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73 lines
2 KiB
Rust
73 lines
2 KiB
Rust
// 3 possible solutions are presented.
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// With `for` loop and a mutable variable.
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fn factorial_for(num: u64) -> u64 {
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let mut result = 1;
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for x in 2..=num {
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result *= x;
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}
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result
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}
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// Equivalent to `factorial_for` but shorter and without a `for` loop and
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// mutable variables.
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fn factorial_fold(num: u64) -> u64 {
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// Case num==0: The iterator 2..=0 is empty
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// -> The initial value of `fold` is returned which is 1.
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// Case num==1: The iterator 2..=1 is also empty
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// -> The initial value 1 is returned.
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// Case num==2: The iterator 2..=2 contains one element
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// -> The initial value 1 is multiplied by 2 and the result
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// is returned.
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// Case num==3: The iterator 2..=3 contains 2 elements
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// -> 1 * 2 is calculated, then the result 2 is multiplied by
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// the second element 3 so the result 6 is returned.
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// And so on…
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#[allow(clippy::unnecessary_fold)]
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(2..=num).fold(1, |acc, x| acc * x)
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}
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// Equivalent to `factorial_fold` but with a built-in method that is suggested
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// by Clippy.
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fn factorial_product(num: u64) -> u64 {
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(2..=num).product()
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}
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fn main() {
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// You can optionally experiment here.
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn factorial_of_0() {
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assert_eq!(factorial_for(0), 1);
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assert_eq!(factorial_fold(0), 1);
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assert_eq!(factorial_product(0), 1);
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}
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#[test]
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fn factorial_of_1() {
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assert_eq!(factorial_for(1), 1);
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assert_eq!(factorial_fold(1), 1);
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assert_eq!(factorial_product(1), 1);
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}
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#[test]
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fn factorial_of_2() {
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assert_eq!(factorial_for(2), 2);
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assert_eq!(factorial_fold(2), 2);
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assert_eq!(factorial_product(2), 2);
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}
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#[test]
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fn factorial_of_4() {
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assert_eq!(factorial_for(4), 24);
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assert_eq!(factorial_fold(4), 24);
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assert_eq!(factorial_product(4), 24);
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}
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}
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