user_input = "42";
+    let x: i64 = pretend_user_input.parse()?;
+    println!("output={:?}", PositiveNonzeroInteger::new(x)?);
+    Ok(())
+}
diff --git a/solutions/13_error_handling/errors6.rs b/solutions/13_error_handling/errors6.rs
new file mode 100644
index 0000000..429d3ea
--- /dev/null
+++ b/solutions/13_error_handling/errors6.rs
@@ -0,0 +1,91 @@
+// Using catch-all error types like `Box<dyn Error>` isn't recommended for
+// library code where callers might want to make decisions based on the error
+// content instead of printing it out or propagating it further. Here, we define
+// a custom error type to make it possible for callers to decide what to do next
+// when our function returns an error.
+
+use std::num::ParseIntError;
+
+#[derive(PartialEq, Debug)]
+enum CreationError {
+    Negative,
+    Zero,
+}
+
+// A custom error type that we will be using in `PositiveNonzeroInteger::parse`.
+#[derive(PartialEq, Debug)]
+enum ParsePosNonzeroError {
+    Creation(CreationError),
+    ParseInt(ParseIntError),
+}
+
+impl ParsePosNonzeroError {
+    fn from_creation(err: CreationError) -> Self {
+        Self::Creation(err)
+    }
+
+    fn from_parseint(err: ParseIntError) -> Self {
+        Self::ParseInt(err)
+    }
+}
+
+#[derive(PartialEq, Debug)]
+struct PositiveNonzeroInteger(u64);
+
+impl PositiveNonzeroInteger {
+    fn new(value: i64) -> Result<Self, CreationError> {
+        match value {
+            x if x < 0 => Err(CreationError::Negative),
+            0 => Err(CreationError::Zero),
+            x => Ok(Self(x as u64)),
+        }
+    }
+
+    fn parse(s: &str) -> Result<Self, ParsePosNonzeroError> {
+        // Return an appropriate error instead of panicking when `parse()`
+        // returns an error.
+        let x: i64 = s.parse().map_err(ParsePosNonzeroError::from_parseint)?;
+        //                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+        Self::new(x).map_err(ParsePosNonzeroError::from_creation)
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_parse_error() {
+        assert!(matches!(
+            PositiveNonzeroInteger::parse("not a number"),
+            Err(ParsePosNonzeroError::ParseInt(_)),
+        ));
+    }
+
+    #[test]
+    fn test_negative() {
+        assert_eq!(
+            PositiveNonzeroInteger::parse("-555"),
+            Err(ParsePosNonzeroError::Creation(CreationError::Negative)),
+        );
+    }
+
+    #[test]
+    fn test_zero() {
+        assert_eq!(
+            PositiveNonzeroInteger::parse("0"),
+            Err(ParsePosNonzeroError::Creation(CreationError::Zero)),
+        );
+    }
+
+    #[test]
+    fn test_positive() {
+        let x = PositiveNonzeroInteger::new(42).unwrap();
+        assert_eq!(x.0, 42);
+        assert_eq!(PositiveNonzeroInteger::parse("42"), Ok(x));
+    }
+}
diff --git a/solutions/14_generics/generics1.rs b/solutions/14_generics/generics1.rs
new file mode 100644
index 0000000..e2195fd
--- /dev/null
+++ b/solutions/14_generics/generics1.rs
@@ -0,0 +1,17 @@
+// `Vec<T>` is generic over the type `T`. In most cases, the compiler is able to
+// infer `T`, for example after pushing a value with a concrete type to the vector.
+// But in this exercise, the compiler needs some help through a type annotation.
+
+fn main() {
+    // `u8` and `i8` can both be converted to `i16`.
+    let mut numbers: Vec<i16> = Vec::new();
+    //             ^^^^^^^^^^ added
+
+    // Don't change the lines below.
+    let n1: u8 = 42;
+    numbers.push(n1.into());
+    let n2: i8 = -1;
+    numbers.push(n2.into());
+
+    println!("{numbers:?}");
+}
diff --git a/solutions/14_generics/generics2.rs b/solutions/14_generics/generics2.rs
new file mode 100644
index 0000000..14f3f7a
--- /dev/null
+++ b/solutions/14_generics/generics2.rs
@@ -0,0 +1,28 @@
+struct Wrapper<T> {
+    value: T,
+}
+
+impl<T> Wrapper<T> {
+    fn new(value: T) -> Self {
+        Wrapper { value }
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn store_u32_in_wrapper() {
+        assert_eq!(Wrapper::new(42).value, 42);
+    }
+
+    #[test]
+    fn store_str_in_wrapper() {
+        assert_eq!(Wrapper::new("Foo").value, "Foo");
+    }
+}
diff --git a/solutions/15_traits/traits1.rs b/solutions/15_traits/traits1.rs
new file mode 100644
index 0000000..790873f
--- /dev/null
+++ b/solutions/15_traits/traits1.rs
@@ -0,0 +1,32 @@
+// The trait `AppendBar` has only one function which appends "Bar" to any object
+// implementing this trait.
+trait AppendBar {
+    fn append_bar(self) -> Self;
+}
+
+impl AppendBar for String {
+    fn append_bar(self) -> Self {
+        self + "Bar"
+    }
+}
+
+fn main() {
+    let s = String::from("Foo");
+    let s = s.append_bar();
+    println!("s: {s}");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn is_foo_bar() {
+        assert_eq!(String::from("Foo").append_bar(), "FooBar");
+    }
+
+    #[test]
+    fn is_bar_bar() {
+        assert_eq!(String::from("").append_bar().append_bar(), "BarBar");
+    }
+}
diff --git a/solutions/15_traits/traits2.rs b/solutions/15_traits/traits2.rs
new file mode 100644
index 0000000..0db93e0
--- /dev/null
+++ b/solutions/15_traits/traits2.rs
@@ -0,0 +1,27 @@
+trait AppendBar {
+    fn append_bar(self) -> Self;
+}
+
+impl AppendBar for Vec<String> {
+    fn append_bar(mut self) -> Self {
+        //        ^^^ this is important
+        self.push(String::from("Bar"));
+        self
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn is_vec_pop_eq_bar() {
+        let mut foo = vec![String::from("Foo")].append_bar();
+        assert_eq!(foo.pop().unwrap(), "Bar");
+        assert_eq!(foo.pop().unwrap(), "Foo");
+    }
+}
diff --git a/solutions/15_traits/traits3.rs b/solutions/15_traits/traits3.rs
new file mode 100644
index 0000000..747d919
--- /dev/null
+++ b/solutions/15_traits/traits3.rs
@@ -0,0 +1,38 @@
+#![allow(dead_code)]
+
+trait Licensed {
+    fn licensing_info(&self) -> String {
+        "Default license".to_string()
+    }
+}
+
+struct SomeSoftware {
+    version_number: i32,
+}
+
+struct OtherSoftware {
+    version_number: String,
+}
+
+impl Licensed for SomeSoftware {}
+impl Licensed for OtherSoftware {}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn is_licensing_info_the_same() {
+        let licensing_info = "Default license";
+        let some_software = SomeSoftware { version_number: 1 };
+        let other_software = OtherSoftware {
+            version_number: "v2.0.0".to_string(),
+        };
+        assert_eq!(some_software.licensing_info(), licensing_info);
+        assert_eq!(other_software.licensing_info(), licensing_info);
+    }
+}
diff --git a/solutions/15_traits/traits4.rs b/solutions/15_traits/traits4.rs
new file mode 100644
index 0000000..3675b8d
--- /dev/null
+++ b/solutions/15_traits/traits4.rs
@@ -0,0 +1,35 @@
+trait Licensed {
+    fn licensing_info(&self) -> String {
+        "Default license".to_string()
+    }
+}
+
+struct SomeSoftware;
+struct OtherSoftware;
+
+impl Licensed for SomeSoftware {}
+impl Licensed for OtherSoftware {}
+
+fn compare_license_types(software1: impl Licensed, software2: impl Licensed) -> bool {
+    //                              ^^^^^^^^^^^^^             ^^^^^^^^^^^^^
+    software1.licensing_info() == software2.licensing_info()
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn compare_license_information() {
+        assert!(compare_license_types(SomeSoftware, OtherSoftware));
+    }
+
+    #[test]
+    fn compare_license_information_backwards() {
+        assert!(compare_license_types(OtherSoftware, SomeSoftware));
+    }
+}
diff --git a/solutions/15_traits/traits5.rs b/solutions/15_traits/traits5.rs
new file mode 100644
index 0000000..1fb426a
--- /dev/null
+++ b/solutions/15_traits/traits5.rs
@@ -0,0 +1,39 @@
+trait SomeTrait {
+    fn some_function(&self) -> bool {
+        true
+    }
+}
+
+trait OtherTrait {
+    fn other_function(&self) -> bool {
+        true
+    }
+}
+
+struct SomeStruct;
+impl SomeTrait for SomeStruct {}
+impl OtherTrait for SomeStruct {}
+
+struct OtherStruct;
+impl SomeTrait for OtherStruct {}
+impl OtherTrait for OtherStruct {}
+
+fn some_func(item: impl SomeTrait + OtherTrait) -> bool {
+    //             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    item.some_function() && item.other_function()
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_some_func() {
+        assert!(some_func(SomeStruct));
+        assert!(some_func(OtherStruct));
+    }
+}
diff --git a/solutions/16_lifetimes/lifetimes1.rs b/solutions/16_lifetimes/lifetimes1.rs
new file mode 100644
index 0000000..ca7b688
--- /dev/null
+++ b/solutions/16_lifetimes/lifetimes1.rs
@@ -0,0 +1,28 @@
+// The Rust compiler needs to know how to check whether supplied references are
+// valid, so that it can let the programmer know if a reference is at risk of
+// going out of scope before it is used. Remember, references are borrows and do
+// not own their own data. What if their owner goes out of scope?
