diff --git a/standard_library_types/iterators4.rs b/standard_library_types/iterators4.rs
new file mode 100644
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+++ b/standard_library_types/iterators4.rs
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+pub fn factorial(num: u64) -> u64 {
+    // Complete this function to return factorial of num
+    // Do not use:
+    // - return
+    // For extra fun don't use:
+    // - imperative style loops (for, while)
+    // - additional variables
+    // For the most fun don't use:
+    // - recursion
+    // Scroll down for hints.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn factorial_of_1() {
+        assert_eq!(1, factorial(1));
+    }
+    #[test]
+    fn factorial_of_2() {
+        assert_eq!(2, factorial(2));
+    }
+
+    #[test]
+    fn factorial_of_4() {
+        assert_eq!(24, factorial(4));
+    }
+}
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+// In an imperative language you might write a for loop to iterate through
+// multiply the values into a mutable variable. Or you might write code more
+// functionally with recursion and a match clause. But you can also use ranges
+// and iterators to solve this in rust.