Rust For Loop

The for loop is the loop Rust programmers reach for first. Instead of managing a counter yourself, you tell for what to go through, and it hands you one item at a time. It is shorter to write, easier to read, and it can never accidentally run past the end.

Looping over a range

A range is a sequence of numbers. 1..5 means 1, 2, 3, 4: it stops before 5. If you want to include the last number, write 1..=5.

fn main() {
    for i in 1..=5 {
        println!("{} x 3 = {}", i, i * 3);
    }
}

1 x 3 = 3
2 x 3 = 6
3 x 3 = 9
4 x 3 = 12
5 x 3 = 15

The variable i is created fresh for each round and holds the current number. You can call it anything you like, but i is the traditional name for a counter.

Backwards and in steps

Add .rev() to go backwards, and .step_by(n) to jump by more than one. Notice the round brackets around the range when you add these:

fn main() {
    for n in (1..=5).rev() {
        print!("{} ", n);
    }
    println!("Go!");

    for even in (0..=10).step_by(2) {
        print!("{} ", even);
    }
    println!();
}

5 4 3 2 1 Go!
0 2 4 6 8 10 

Looping over a list

for can walk through a list of values just as easily. Here is an array of names. We will learn about arrays properly later; for now, enjoy how simple the loop is:

fn main() {
    let friends = ["Asha", "Ravi", "Leela"];
    for name in friends {
        println!("Hello, {}!", name);
    }

    for (position, name) in friends.iter().enumerate() {
        println!("{}. {}", position + 1, name);
    }
}

Hello, Asha!
Hello, Ravi!
Hello, Leela!
1. Asha
2. Ravi
3. Leela

enumerate gives you each item together with its position. Positions start at 0, which is why we print position + 1 for people, who like to count from 1.

Adding things up

A very common pattern is to keep a running total while you loop:

fn main() {
    let mut total = 0;
    for n in 1..=100 {
        total += n;
    }
    println!("1 + 2 + ... + 100 = {}", total);
}

1 + 2 + ... + 100 = 5050

Practice exercises

1. Print the multiplication table of 7 from 7 x 1 to 7 x 10.
2. Add up all the odd numbers from 1 to 99.
3. Make an array of five fruits and print them numbered from 1 to 5.