+
+fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
+    //    ^^^^     ^^          ^^          ^^
+    if x.len() > y.len() {
+        x
+    } else {
+        y
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_longest() {
+        assert_eq!(longest("abcd", "123"), "abcd");
+        assert_eq!(longest("abc", "1234"), "1234");
+    }
+}
diff --git a/solutions/16_lifetimes/lifetimes2.rs b/solutions/16_lifetimes/lifetimes2.rs
new file mode 100644
index 0000000..b0f2ef1
--- /dev/null
+++ b/solutions/16_lifetimes/lifetimes2.rs
@@ -0,0 +1,33 @@
+fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
+    if x.len() > y.len() {
+        x
+    } else {
+        y
+    }
+}
+
+fn main() {
+    let string1 = String::from("long string is long");
+    // Solution1: You can move `strings2` out of the inner block so that it is
+    // not dropped before the print statement.
+    let string2 = String::from("xyz");
+    let result;
+    {
+        result = longest(&string1, &string2);
+    }
+    println!("The longest string is '{result}'");
+    // `string2` dropped at the end of the function.
+
+    // =========================================================================
+
+    let string1 = String::from("long string is long");
+    let result;
+    {
+        let string2 = String::from("xyz");
+        result = longest(&string1, &string2);
+        // Solution2: You can move the print statement into the inner block so
+        // that it is executed before `string2` is dropped.
+        println!("The longest string is '{result}'");
+        // `string2` dropped here (end of the inner scope).
+    }
+}
diff --git a/solutions/16_lifetimes/lifetimes3.rs b/solutions/16_lifetimes/lifetimes3.rs
new file mode 100644
index 0000000..16a5a68
--- /dev/null
+++ b/solutions/16_lifetimes/lifetimes3.rs
@@ -0,0 +1,18 @@
+// Lifetimes are also needed when structs hold references.
+
+struct Book<'a> {
+    //     ^^^^ added a lifetime annotation
+    author: &'a str,
+    //       ^^
+    title: &'a str,
+    //      ^^
+}
+
+fn main() {
+    let book = Book {
+        author: "George Orwell",
+        title: "1984",
+    };
+
+    println!("{} by {}", book.title, book.author);
+}
diff --git a/solutions/17_tests/tests1.rs b/solutions/17_tests/tests1.rs
new file mode 100644
index 0000000..c52b8b1
--- /dev/null
+++ b/solutions/17_tests/tests1.rs
@@ -0,0 +1,24 @@
+// Tests are important to ensure that your code does what you think it should
+// do.
+
+fn is_even(n: i64) -> bool {
+    n % 2 == 0
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    // When writing unit tests, it is common to import everything from the outer
+    // module (`super`) using a wildcard.
+    use super::*;
+
+    #[test]
+    fn you_can_assert() {
+        assert!(is_even(0));
+        assert!(!is_even(-1));
+        //      ^ You can assert `false` using the negation operator `!`.
+    }
+}
diff --git a/solutions/17_tests/tests2.rs b/solutions/17_tests/tests2.rs
new file mode 100644
index 0000000..39a0005
--- /dev/null
+++ b/solutions/17_tests/tests2.rs
@@ -0,0 +1,22 @@
+// Calculates the power of 2 using a bit shift.
+// `1 << n` is equivalent to "2 to the power of n".
+fn power_of_2(n: u8) -> u64 {
+    1 << n
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn you_can_assert_eq() {
+        assert_eq!(power_of_2(0), 1);
+        assert_eq!(power_of_2(1), 2);
+        assert_eq!(power_of_2(2), 4);
+        assert_eq!(power_of_2(3), 8);
+    }
+}
diff --git a/solutions/17_tests/tests3.rs b/solutions/17_tests/tests3.rs
new file mode 100644
index 0000000..503f9bc
--- /dev/null
+++ b/solutions/17_tests/tests3.rs
@@ -0,0 +1,45 @@
+struct Rectangle {
+    width: i32,
+    height: i32,
+}
+
+impl Rectangle {
+    // Don't change this function.
+    fn new(width: i32, height: i32) -> Self {
+        if width <= 0 || height <= 0 {
+            // Returning a `Result` would be better here. But we want to learn
+            // how to test functions that can panic.
+            panic!("Rectangle width and height can't be negative");
+        }
+
+        Rectangle { width, height }
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn correct_width_and_height() {
+        let rect = Rectangle::new(10, 20);
+        assert_eq!(rect.width, 10); // Check width
+        assert_eq!(rect.height, 20); // Check height
+    }
+
+    #[test]
+    #[should_panic] // Added this attribute to check that the test panics.
+    fn negative_width() {
+        let _rect = Rectangle::new(-10, 10);
+    }
+
+    #[test]
+    #[should_panic] // Added this attribute to check that the test panics.
+    fn negative_height() {
+        let _rect = Rectangle::new(10, -10);
+    }
+}
diff --git a/solutions/18_iterators/iterators1.rs b/solutions/18_iterators/iterators1.rs
new file mode 100644
index 0000000..93a6008
--- /dev/null
+++ b/solutions/18_iterators/iterators1.rs
@@ -0,0 +1,26 @@
+// When performing operations on elements within a collection, iterators are
+// essential. This module helps you get familiar with the structure of using an
+// iterator and how to go through elements within an iterable collection.
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn iterators() {
+        let my_fav_fruits = ["banana", "custard apple", "avocado", "peach", "raspberry"];
+
+        // Create an iterator over the array.
+        let mut fav_fruits_iterator = my_fav_fruits.iter();
+
+        assert_eq!(fav_fruits_iterator.next(), Some(&"banana"));
+        assert_eq!(fav_fruits_iterator.next(), Some(&"custard apple"));
+        assert_eq!(fav_fruits_iterator.next(), Some(&"avocado"));
+        assert_eq!(fav_fruits_iterator.next(), Some(&"peach"));
+        assert_eq!(fav_fruits_iterator.next(), Some(&"raspberry"));
+        assert_eq!(fav_fruits_iterator.next(), None);
+        //                                     ^^^^ reached the end
+    }
+}
diff --git a/solutions/18_iterators/iterators2.rs b/solutions/18_iterators/iterators2.rs
new file mode 100644
index 0000000..db05f29
--- /dev/null
+++ b/solutions/18_iterators/iterators2.rs
@@ -0,0 +1,56 @@
+// In this exercise, you'll learn some of the unique advantages that iterators
+// can offer.
+
+// "hello" -> "Hello"
+fn capitalize_first(input: &str) -> String {
+    let mut chars = input.chars();
+    match chars.next() {
+        None => String::new(),
+        Some(first) => first.to_uppercase().to_string() + chars.as_str(),
+    }
+}
+
+// Apply the `capitalize_first` function to a slice of string slices.
+// Return a vector of strings.
+// ["hello", "world"] -> ["Hello", "World"]
+fn capitalize_words_vector(words: &[&str]) -> Vec<String> {
+    words.iter().map(|word| capitalize_first(word)).collect()
+}
+
+// Apply the `capitalize_first` function again to a slice of string
+// slices. Return a single string.
