03_control_flow.md

Control flow

Most of the control flow constructs except for and while loops are expressions.

Block

Blocks create their own scope. They are an expression with the value of the last expression in the block:

fn main() {
    let x = {
        let y = 5;
        println!("y = {}", y);
        y
    };
    println!("x = {}", x);
}

Note that the last expression cannot be followed by ; or it would be considered a statement instead.

If

Works like in most languages, except is an expression:

fn main() {
    let ternary = if x < 5 { "small" } else { "big" };
}

fn rng(x: i32) -> i32 {
    if x < 5 {
        x
    } else if x < 10 {
        x * 2
    } else {
        x * 3
    }
}

Match

A switch-like expression that matches patterns and values. Must be exhaustive — all possible patterns and values must be matched at least in one arm:

fn main() {
    let text = match val {
        Some(5) => "five",
        Some(x) => "some other number",
        None => "nothing",
    };
}

If let

Syntax sugar for match that matches one pattern does nothing with the rest:

fn main() {
    if let Some(x) = val {
        println!("x = {}", x);
    }
}

Can also have else and else if like regular if expressions.

Loop

Loops forever. The break statement can be called with a value to return a value from the loop.

fn main() {
    let mut counter = 0;
    let result = loop {
        counter += 1;

        if counter == 10 {
            break counter * 2;
        }
    };
}

For

Looping through iterables is done with a for statement:

fn main() {
    for number in 1..4 {
        println!("{}", number);
    }
}

While

Runs while the condition evaluates to true:

fn main() {
    let a = [10, 20, 30, 40, 50];
    let mut index = 0;

    while index < 5 {
        println!("the value is: {}", a[index]);
        index += 1;
    }
}

While let

Runs while the pattern matches, like in if let:

fn main() {
    let mut stack: Vec<u32> = vec![3, 33, 44];
    while let Some(num) = stack.pop() {
        println!("popped {}", num);
    }
}