Destructuring in for Loops

Goal of This Episode

Learn to destructure tuples or structs directly in the variable position of a for loop.

Concept

We’ve already seen let destructure tuples and structs. It turns out the variable position of a for loop can too — just write the same destructuring syntax right there.

Iterating over an array of tuples:

fn main() {
    let pairs = [(1, "one"), (2, "two"), (3, "three")];

    for (num, name) in pairs {
        println!("{} = {}", num, name);
    }
}

(num, name) is the pattern. Each element of the array is a tuple, and the loop splits it into num and name.

Iterating over structs works the same way:

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let points = [
        Point { x: 0, y: 0 },
        Point { x: 1, y: 2 },
        Point { x: 3, y: 4 },
    ];

    for Point { x, y } in points {
        println!("({}, {})", x, y);
    }
}

Think of it as let destructuring combined with a for loop: each time the loop takes out an element, it splits it apart with let-destructuring syntax.

Example Code

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    // Iterate over a tuple array, destructuring
    let scores = [("Alice", 85), ("Bob", 92), ("Carol", 78)];
    for (name, score) in scores {
        println!("{}: {}", name, score);
    }

    // Iterate over a struct array, destructuring
    let points = [
        Point { x: 0, y: 0 },
        Point { x: 3, y: 4 },
        Point { x: -1, y: 2 },
    ];
    for Point { x, y } in points {
        println!("({}, {})", x, y);
    }

    // Use .. to ignore unwanted fields
    let more_points = [
        Point { x: 1, y: 10 },
        Point { x: 2, y: 20 },
    ];
    for Point { x, .. } in more_points {
        println!("x = {}", x);
    }
}

Recap

• The variable position of a for loop accepts destructuring patterns directly.
• Iterating over an array of tuples: for (a, b) in pairs.
• Iterating over an array of structs: for Point { x, y } in points.