+// ["hello", " ", "world"] -> "Hello World"
+fn capitalize_words_string(words: &[&str]) -> String {
+    words.iter().map(|word| capitalize_first(word)).collect()
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_success() {
+        assert_eq!(capitalize_first("hello"), "Hello");
+    }
+
+    #[test]
+    fn test_empty() {
+        assert_eq!(capitalize_first(""), "");
+    }
+
+    #[test]
+    fn test_iterate_string_vec() {
+        let words = vec!["hello", "world"];
+        assert_eq!(capitalize_words_vector(&words), ["Hello", "World"]);
+    }
+
+    #[test]
+    fn test_iterate_into_string() {
+        let words = vec!["hello", " ", "world"];
+        assert_eq!(capitalize_words_string(&words), "Hello World");
+    }
+}
diff --git a/solutions/18_iterators/iterators3.rs b/solutions/18_iterators/iterators3.rs
new file mode 100644
index 0000000..d66d1ef
--- /dev/null
+++ b/solutions/18_iterators/iterators3.rs
@@ -0,0 +1,73 @@
+#[derive(Debug, PartialEq, Eq)]
+enum DivisionError {
+    DivideByZero,
+    NotDivisible,
+}
+
+fn divide(a: i64, b: i64) -> Result<i64, DivisionError> {
+    if b == 0 {
+        return Err(DivisionError::DivideByZero);
+    }
+
+    if a % b != 0 {
+        return Err(DivisionError::NotDivisible);
+    }
+
+    Ok(a / b)
+}
+
+fn result_with_list() -> Result<Vec<i64>, DivisionError> {
+    //                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    let numbers = [27, 297, 38502, 81];
+    let division_results = numbers.into_iter().map(|n| divide(n, 27));
+    // Collects to the expected return type. Returns the first error in the
+    // division results (if one exists).
+    division_results.collect()
+}
+
+fn list_of_results() -> Vec<Result<i64, DivisionError>> {
+    //               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    let numbers = [27, 297, 38502, 81];
+    let division_results = numbers.into_iter().map(|n| divide(n, 27));
+    // Collects to the expected return type.
+    division_results.collect()
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_success() {
+        assert_eq!(divide(81, 9), Ok(9));
+    }
+
+    #[test]
+    fn test_divide_by_0() {
+        assert_eq!(divide(81, 0), Err(DivisionError::DivideByZero));
+    }
+
+    #[test]
+    fn test_not_divisible() {
+        assert_eq!(divide(81, 6), Err(DivisionError::NotDivisible));
+    }
+
+    #[test]
+    fn test_divide_0_by_something() {
+        assert_eq!(divide(0, 81), Ok(0));
+    }
+
+    #[test]
+    fn test_result_with_list() {
+        assert_eq!(result_with_list().unwrap(), [1, 11, 1426, 3]);
+    }
+
+    #[test]
+    fn test_list_of_results() {
+        assert_eq!(list_of_results(), [Ok(1), Ok(11), Ok(1426), Ok(3)]);
+    }
+}
diff --git a/solutions/18_iterators/iterators4.rs b/solutions/18_iterators/iterators4.rs
new file mode 100644
index 0000000..4c3c49d
--- /dev/null
+++ b/solutions/18_iterators/iterators4.rs
@@ -0,0 +1,71 @@
+// 3 possible solutions are presented.
+
+// With `for` loop and a mutable variable.
+fn factorial_for(num: u64) -> u64 {
+    let mut result = 1;
+
+    for x in 2..=num {
+        result *= x;
+    }
+
+    result
+}
+
+// Equivalent to `factorial_for` but shorter and without a `for` loop and
+// mutable variables.
+fn factorial_fold(num: u64) -> u64 {
+    // Case num==0: The iterator 2..=0 is empty
+    //              -> The initial value of `fold` is returned which is 1.
+    // Case num==1: The iterator 2..=1 is also empty
+    //              -> The initial value 1 is returned.
+    // Case num==2: The iterator 2..=2 contains one element
+    //              -> The initial value 1 is multiplied by 2 and the result
+    //                 is returned.
+    // Case num==3: The iterator 2..=3 contains 2 elements
+    //              -> 1 * 2 is calculated, then the result 2 is multiplied by
+    //                 the second element 3 so the result 6 is returned.
+    // And so on…
+    (2..=num).fold(1, |acc, x| acc * x)
+}
+
+// Equivalent to `factorial_fold` but with a built-in method that is suggested
+// by Clippy.
+fn factorial_product(num: u64) -> u64 {
+    (2..=num).product()
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn factorial_of_0() {
+        assert_eq!(factorial_for(0), 1);
+        assert_eq!(factorial_fold(0), 1);
+        assert_eq!(factorial_product(0), 1);
+    }
+
+    #[test]
+    fn factorial_of_1() {
+        assert_eq!(factorial_for(1), 1);
+        assert_eq!(factorial_fold(1), 1);
+        assert_eq!(factorial_product(1), 1);
+    }
+    #[test]
+    fn factorial_of_2() {
+        assert_eq!(factorial_for(2), 2);
+        assert_eq!(factorial_fold(2), 2);
+        assert_eq!(factorial_product(2), 2);
+    }
+
+    #[test]
+    fn factorial_of_4() {
+        assert_eq!(factorial_for(4), 24);
+        assert_eq!(factorial_fold(4), 24);
+        assert_eq!(factorial_product(4), 24);
+    }
+}
diff --git a/solutions/18_iterators/iterators5.rs b/solutions/18_iterators/iterators5.rs
new file mode 100644
index 0000000..85d9a4f
--- /dev/null
+++ b/solutions/18_iterators/iterators5.rs
@@ -0,0 +1,183 @@
+// Let's define a simple model to track Rustlings' exercise progress. Progress
+// will be modelled using a hash map. The name of the exercise is the key and
+// the progress is the value. Two counting functions were created to count the
+// number of exercises with a given progress. Recreate this counting
+// functionality using iterators. Try to not use imperative loops (for/while).
+
+use std::collections::HashMap;
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+enum Progress {
+    None,
+    Some,
+    Complete,
+}
+
+fn count_for(map: &HashMap<String, Progress>, value: Progress) -> usize {
+    let mut count = 0;
+    for val in map.values() {
+        if *val == value {
+            count += 1;
+        }
+    }
+    count
+}
+
+fn count_iterator(map: &HashMap<String, Progress>, value: Progress) -> usize {
+    // `map` is a hash map with `String` keys and `Progress` values.
+    // map = { "variables1": Complete, "from_str": None, … }
+    map.values().filter(|val| **val == value).count()
+}
+
+fn count_collection_for(collection: &[HashMap<String, Progress>], value: Progress) -> usize {
+    let mut count = 0;
+    for map in collection {
+        count += count_for(map, value);
+    }
+    count
+}
+
+fn count_collection_iterator(collection: &[HashMap<String, Progress>], value: Progress) -> usize {
+    // `collection` is a slice of hash maps.
+    // collection = [{ "variables1": Complete, "from_str": None, … },
+    //               { "variables2": Complete, … }, … ]
+    collection
+        .iter()
+        .map(|map| count_iterator(map, value))
+        .sum()
+}
+
+// Equivalent to `count_collection_iterator`+`count_iterator`, iterating as if
+// the collection was a single container instead of a container of containers
+// (and more accurately, a single iterator instead of an iterator of iterators).
+fn count_collection_iterator_flat(
+    collection: &[HashMap<String, Progress>],
+    value: Progress,
+) -> usize {
+    // `collection` is a slice of hash maps.
