Functions

You declare a function as follows:

fn say_hello() {
    println!("Hello");
}

fn main() {
    say_hello();
}

Functions with parameters

fn main() {
    print_sum(5, 7);
}

fn print_sum(number1: i32, number2: i32) {
    println!("Sum of {} and {} is {}", number1, number2, number1 + number2);
}

Statements vs Expressions

In Rust,

• Statements don't return a value
• Expressions always evaluate to a value

Returning a value from a function

When returning a value from a function, you would typically do so using the last expression in the function. In this case, you must not end it with a semicolon. Example:

fn main() {
    let num1 = 5;
    let num2 = 7;
    
    let sum  = sum(5, 7);
    println!("Sum of {} and {} is {}", num1, num2, sum);
}

fn sum(number1: i32, number2: i32) -> i32 {
    number1 + number2
}

If you were to end the last line above with a semicolon (as shown below), that line becomes a statement, and you'll get an error.

fn main() {
    let num1 = 5;
    let num2 = 7;
    
    let sum  = sum(5, 7);
    println!("Sum of {} and {} is {}", num1, num2, sum);
}

fn sum(number1: i32, number2: i32) -> i32 {
    number1 + number2;
}

In the above, you get an error about the unit type: (). This means that you're trying to use a statement where an expression is expected. Statements don't return a value , which is expressed by the (), the unit type.

Lastly, you can return from a function early using return keyword and specifying a value. That is, before the function's last line, for example, from the body of an if block within the function.

fn main() {
    let number1 = 3;
    let number2 = 5;
    let number3 = 15;
    let number4 = 16;
    
    println!("{}", fizz_buzz(number1));
    println!("{}", fizz_buzz(number2));
    println!("{}", fizz_buzz(number3));
    println!("{}", fizz_buzz(number4));
   
}

fn fizz_buzz(number: i32) -> String {
    if number % 15 == 0 {
        return String::from("Fizz Buzz");
    } else if number % 5 == 0 {
        return String::from("Buzz");
    } else if number % 3 ==0 {
        return String::from("Fizz");
    }
    number.to_string()
}