iterators4 solution

3 files changed, 80 insertions, 10 deletions

diff --git a/exercises/18_iterators/iterators4.rs b/exercises/18_iterators/iterators4.rs
index ae4d502..08ba365 100644
--- a/exercises/18_iterators/iterators4.rs
+++ b/exercises/18_iterators/iterators4.rs
@@ -1,9 +1,9 @@
-fn factorial(num: u64) -> u64 {
-    // Complete this function to return the factorial of num
+fn factorial(num: u8) -> u64 {
+    // TODO: Complete this function to return the factorial of `num`.
     // Do not use:
     // - early returns (using the `return` keyword explicitly)
     // Try not to use:
-    // - imperative style loops (for, while)
+    // - imperative style loops (for/while)
     // - additional variables
     // For an extra challenge, don't use:
     // - recursion
@@ -19,20 +19,20 @@ mod tests {
 
     #[test]
     fn factorial_of_0() {
-        assert_eq!(1, factorial(0));
+        assert_eq!(factorial(0), 1);
     }
 
     #[test]
     fn factorial_of_1() {
-        assert_eq!(1, factorial(1));
+        assert_eq!(factorial(1), 1);
     }
     #[test]
     fn factorial_of_2() {
-        assert_eq!(2, factorial(2));
+        assert_eq!(factorial(2), 2);
     }
 
     #[test]
     fn factorial_of_4() {
-        assert_eq!(24, factorial(4));
+        assert_eq!(factorial(4), 24);
     }
 }
diff --git a/rustlings-macros/info.toml b/rustlings-macros/info.toml
index 8b1feb4..72f956b 100644
--- a/rustlings-macros/info.toml
+++ b/rustlings-macros/info.toml
@@ -942,10 +942,10 @@ dir = "18_iterators"
 hint = """
 In an imperative language, you might write a `for` loop that updates a mutable
 variable. Or, you might write code utilizing recursion and a match clause. In
-Rust you can take another functional approach, computing the factorial
+Rust, you can take another functional approach, computing the factorial
 elegantly with ranges and iterators.
 
-Hint 2: Check out the `fold` and `rfold` methods!"""
+Check out the `fold` and `rfold` methods!"""
 
 [[exercises]]
 name = "iterators5"
diff --git a/solutions/18_iterators/iterators4.rs b/solutions/18_iterators/iterators4.rs
index 4e18198..4c3c49d 100644
--- a/solutions/18_iterators/iterators4.rs
+++ b/solutions/18_iterators/iterators4.rs
@@ -1 +1,71 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+// 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);
+    }
+}