+    // collection = [{ "variables1": Complete, "from_str": None, … },
+    //               { "variables2": Complete, … }, … ]
+    collection
+        .iter()
+        .flat_map(HashMap::values) // or just `.flatten()` when wanting the default iterator (`HashMap::iter`)
+        .filter(|val| **val == value)
+        .count()
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn get_map() -> HashMap<String, Progress> {
+        use Progress::*;
+
+        let mut map = HashMap::new();
+        map.insert(String::from("variables1"), Complete);
+        map.insert(String::from("functions1"), Complete);
+        map.insert(String::from("hashmap1"), Complete);
+        map.insert(String::from("arc1"), Some);
+        map.insert(String::from("as_ref_mut"), None);
+        map.insert(String::from("from_str"), None);
+
+        map
+    }
+
+    fn get_vec_map() -> Vec<HashMap<String, Progress>> {
+        use Progress::*;
+
+        let map = get_map();
+
+        let mut other = HashMap::new();
+        other.insert(String::from("variables2"), Complete);
+        other.insert(String::from("functions2"), Complete);
+        other.insert(String::from("if1"), Complete);
+        other.insert(String::from("from_into"), None);
+        other.insert(String::from("try_from_into"), None);
+
+        vec![map, other]
+    }
+
+    #[test]
+    fn count_complete() {
+        let map = get_map();
+        assert_eq!(count_iterator(&map, Progress::Complete), 3);
+    }
+
+    #[test]
+    fn count_some() {
+        let map = get_map();
+        assert_eq!(count_iterator(&map, Progress::Some), 1);
+    }
+
+    #[test]
+    fn count_none() {
+        let map = get_map();
+        assert_eq!(count_iterator(&map, Progress::None), 2);
+    }
+
+    #[test]
+    fn count_complete_equals_for() {
+        let map = get_map();
+        let progress_states = [Progress::Complete, Progress::Some, Progress::None];
+        for progress_state in progress_states {
+            assert_eq!(
+                count_for(&map, progress_state),
+                count_iterator(&map, progress_state),
+            );
+        }
+    }
+
+    #[test]
+    fn count_collection_complete() {
+        let collection = get_vec_map();
+        assert_eq!(
+            count_collection_iterator(&collection, Progress::Complete),
+            6,
+        );
+        assert_eq!(
+            count_collection_iterator_flat(&collection, Progress::Complete),
+            6,
+        );
+    }
+
+    #[test]
+    fn count_collection_some() {
+        let collection = get_vec_map();
+        assert_eq!(count_collection_iterator(&collection, Progress::Some), 1);
+        assert_eq!(
+            count_collection_iterator_flat(&collection, Progress::Some),
+            1
+        );
+    }
+
+    #[test]
+    fn count_collection_none() {
+        let collection = get_vec_map();
+        assert_eq!(count_collection_iterator(&collection, Progress::None), 4);
+        assert_eq!(
+            count_collection_iterator_flat(&collection, Progress::None),
+            4
+        );
+    }
+
+    #[test]
+    fn count_collection_equals_for() {
+        let collection = get_vec_map();
+        let progress_states = [Progress::Complete, Progress::Some, Progress::None];
+
+        for progress_state in progress_states {
+            assert_eq!(
+                count_collection_for(&collection, progress_state),
+                count_collection_iterator(&collection, progress_state),
+            );
+            assert_eq!(
+                count_collection_for(&collection, progress_state),
+                count_collection_iterator_flat(&collection, progress_state),
+            );
+        }
+    }
+}
diff --git a/solutions/19_smart_pointers/arc1.rs b/solutions/19_smart_pointers/arc1.rs
new file mode 100644
index 0000000..bd76189
--- /dev/null
+++ b/solutions/19_smart_pointers/arc1.rs
@@ -0,0 +1,45 @@
+// In this exercise, we are given a `Vec` of `u32` called `numbers` with values
+// ranging from 0 to 99. We would like to use this set of numbers within 8
+// different threads simultaneously. Each thread is going to get the sum of
+// every eighth value with an offset.
+//
+// The first thread (offset 0), will sum 0, 8, 16, …
+// The second thread (offset 1), will sum 1, 9, 17, …
+// The third thread (offset 2), will sum 2, 10, 18, …
+// …
+// The eighth thread (offset 7), will sum 7, 15, 23, …
+//
+// Each thread should own a reference-counting pointer to the vector of
+// numbers. But `Rc` isn't thread-safe. Therefore, we need to use `Arc`.
+//
+// Don't get distracted by how threads are spawned and joined. We will practice
+// that later in the exercises about threads.
+
+// Don't change the lines below.
+#![forbid(unused_imports)]
+use std::{sync::Arc, thread};
+
+fn main() {
+    let numbers: Vec<_> = (0..100u32).collect();
+
+    let shared_numbers = Arc::new(numbers);
+    //                   ^^^^^^^^^^^^^^^^^
+
+    let mut join_handles = Vec::new();
+
+    for offset in 0..8 {
+        let child_numbers = Arc::clone(&shared_numbers);
+        //                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+        let handle = thread::spawn(move || {
+            let sum: u32 = child_numbers.iter().filter(|&&n| n % 8 == offset).sum();
+            println!("Sum of offset {offset} is {sum}");
+        });
+
+        join_handles.push(handle);
+    }
+
+    for handle in join_handles.into_iter() {
+        handle.join().unwrap();
+    }
+}
diff --git a/solutions/19_smart_pointers/box1.rs b/solutions/19_smart_pointers/box1.rs
new file mode 100644
index 0000000..189cc56
--- /dev/null
+++ b/solutions/19_smart_pointers/box1.rs
@@ -0,0 +1,47 @@
+// At compile time, Rust needs to know how much space a type takes up. This
+// becomes problematic for recursive types, where a value can have as part of
+// itself another value of the same type. To get around the issue, we can use a
+// `Box` - a smart pointer used to store data on the heap, which also allows us
+// to wrap a recursive type.
+//
+// The recursive type we're implementing in this exercise is the "cons list", a
+// data structure frequently found in functional programming languages. Each
+// item in a cons list contains two elements: The value of the current item and
+// the next item. The last item is a value called `Nil`.
+
+#[derive(PartialEq, Debug)]
+enum List {
+    Cons(i32, Box<List>),
+    Nil,
+}
+
+fn create_empty_list() -> List {
+    List::Nil
+}
+
+fn create_non_empty_list() -> List {
+    List::Cons(42, Box::new(List::Nil))
+}
+
+fn main() {
+    println!("This is an empty cons list: {:?}", create_empty_list());
+    println!(
+        "This is a non-empty cons list: {:?}",
+        create_non_empty_list(),
+    );
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_create_empty_list() {
+        assert_eq!(create_empty_list(), List::Nil);
+    }
+
+    #[test]
+    fn test_create_non_empty_list() {
+        assert_ne!(create_empty_list(), create_non_empty_list());
+    }
+}
diff --git a/solutions/19_smart_pointers/cow1.rs b/solutions/19_smart_pointers/cow1.rs
new file mode 100644
index 0000000..0a21a91
--- /dev/null
+++ b/solutions/19_smart_pointers/cow1.rs
@@ -0,0 +1,68 @@
+// This exercise explores the `Cow` (Clone-On-Write) smart pointer. It can
+// enclose and provide immutable access to borrowed data and clone the data
+// lazily when mutation or ownership is required. The type is designed to work
+// with general borrowed data via the `Borrow` trait.
+
+use std::borrow::Cow;
+
+fn abs_all(input: &mut Cow<[i32]>) {
+    for ind in 0..input.len() {
+        let value = input[ind];
+        if value < 0 {
+            // Clones into a vector if not already owned.
+            input.to_mut()[ind] = -value;
+        }
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn reference_mutation() {
+        // Clone occurs because `input` needs to be mutated.
+        let vec = vec![-1, 0, 1];
+        let mut input = Cow::from(&vec);
+        abs_all(&mut input);
+        assert!(matches!(input, Cow::Owned(_)));
+    }
+
+    #[test]
+    fn reference_no_mutation() {
+        // No clone occurs because `input` doesn't need to be mutated.
+        let vec = vec![0, 1, 2];
+        let mut input = Cow::from(&vec);
+        abs_all(&mut input);
+        assert!(matches!(input, Cow::Borrowed(_)));
+        //                      ^^^^^^^^^^^^^^^^
+    }
+
+    #[test]
+    fn owned_no_mutation() {
+        // We can also pass `vec` without `&` so `Cow` owns it directly. In this
+        // case, no mutation occurs and thus also no clone. But the result is
+        // still owned because it was never borrowed or mutated.
+        let vec = vec![0, 1, 2];
+        let mut input = Cow::from(vec);
+        abs_all(&mut input);
+        assert!(matches!(input, Cow::Owned(_)));
+        //                      ^^^^^^^^^^^^^
+    }
+
+    #[test]
+    fn owned_mutation() {
+        // Of course this is also the case if a mutation does occur. In this
+        // case, the call to `to_mut()` in the `abs_all` function returns a
+        // reference to the same data as before.
