Nested Loops

Goal of This Episode

Put a loop inside a loop — use nested loops to print the multiplication table.

Main Text

Last episode we learned the for loop. Today we’ll try something more advanced — putting one loop inside another loop.

What Are Nested Loops?

“Nested” means “one layer inside another,” like Russian nesting dolls. Each time the outer loop runs once, the inner loop runs all the way through.

The Multiplication Table

Let’s take on a challenge: print the 9×9 multiplication table with nested loops:

fn main() {
    for i in 1..=9 {
        for j in 1..=9 {
            print!("{} x {} = {}   ", i, j, i * j);
        }
        println!(); // New line
    }
}

How Does It Work?

1. The outer loop runs i from 1 to 9.
2. When i = 1, the inner loop runs j from 1 to 9 → printing 1×1, 1×2, … 1×9.
3. After the inner loop finishes, println!() starts a new line.
4. The outer loop moves to i = 2, and the inner loop runs 1 to 9 again → printing 2×1, 2×2, … 2×9.
5. And so on…

print! vs println!

We used something new here: print!. It’s a lot like println!, except that print! doesn’t start a new line after printing, whereas println! does.

Visualizing It

One outer-loop iteration = one row:

i=1 → [j=1, j=2, j=3, ... j=9] → new line
i=2 → [j=1, j=2, j=3, ... j=9] → new line
...
i=9 → [j=1, j=2, j=3, ... j=9] → new line

break Only Exits the Innermost Layer

Using break inside nested loops only exits the innermost loop — the outer one keeps going:

fn main() {
    for i in 1..=3 {
        for j in 1..=3 {
            if j == 2 {
                break; // Only exits the inner loop
            }
            println!("i={}, j={}", i, j);
        }
    }
}

Each time j reaches 2 it breaks, but the outer i still runs through 1, 2, 3.

Loop Labels: Breaking Out of a Specific Layer

What if you want to jump straight out of the outer loop? Use a loop label:

fn main() {
    'outer: for i in 1..=3 {
        for j in 1..=3 {
            if j == 2 {
                break 'outer; // Exits the outer loop
            }
            println!("i={}, j={}", i, j);
        }
    }
    println!("Done!");
}

'outer: is a label, placed in front of a loop. break 'outer says “exit the loop labeled 'outer.” Note that label names start with ' (a single quote).

Recap

• Nested loops are loops inside loops: each outer-loop iteration runs the inner loop all the way through.
• The difference between print! and println!: print! doesn’t start a new line.
• In nested loops, break only exits the innermost loop.
• Use a loop label ('outer: + break 'outer) to break out of a specific outer loop.