Functions

They allow us to group and reuse statements, with the possibility to return values

• Functions are defined with the fn keyword, followed by the function's name and parameters (placed in ())
• The function's body is placed between a pair of {}
• Rust functions must be called in order to be used

Here is the definition and usage of a function that prints "Hello World!":

                fn main() {
                    hello_world();
                }

                fn hello_world() {
                    println!("Hello World!"):
                }
        

Parameters

• We can define parameters: special variables that work as inputs for that function
• When declaring them, we must explicitly declare their type:
  param: Type
• If there are more than 1, separate them with commas

Example:

            fn main() {
                let a = 6;
                let b = 8;

                pythagoras(a, b); // Prints "a^2 + b^2 = 100"
            }

            fn pythagoras(a: i32, b: i32) {
                println!("a^2 + b^2 = {}", a*a + b*b);
            }
        

Returning values

We often need a function to return a value after its execution. To do so:

• We need to specify the return type in the function's definition with -> Type
• We need to actually return something at the end of the execution:
  • To return early from a function, we use the return keyword with a value;
  • If the function returns something at the very end of the body, we can omit the return keyword and the colon at the end of the line

Let's change the previous example:

            fn main() {
                let a = 6;
                let b = 8;

                let res = pythagoras(a, b);

                println!("a^2 + b^2 = {res}");
            }

            fn pythagoras(a: i32, b: i32) -> i32 {
                a*a + b*b
            }
        

As in other programming languages, we can use recursion to perform tasks:

            fn main(){
                let n = 4;

                let res = fibonacci(n);

                println!("{res}"); // Prints "5"
            }

            //Function that calculates the nth Fibonacci number
            fn fibonacci(n: u64) -> u64 {
                if n <= 1 {
                    return 1;
                }

                fibonacci(n - 1) + fibonacci(n - 2)
            }