+        let vec = vec![-1, 0, 1];
+        let mut input = Cow::from(vec);
+        abs_all(&mut input);
+        assert!(matches!(input, Cow::Owned(_)));
+        //                      ^^^^^^^^^^^^^
+    }
+}
diff --git a/solutions/19_smart_pointers/rc1.rs b/solutions/19_smart_pointers/rc1.rs
new file mode 100644
index 0000000..512eb9c
--- /dev/null
+++ b/solutions/19_smart_pointers/rc1.rs
@@ -0,0 +1,105 @@
+// In this exercise, we want to express the concept of multiple owners via the
+// `Rc<T>` type. This is a model of our solar system - there is a `Sun` type and
+// multiple `Planet`s. The planets take ownership of the sun, indicating that
+// they revolve around the sun.
+
+use std::rc::Rc;
+
+#[derive(Debug)]
+struct Sun;
+
+#[allow(dead_code)]
+#[derive(Debug)]
+enum Planet {
+    Mercury(Rc<Sun>),
+    Venus(Rc<Sun>),
+    Earth(Rc<Sun>),
+    Mars(Rc<Sun>),
+    Jupiter(Rc<Sun>),
+    Saturn(Rc<Sun>),
+    Uranus(Rc<Sun>),
+    Neptune(Rc<Sun>),
+}
+
+impl Planet {
+    fn details(&self) {
+        println!("Hi from {self:?}!");
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn rc1() {
+        let sun = Rc::new(Sun);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 1 reference
+
+        let mercury = Planet::Mercury(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 2 references
+        mercury.details();
+
+        let venus = Planet::Venus(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 3 references
+        venus.details();
+
+        let earth = Planet::Earth(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 4 references
+        earth.details();
+
+        let mars = Planet::Mars(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 5 references
+        mars.details();
+
+        let jupiter = Planet::Jupiter(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 6 references
+        jupiter.details();
+
+        let saturn = Planet::Saturn(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 7 references
+        saturn.details();
+
+        // TODO
+        let uranus = Planet::Uranus(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 8 references
+        uranus.details();
+
+        // TODO
+        let neptune = Planet::Neptune(Rc::clone(&sun));
+        println!("reference count = {}", Rc::strong_count(&sun)); // 9 references
+        neptune.details();
+
+        assert_eq!(Rc::strong_count(&sun), 9);
+
+        drop(neptune);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 8 references
+
+        drop(uranus);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 7 references
+
+        drop(saturn);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 6 references
+
+        drop(jupiter);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 5 references
+
+        drop(mars);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 4 references
+
+        drop(earth);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 3 references
+
+        drop(venus);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 2 references
+
+        drop(mercury);
+        println!("reference count = {}", Rc::strong_count(&sun)); // 1 reference
+
+        assert_eq!(Rc::strong_count(&sun), 1);
+    }
+}
diff --git a/solutions/20_threads/threads1.rs b/solutions/20_threads/threads1.rs
new file mode 100644
index 0000000..7f3dd29
--- /dev/null
+++ b/solutions/20_threads/threads1.rs
@@ -0,0 +1,37 @@
+// This program spawns multiple threads that each run for at least 250ms, and
+// each thread returns how much time they took to complete. The program should
+// wait until all the spawned threads have finished and should collect their
+// return values into a vector.
+
+use std::{
+    thread,
+    time::{Duration, Instant},
+};
+
+fn main() {
+    let mut handles = Vec::new();
+    for i in 0..10 {
+        let handle = thread::spawn(move || {
+            let start = Instant::now();
+            thread::sleep(Duration::from_millis(250));
+            println!("Thread {i} done");
+            start.elapsed().as_millis()
+        });
+        handles.push(handle);
+    }
+
+    let mut results = Vec::new();
+    for handle in handles {
+        // Collect the results of all threads into the `results` vector.
+        results.push(handle.join().unwrap());
+    }
+
+    if results.len() != 10 {
+        panic!("Oh no! Some thread isn't done yet!");
+    }
+
+    println!();
+    for (i, result) in results.into_iter().enumerate() {
+        println!("Thread {i} took {result}ms");
+    }
+}
diff --git a/solutions/20_threads/threads2.rs b/solutions/20_threads/threads2.rs
new file mode 100644
index 0000000..bc268d6
--- /dev/null
+++ b/solutions/20_threads/threads2.rs
@@ -0,0 +1,41 @@
+// Building on the last exercise, we want all of the threads to complete their
+// work. But this time, the spawned threads need to be in charge of updating a
+// shared value: `JobStatus.jobs_done`
+
+use std::{
+    sync::{Arc, Mutex},
+    thread,
+    time::Duration,
+};
+
+struct JobStatus {
+    jobs_done: u32,
+}
+
+fn main() {
+    // `Arc` isn't enough if you want a **mutable** shared state.
+    // We need to wrap the value with a `Mutex`.
+    let status = Arc::new(Mutex::new(JobStatus { jobs_done: 0 }));
+    //                    ^^^^^^^^^^^                          ^
+
+    let mut handles = Vec::new();
+    for _ in 0..10 {
+        let status_shared = Arc::clone(&status);
+        let handle = thread::spawn(move || {
+            thread::sleep(Duration::from_millis(250));
+
+            // Lock before you update a shared value.
+            status_shared.lock().unwrap().jobs_done += 1;
+            //           ^^^^^^^^^^^^^^^^
+        });
+        handles.push(handle);
+    }
+
+    // Waiting for all jobs to complete.
+    for handle in handles {
+        handle.join().unwrap();
+    }
+
+    println!("Jobs done: {}", status.lock().unwrap().jobs_done);
+    //                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+}
diff --git a/solutions/20_threads/threads3.rs b/solutions/20_threads/threads3.rs
new file mode 100644
index 0000000..cd2dfbe
--- /dev/null
+++ b/solutions/20_threads/threads3.rs
@@ -0,0 +1,66 @@
+use std::{sync::mpsc, thread, time::Duration};
+
+struct Queue {
+    length: u32,
+    first_half: Vec<u32>,
+    second_half: Vec<u32>,
+}
+
+impl Queue {
+    fn new() -> Self {
+        Self {
+            length: 10,
+            first_half: vec![1, 2, 3, 4, 5],
+            second_half: vec![6, 7, 8, 9, 10],
+        }
+    }
+}
+
+fn send_tx(q: Queue, tx: mpsc::Sender<u32>) {
+    // Clone the sender `tx` first.
+    let tx_clone = tx.clone();
+    thread::spawn(move || {
+        for val in q.first_half {
+            println!("Sending {val:?}");
+            // Then use the clone in the first thread. This means that
+            // `tx_clone` is moved to the first thread and `tx` to the second.
+            tx_clone.send(val).unwrap();
+            thread::sleep(Duration::from_millis(250));
+        }
+    });
+
+    thread::spawn(move || {
+        for val in q.second_half {
+            println!("Sending {val:?}");
+            tx.send(val).unwrap();
+            thread::sleep(Duration::from_millis(250));
+        }
+    });
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn threads3() {
+        let (tx, rx) = mpsc::channel();
+        let queue = Queue::new();
+        let queue_length = queue.length;
+
+        send_tx(queue, tx);
+
+        let mut total_received: u32 = 0;
+        for received in rx {
+            println!("Got: {received}");
+            total_received += 1;
+        }
+
+        println!("Number of received values: {total_received}");
+        assert_eq!(total_received, queue_length);
+    }
+}
diff --git a/solutions/21_macros/macros1.rs b/solutions/21_macros/macros1.rs
new file mode 100644
index 0000000..1b86156
--- /dev/null
+++ b/solutions/21_macros/macros1.rs
@@ -0,0 +1,10 @@
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    };
+}
+
+fn main() {
+    my_macro!();
+    //      ^
+}
diff --git a/solutions/21_macros/macros2.rs b/solutions/21_macros/macros2.rs
new file mode 100644
index 0000000..b6fd5d2
--- /dev/null
+++ b/solutions/21_macros/macros2.rs
@@ -0,0 +1,10 @@
+// Moved the macro definition to be before its call.
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    };
+}
+
+fn main() {
+    my_macro!();
+}
diff --git a/solutions/21_macros/macros3.rs b/solutions/21_macros/macros3.rs
new file mode 100644
index 0000000..df35be4
--- /dev/null
+++ b/solutions/21_macros/macros3.rs
@@ -0,0 +1,13 @@
+// Added the attribute `macro_use` attribute.
+#[macro_use]
+mod macros {
+    macro_rules! my_macro {
+        () => {
+            println!("Check out my macro!");
+        };
+    }
+}
+
+fn main() {
+    my_macro!();
+}
diff --git a/solutions/21_macros/macros4.rs b/solutions/21_macros/macros4.rs
new file mode 100644
index 0000000..41bcad1
--- /dev/null
+++ b/solutions/21_macros/macros4.rs
@@ -0,0 +1,15 @@
+// Added semicolons to separate the macro arms.
