Nested Loops in Rust: for Loops Inside for Loops

A nested loop has one loop inside of another. These are typically used for working with two dimensions such as printing stars in rows and columns as shown below. When a loop is nested inside another loop, the inner loop runs many times inside the outer loop. In each iteration of the outer loop, the inner loop will be re-started. The inner loop must finish all of its iterations before the outer loop can continue to its next iteration.

Example:

for row in 0..3 {
	for col in 0..5 {
		print!("*");
	}
	println!();
}

// This code prints a rectangle with 3 rows and 5 columns, filled with '*'

Iterating through multidimensional arrays

Nested loops are also helpful when we want to iterate through a multi dimensional array, for example:

let arr = vec![
	vec![1, 2],
	vec![3, 4],
	vec![5, 6],
];

for i in 0..3 {
	for j in 0..2 {
		println!("{}", arr[i][j]);
	}
}

// This outputs each sub-element in arr one at a time

Breaking nested loops

In a nested loop, a break statement only stops the loop it is placed in. Therefore, if a break is placed in the inner loop, the outer loop still continues. For example:

let arr = vec![
	vec![1, 2],
	vec![3, 4],
	vec![5, 6],
];

for i in 0..3 {
	for j in 0..2 {
		if arr[i][j] == 3 {
			break;
		}
		println!("{}", arr[i][j]);
	}
}

// prints the numbers 1, 2, 5 and 6 on different lines

When i is 1 and j is 0, we execute the break. This means we stop the inner loop and go back to the outer loop to continue from the next iteration, which is i = 2. And as you can see, we print all the elements of row 2.

However, if the break is placed in the outer loop, all of the looping stops. For example:

let arr = vec![
	vec![1, 2],
	vec![3, 4],
	vec![5, 6],
];

for i in 0..3 {
	if arr[i][0] == 3 {
		break;
	}
	for j in 0..2 {
		println!("{}", arr[i][j]);
	}
}

// prints the numbers 1 and 2 on different lines

Assignment
Follow the Coding Tutorial and let's write some nested loops.

Hint
Look at the examples above if you get stuck.