Rust by Example

8 Functions

Functions are declared using the fn keyword. Its arguments are type annotated, just like variables, and, if the function returns a value, the return type must be specified after an arrow ->.

The final expression in the function will be used as return value. Alternatively, the return statement can be used to return a value earlier from within the function, even from inside loops or ifs.

Let's rewrite FizzBuzz using functions!

// Unlike C/C++, there's no restriction on the order of function definitions fn main() { // We can use this function here, and define it somewhere later fizzbuzz_to(100); } // Function that returns a boolean value fn is_divisible_by(lhs: u32, rhs: u32) -> bool { // Corner case, early return if rhs == 0 { return false; } // This is an expression, the `return` keyword is not necessary here lhs % rhs == 0 } // Functions that "don't" return a value, actually return the unit type `()` fn fizzbuzz(n: u32) -> () { if is_divisible_by(n, 15) { println!("fizzbuzz"); } else if is_divisible_by(n, 3) { println!("fizz"); } else if is_divisible_by(n, 5) { println!("buzz"); } else { println!("{}", n); } } // When a function returns `()`, the return type can be omitted from the // signature fn fizzbuzz_to(n: u32) { for n in 1..n + 1 { fizzbuzz(n); } }