+#[rustfmt::skip]
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    };
+    ($val:expr) => {
+        println!("Look at this other macro: {}", $val);
+    };
+}
+
+fn main() {
+    my_macro!();
+    my_macro!(7777);
+}
diff --git a/solutions/22_clippy/clippy1.rs b/solutions/22_clippy/clippy1.rs
new file mode 100644
index 0000000..b9d1ec1
--- /dev/null
+++ b/solutions/22_clippy/clippy1.rs
@@ -0,0 +1,17 @@
+// The Clippy tool is a collection of lints to analyze your code so you can
+// catch common mistakes and improve your Rust code.
+//
+// For these exercises, the code will fail to compile when there are Clippy
+// warnings. Check Clippy's suggestions from the output to solve the exercise.
+
+use std::f32::consts::PI;
+
+fn main() {
+    // Use the more accurate `PI` constant.
+    let pi = PI;
+    let radius: f32 = 5.0;
+
+    let area = pi * radius.powi(2);
+
+    println!("The area of a circle with radius {radius:.2} is {area:.5}");
+}
diff --git a/solutions/22_clippy/clippy2.rs b/solutions/22_clippy/clippy2.rs
new file mode 100644
index 0000000..7f63562
--- /dev/null
+++ b/solutions/22_clippy/clippy2.rs
@@ -0,0 +1,10 @@
+fn main() {
+    let mut res = 42;
+    let option = Some(12);
+    // Use `if-let` instead of iteration.
+    if let Some(x) = option {
+        res += x;
+    }
+
+    println!("{res}");
+}
diff --git a/solutions/22_clippy/clippy3.rs b/solutions/22_clippy/clippy3.rs
new file mode 100644
index 0000000..811d184
--- /dev/null
+++ b/solutions/22_clippy/clippy3.rs
@@ -0,0 +1,31 @@
+use std::mem;
+
+#[rustfmt::skip]
+#[allow(unused_variables, unused_assignments)]
+fn main() {
+    let my_option: Option<()> = None;
+    // `unwrap` of an `Option` after checking if it is `None` will panic.
+    // Use `if-let` instead.
+    if let Some(value) = my_option {
+        println!("{value:?}");
+    }
+
+    // A comma was missing.
+    let my_arr = &[
+        -1, -2, -3,
+        -4, -5, -6,
+    ];
+    println!("My array! Here it is: {:?}", my_arr);
+
+    let mut my_empty_vec = vec![1, 2, 3, 4, 5];
+    // `resize` mutates a vector instead of returning a new one.
+    // `resize(0, …)` clears a vector, so it is better to use `clear`.
+    my_empty_vec.clear();
+    println!("This Vec is empty, see? {my_empty_vec:?}");
+
+    let mut value_a = 45;
+    let mut value_b = 66;
+    // Use `mem::swap` to correctly swap two values.
+    mem::swap(&mut value_a, &mut value_b);
+    println!("value a: {}; value b: {}", value_a, value_b);
+}
diff --git a/solutions/23_conversions/as_ref_mut.rs b/solutions/23_conversions/as_ref_mut.rs
new file mode 100644
index 0000000..91b12ba
--- /dev/null
+++ b/solutions/23_conversions/as_ref_mut.rs
@@ -0,0 +1,59 @@
+// AsRef and AsMut allow for cheap reference-to-reference conversions. Read more
+// about them at https://doc.rust-lang.org/std/convert/trait.AsRef.html and
+// https://doc.rust-lang.org/std/convert/trait.AsMut.html, respectively.
+
+// Obtain the number of bytes (not characters) in the given argument.
+fn byte_counter<T: AsRef<str>>(arg: T) -> usize {
+    arg.as_ref().as_bytes().len()
+}
+
+// Obtain the number of characters (not bytes) in the given argument.
+fn char_counter<T: AsRef<str>>(arg: T) -> usize {
+    arg.as_ref().chars().count()
+}
+
+// Squares a number using `as_mut()`.
+fn num_sq<T: AsMut<u32>>(arg: &mut T) {
+    let arg = arg.as_mut();
+    *arg = *arg * *arg;
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn different_counts() {
+        let s = "Café au lait";
+        assert_ne!(char_counter(s), byte_counter(s));
+    }
+
+    #[test]
+    fn same_counts() {
+        let s = "Cafe au lait";
+        assert_eq!(char_counter(s), byte_counter(s));
+    }
+
+    #[test]
+    fn different_counts_using_string() {
+        let s = String::from("Café au lait");
+        assert_ne!(char_counter(s.clone()), byte_counter(s));
+    }
+
+    #[test]
+    fn same_counts_using_string() {
+        let s = String::from("Cafe au lait");
+        assert_eq!(char_counter(s.clone()), byte_counter(s));
+    }
+
+    #[test]
+    fn mut_box() {
+        let mut num: Box<u32> = Box::new(3);
+        num_sq(&mut num);
+        assert_eq!(*num, 9);
+    }
+}
diff --git a/solutions/23_conversions/from_into.rs b/solutions/23_conversions/from_into.rs
new file mode 100644
index 0000000..cec23cb
--- /dev/null
+++ b/solutions/23_conversions/from_into.rs
@@ -0,0 +1,136 @@
+// The `From` trait is used for value-to-value conversions. If `From` is
+// implemented, an implementation of `Into` is automatically provided.
+// You can read more about it in the documentation:
+// https://doc.rust-lang.org/std/convert/trait.From.html
+
+#[derive(Debug)]
+struct Person {
+    name: String,
+    age: u8,
+}
+
+// We implement the Default trait to use it as a fallback when the provided
+// string is not convertible into a `Person` object.
+impl Default for Person {
+    fn default() -> Self {
+        Self {
+            name: String::from("John"),
+            age: 30,
+        }
+    }
+}
+
+impl From<&str> for Person {
+    fn from(s: &str) -> Self {
+        let mut split = s.split(',');
+        let (Some(name), Some(age), None) = (split.next(), split.next(), split.next()) else {
+            //                      ^^^^ there should be no third element
+            return Self::default();
+        };
+
+        if name.is_empty() {
+            return Self::default();
+        }
+
+        let Ok(age) = age.parse() else {
+            return Self::default();
+        };
+
+        Self {
+            name: name.into(),
+            age,
+        }
+    }
+}
+
+fn main() {
+    // Use the `from` function.
+    let p1 = Person::from("Mark,20");
+    println!("{p1:?}");
+
+    // Since `From` is implemented for Person, we are able to use `Into`.
+    let p2: Person = "Gerald,70".into();
+    println!("{p2:?}");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_default() {
+        let dp = Person::default();
+        assert_eq!(dp.name, "John");
+        assert_eq!(dp.age, 30);
+    }
+
+    #[test]
+    fn test_bad_convert() {
+        let p = Person::from("");
+        assert_eq!(p.name, "John");
+        assert_eq!(p.age, 30);
+    }
+
+    #[test]
+    fn test_good_convert() {
+        let p = Person::from("Mark,20");
+        assert_eq!(p.name, "Mark");
+        assert_eq!(p.age, 20);
+    }
+
+    #[test]
+    fn test_bad_age() {
+        let p = Person::from("Mark,twenty");
+        assert_eq!(p.name, "John");
+        assert_eq!(p.age, 30);
+    }
+
+    #[test]
+    fn test_missing_comma_and_age() {
+        let p: Person = Person::from("Mark");
+        assert_eq!(p.name, "John");
+        assert_eq!(p.age, 30);
+    }
+
+    #[test]
+    fn test_missing_age() {
+        let p: Person = Person::from("Mark,");
+        assert_eq!(p.name, "John");
+        assert_eq!(p.age, 30);
+    }
+
+    #[test]
+    fn test_missing_name() {
+        let p: Person = Person::from(",1");
+        assert_eq!(p.name, "John");
+        assert_eq!(p.age, 30);
+    }
+
+    #[test]
+    fn test_missing_name_and_age() {
+        let p: Person = Person::from(",");
+        assert_eq!(p.name, "John");
+        assert_eq!(p.age, 30);
+    }
+
+    #[test]
+    fn test_missing_name_and_invalid_age() {
+        let p: Person = Person::from(",one");
+        assert_eq!(p.name, "John");
+        assert_eq!(p.age, 30);
+    }
+
+    #[test]
+    fn test_trailing_comma() {
+        let p: Person = Person::from("Mike,32,");
+        assert_eq!(p.name, "John");
+        assert_eq!(p.age, 30);
+    }
+
+    #[test]
+    fn test_trailing_comma_and_some_string() {
+        let p: Person = Person::from("Mike,32,dog");
+        assert_eq!(p.name, "John");
+        assert_eq!(p.age, 30);
+    }
+}
diff --git a/solutions/23_conversions/from_str.rs b/solutions/23_conversions/from_str.rs
new file mode 100644
index 0000000..005b501
--- /dev/null
+++ b/solutions/23_conversions/from_str.rs
@@ -0,0 +1,117 @@
+// This is similar to the previous `from_into` exercise. But this time, we'll
+// implement `FromStr` and return errors instead of falling back to a default
+// value. Additionally, upon implementing `FromStr`, you can use the `parse`
+// method on strings to generate an object of the implementor type. You can read
+// more about it in the documentation:
+// https://doc.rust-lang.org/std/str/trait.FromStr.html
+
+use std::num::ParseIntError;
+use std::str::FromStr;
+
+#[derive(Debug, PartialEq)]
+struct Person {
+    name: String,
+    age: u8,
+}
+
+// We will use this error type for the `FromStr` implementation.
