Step 5 of 7

for loops and ranges

To repeat something a known number of times, or once for each value in a sequence, use for. The simplest sequence is a range of numbers:

fn main() {
    for round in 1..4 {
        println!("round {round}");
    }
    let mut total = 0;
    for n in 1..=100 {
        total += n;
    }
    println!("sum of 1 to 100 = {total}");
}

round 1
round 2
round 3
sum of 1 to 100 = 5050

• for round in 1..4 runs the body once for each value in the range, with round set to that value. The loop variable is a fresh variable each time, and it only exists inside the loop.
• a..b is a half-open range: from a up to but not including b. So 0..n runs exactly n times, with the values 0 to n - 1.
• a..=b is an inclusive range: it includes b.
• If you don't need the value, call it _: for _ in 0..3 { println!("hip hip hooray"); }.

Backwards, and in steps

fn main() {
    for i in (1..=3).rev() {
        print!("{i} ");
    }
    println!("liftoff");
    for even in (0..=10).step_by(2) {
        print!("{even} ");
    }
    println!();
}

3 2 1 liftoff
0 2 4 6 8 10

• .rev() runs a range backwards. The parentheses around the range are needed: 1..=3.rev() would try to call .rev() on the number 3.
• .step_by(2) takes every second value.
• println!() with nothing inside just ends the line.

Why for is usually the best choice

You could count with while:

let mut i = 0;
while i < 5 {
    println!("{i}");
    i += 1;
}

But for i in 0..5 can't forget the i += 1, can't get stuck, and shows the whole range at a glance. In Rust, for is the everyday loop; while and loop are for when you don't know in advance how many iterations you need. Later you'll use for with lists and text too: it works with anything that produces a sequence of values (an iterator).

Common mistakes

• Off-by-one with ..: for i in 1..10 stops at 9. Use 1..=10 to include 10.
• Counting down with a "backwards" range: for i in 5..1 runs zero times. To count 5, 4, 3, 2, 1, write (1..=5).rev().
• Changing the loop variable to skip ahead: the next value comes from the range anyway (and the loop variable is immutable). Use .step_by() or continue (next step).

Your task

Print the 7 times table, from 7 x 1 = 7 to 7 x 10 = 70, one line each, using a for loop with a single println! inside.

Challenge 2

Print the even numbers from 20 down to 0 on one line (separated by spaces), then Go! on the next line:

20 18 16 14 12 10 8 6 4 2 0
Go!

Use one for loop over a range with .rev() and .step_by(2).

Challenge 3

Write fn sum_of_multiples(limit: u32) -> u32 that adds up every number below limit that is a multiple of 3 or of 5. For example, below 10 these are 3, 5, 6 and 9, which sum to 23. Use a for loop.

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