11.2 Using functions

11.2.1Accepting parameters¶

Each function parameter is declared with its type, separated by a colon. Multiple parameters are separated by commas.

Here is an example with one integer parameter:

fn foo(n: u8) {
  println!("The number is {n}");
}

foo(5);

The number is 5

Here is an example with two parameters:

fn foo(n: u8, c: char) {
  println!("The number is {n}.");
  println!("The character is {c}.");
}

foo(5, 'Z');

The number is 5.

The character is Z.

11.2.2Parameter mutability¶

Any parameter can be declared mutable in order to allow its modification inside the function:

fn foo(mut n: u8) {
  n += 10;
  println!("The number is {n}.");
}

foo(5);

The number is 15.

11.2.3Returning a value¶

The most used form for returning a value is by writing an expression at the end of the function, without a semicolon. The type of the returned value must be declared using the -> symbol:

fn foo() -> u8 {
  let x = 2;
  x + 4
}

foo()

Since the if statement can also be used to build an expression, we can use it to return a value conditionally:

fn foo(n: u8) -> u8 {
  if n > 100 {
    n / 2
  } else {
    n
  }
}

foo(128)

11.2.4The return keyword¶

The return keyword is used to leave the function. It can be used anywhere inside the function’s scope:

fn foo(n: i8) {
  if n < 5 {
    return;
  }
  println!("The number is {n}.");
}

foo(-3);
foo(6);

The number is 6.

The return keyword can also return a value:

fn foo() -> u8 {
  let x = 2;
  return x + 4;
}

foo()