+#[derive(Debug, PartialEq)]
+enum ParsePersonError {
+    // Incorrect number of fields
+    BadLen,
+    // Empty name field
+    NoName,
+    // Wrapped error from parse::<u8>()
+    ParseInt(ParseIntError),
+}
+
+impl FromStr for Person {
+    type Err = ParsePersonError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let mut split = s.split(',');
+        let (Some(name), Some(age), None) = (split.next(), split.next(), split.next()) else {
+            //                      ^^^^ there should be no third element
+            return Err(ParsePersonError::BadLen);
+        };
+
+        if name.is_empty() {
+            return Err(ParsePersonError::NoName);
+        }
+
+        let age = age.parse().map_err(ParsePersonError::ParseInt)?;
+
+        Ok(Self {
+            name: name.into(),
+            age,
+        })
+    }
+}
+
+fn main() {
+    let p = "Mark,20".parse::<Person>();
+    println!("{p:?}");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use ParsePersonError::*;
+
+    #[test]
+    fn empty_input() {
+        assert_eq!("".parse::<Person>(), Err(BadLen));
+    }
+
+    #[test]
+    fn good_input() {
+        let p = "John,32".parse::<Person>();
+        assert!(p.is_ok());
+        let p = p.unwrap();
+        assert_eq!(p.name, "John");
+        assert_eq!(p.age, 32);
+    }
+
+    #[test]
+    fn missing_age() {
+        assert!(matches!("John,".parse::<Person>(), Err(ParseInt(_))));
+    }
+
+    #[test]
+    fn invalid_age() {
+        assert!(matches!("John,twenty".parse::<Person>(), Err(ParseInt(_))));
+    }
+
+    #[test]
+    fn missing_comma_and_age() {
+        assert_eq!("John".parse::<Person>(), Err(BadLen));
+    }
+
+    #[test]
+    fn missing_name() {
+        assert_eq!(",1".parse::<Person>(), Err(NoName));
+    }
+
+    #[test]
+    fn missing_name_and_age() {
+        assert!(matches!(",".parse::<Person>(), Err(NoName | ParseInt(_))));
+    }
+
+    #[test]
+    fn missing_name_and_invalid_age() {
+        assert!(matches!(
+            ",one".parse::<Person>(),
+            Err(NoName | ParseInt(_)),
+        ));
+    }
+
+    #[test]
+    fn trailing_comma() {
+        assert_eq!("John,32,".parse::<Person>(), Err(BadLen));
+    }
+
+    #[test]
+    fn trailing_comma_and_some_string() {
+        assert_eq!("John,32,man".parse::<Person>(), Err(BadLen));
+    }
+}
diff --git a/solutions/23_conversions/try_from_into.rs b/solutions/23_conversions/try_from_into.rs
new file mode 100644
index 0000000..ee802eb
--- /dev/null
+++ b/solutions/23_conversions/try_from_into.rs
@@ -0,0 +1,193 @@
+// `TryFrom` is a simple and safe type conversion that may fail in a controlled
+// way under some circumstances. Basically, this is the same as `From`. The main
+// difference is that this should return a `Result` type instead of the target
+// type itself. You can read more about it in the documentation:
+// https://doc.rust-lang.org/std/convert/trait.TryFrom.html
+
+#![allow(clippy::useless_vec)]
+use std::convert::{TryFrom, TryInto};
+
+#[derive(Debug, PartialEq)]
+struct Color {
+    red: u8,
+    green: u8,
+    blue: u8,
+}
+
+// We will use this error type for the `TryFrom` conversions.
+#[derive(Debug, PartialEq)]
+enum IntoColorError {
+    // Incorrect length of slice
+    BadLen,
+    // Integer conversion error
+    IntConversion,
+}
+
+impl TryFrom<(i16, i16, i16)> for Color {
+    type Error = IntoColorError;
+
+    fn try_from(tuple: (i16, i16, i16)) -> Result<Self, Self::Error> {
+        let (Ok(red), Ok(green), Ok(blue)) = (
+            u8::try_from(tuple.0),
+            u8::try_from(tuple.1),
+            u8::try_from(tuple.2),
+        ) else {
+            return Err(IntoColorError::IntConversion);
+        };
+
+        Ok(Self { red, green, blue })
+    }
+}
+
+impl TryFrom<[i16; 3]> for Color {
+    type Error = IntoColorError;
+
+    fn try_from(arr: [i16; 3]) -> Result<Self, Self::Error> {
+        // Reuse the implementation for a tuple.
+        Self::try_from((arr[0], arr[1], arr[2]))
+    }
+}
+
+impl TryFrom<&[i16]> for Color {
+    type Error = IntoColorError;
+
+    fn try_from(slice: &[i16]) -> Result<Self, Self::Error> {
+        // Check the length.
+        if slice.len() != 3 {
+            return Err(IntoColorError::BadLen);
+        }
+
+        // Reuse the implementation for a tuple.
+        Self::try_from((slice[0], slice[1], slice[2]))
+    }
+}
+
+fn main() {
+    // Using the `try_from` function.
+    let c1 = Color::try_from((183, 65, 14));
+    println!("{c1:?}");
+
+    // Since `TryFrom` is implemented for `Color`, we can use `TryInto`.
+    let c2: Result<Color, _> = [183, 65, 14].try_into();
+    println!("{c2:?}");
+
+    let v = vec![183, 65, 14];
+    // With slice we should use the `try_from` function
+    let c3 = Color::try_from(&v[..]);
+    println!("{c3:?}");
+    // or put the slice within round brackets and use `try_into`.
+    let c4: Result<Color, _> = (&v[..]).try_into();
+    println!("{c4:?}");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use IntoColorError::*;
+
+    #[test]
+    fn test_tuple_out_of_range_positive() {
+        assert_eq!(Color::try_from((256, 1000, 10000)), Err(IntConversion));
+    }
+
+    #[test]
+    fn test_tuple_out_of_range_negative() {
+        assert_eq!(Color::try_from((-1, -10, -256)), Err(IntConversion));
+    }
+
+    #[test]
+    fn test_tuple_sum() {
+        assert_eq!(Color::try_from((-1, 255, 255)), Err(IntConversion));
+    }
+
+    #[test]
+    fn test_tuple_correct() {
+        let c: Result<Color, _> = (183, 65, 14).try_into();
+        assert!(c.is_ok());
+        assert_eq!(
+            c.unwrap(),
+            Color {
+                red: 183,
+                green: 65,
+                blue: 14,
+            }
+        );
+    }
+
+    #[test]
+    fn test_array_out_of_range_positive() {
+        let c: Result<Color, _> = [1000, 10000, 256].try_into();
+        assert_eq!(c, Err(IntConversion));
+    }
+
+    #[test]
+    fn test_array_out_of_range_negative() {
+        let c: Result<Color, _> = [-10, -256, -1].try_into();
+        assert_eq!(c, Err(IntConversion));
+    }
+
+    #[test]
+    fn test_array_sum() {
+        let c: Result<Color, _> = [-1, 255, 255].try_into();
+        assert_eq!(c, Err(IntConversion));
+    }
+
+    #[test]
+    fn test_array_correct() {
+        let c: Result<Color, _> = [183, 65, 14].try_into();
+        assert!(c.is_ok());
+        assert_eq!(
+            c.unwrap(),
+            Color {
+                red: 183,
+                green: 65,
+                blue: 14
+            }
+        );
+    }
+
+    #[test]
+    fn test_slice_out_of_range_positive() {
+        let arr = [10000, 256, 1000];
+        assert_eq!(Color::try_from(&arr[..]), Err(IntConversion));
+    }
+
+    #[test]
+    fn test_slice_out_of_range_negative() {
+        let arr = [-256, -1, -10];
+        assert_eq!(Color::try_from(&arr[..]), Err(IntConversion));
+    }
+
+    #[test]
+    fn test_slice_sum() {
+        let arr = [-1, 255, 255];
+        assert_eq!(Color::try_from(&arr[..]), Err(IntConversion));
+    }
+
+    #[test]
+    fn test_slice_correct() {
+        let v = vec![183, 65, 14];
+        let c: Result<Color, _> = Color::try_from(&v[..]);
+        assert!(c.is_ok());
+        assert_eq!(
+            c.unwrap(),
+            Color {
+                red: 183,
+                green: 65,
+                blue: 14,
+            }
+        );
+    }
+
+    #[test]
+    fn test_slice_excess_length() {
+        let v = vec![0, 0, 0, 0];
+        assert_eq!(Color::try_from(&v[..]), Err(BadLen));
+    }
+
+    #[test]
+    fn test_slice_insufficient_length() {
+        let v = vec![0, 0];
+        assert_eq!(Color::try_from(&v[..]), Err(BadLen));
+    }
+}
diff --git a/solutions/23_conversions/using_as.rs b/solutions/23_conversions/using_as.rs
new file mode 100644
index 0000000..14b92eb
--- /dev/null
+++ b/solutions/23_conversions/using_as.rs
@@ -0,0 +1,24 @@
+// Type casting in Rust is done via the usage of the `as` operator.
+// Note that the `as` operator is not only used when type casting. It also helps
+// with renaming imports.
+
+fn average(values: &[f64]) -> f64 {
+    let total = values.iter().sum::<f64>();
+    total / values.len() as f64
+    //                   ^^^^^^
+}
+
+fn main() {
+    let values = [3.5, 0.3, 13.0, 11.7];
+    println!("{}", average(&values));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn returns_proper_type_and_value() {
+        assert_eq!(average(&[3.5, 0.3, 13.0, 11.7]), 7.125);
+    }
+}
diff --git a/solutions/quizzes/quiz1.rs b/solutions/quizzes/quiz1.rs
new file mode 100644
index 0000000..bc76166
--- /dev/null
+++ b/solutions/quizzes/quiz1.rs
@@ -0,0 +1,31 @@
+// Mary is buying apples. The price of an apple is calculated as follows:
+// - An apple costs 2 rustbucks.
+// - If Mary buys more than 40 apples, each apple only costs 1 rustbuck!
+// Write a function that calculates the price of an order of apples given the
+// quantity bought.
+
+fn calculate_price_of_apples(n_apples: u64) -> u64 {
+    if n_apples > 40 {
+        n_apples
+    } else {
+        2 * n_apples
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+// Don't change the tests!
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn verify_test() {
+        assert_eq!(calculate_price_of_apples(35), 70);
+        assert_eq!(calculate_price_of_apples(40), 80);
+        assert_eq!(calculate_price_of_apples(41), 41);
+        assert_eq!(calculate_price_of_apples(65), 65);
+    }
+}
diff --git a/solutions/quizzes/quiz2.rs b/solutions/quizzes/quiz2.rs
new file mode 100644
index 0000000..0d2a513
--- /dev/null
+++ b/solutions/quizzes/quiz2.rs
@@ -0,0 +1,107 @@
+// This is a quiz for the following sections:
+// - Strings
+// - Vecs
+// - Move semantics
+// - Modules
+// - Enums
+//
+// Let's build a little machine in the form of a function. As input, we're going
+// to give a list of strings and commands. These commands determine what action
+// is going to be applied to the string. It can either be:
+// - Uppercase the string
+// - Trim the string
+// - Append "bar" to the string a specified amount of times
+//
+// The exact form of this will be:
+// - The input is going to be a vector of a 2-length tuple,
+//   the first element is the string, the second one is the command.
+// - The output element is going to be a vector of strings.
+
+enum Command {
+    Uppercase,
+    Trim,
+    Append(usize),
+}
+
+mod my_module {
+    use super::Command;
+
+    // The solution with a loop. Check out `transformer_iter` for a version
+    // with iterators.
+    pub fn transformer(input: Vec<(String, Command)>) -> Vec<String> {
+        let mut output = Vec::new();
+
+        for (mut string, command) in input {
+            // Create the new string.
+            let new_string = match command {
+                Command::Uppercase => string.to_uppercase(),
+                Command::Trim => string.trim().to_string(),
+                Command::Append(n) => {
+                    for _ in 0..n {
+                        string += "bar";
+                    }
+                    string
+                }
+            };
+
+            // Push the new string to the output vector.
+            output.push(new_string);
+        }
+
+        output
+    }
+
+    // Equivalent to `transform` but uses an iterator instead of a loop for
+    // comparison. Don't worry, we will practice iterators later ;)
+    pub fn transformer_iter(input: Vec<(String, Command)>) -> Vec<String> {
+        input
+            .into_iter()
+            .map(|(mut string, command)| match command {
+                Command::Uppercase => string.to_uppercase(),
+                Command::Trim => string.trim().to_string(),
+                Command::Append(n) => {
+                    for _ in 0..n {
+                        string += "bar";
+                    }
+                    string
+                }
+            })
+            .collect()
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    // Import `transformer`.
+    use super::my_module::transformer;
+
+    use super::my_module::transformer_iter;
+    use super::Command;
+
+    #[test]
+    fn it_works() {
+        for transformer in [transformer, transformer_iter] {
+            let input = vec![
+                ("hello".to_string(), Command::Uppercase),
+                (" all roads lead to rome! ".to_string(), Command::Trim),
+                ("foo".to_string(), Command::Append(1)),
+                ("bar".to_string(), Command::Append(5)),
+            ];
+            let output = transformer(input);
+
+            assert_eq!(
+                output,
+                [
+                    "HELLO",
+                    "all roads lead to rome!",
+                    "foobar",
+                    "barbarbarbarbarbar",
+                ]
+            );
+        }
+    }
+}
diff --git a/solutions/quizzes/quiz3.rs b/solutions/quizzes/quiz3.rs
new file mode 100644
index 0000000..e3413fd
--- /dev/null
+++ b/solutions/quizzes/quiz3.rs
@@ -0,0 +1,69 @@
+// This quiz tests:
+// - Generics
+// - Traits
+//
+// An imaginary magical school has a new report card generation system written
+// in Rust! Currently, the system only supports creating report cards where the
+// student's grade is represented numerically (e.g. 1.0 -> 5.5). However, the
+// school also issues alphabetical grades (A+ -> F-) and needs to be able to
+// print both types of report card!
+//
+// Make the necessary code changes in the struct `ReportCard` and the impl
+// block to support alphabetical report cards in addition to numerical ones.
+
+use std::fmt::Display;
+
+// Make the struct generic over `T`.
+struct ReportCard<T> {
+    //           ^^^
+    grade: T,
+    //     ^
+    student_name: String,
+    student_age: u8,
+}
+
+// To be able to print the grade, it has to implement the `Display` trait.
+impl<T: Display> ReportCard<T> {
+    //  ^^^^^^^ require that `T` implements `Display`.
+    fn print(&self) -> String {
+        format!(
+            "{} ({}) - achieved a grade of {}",
+            &self.student_name, &self.student_age, &self.grade,
+        )
+    }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn generate_numeric_report_card() {
+        let report_card = ReportCard {
+            grade: 2.1,
+            student_name: "Tom Wriggle".to_string(),
+            student_age: 12,
+        };
+        assert_eq!(
+            report_card.print(),
+            "Tom Wriggle (12) - achieved a grade of 2.1",
+        );
+    }
+
+    #[test]
+    fn generate_alphabetic_report_card() {
+        let report_card = ReportCard {
+            grade: "A+",
+            student_name: "Gary Plotter".to_string(),
+            student_age: 11,
+        };
+        assert_eq!(
+            report_card.print(),
+            "Gary Plotter (11) - achieved a grade of A+",
+        );
+    }
+}