Nested loops in Swift in Swift

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("*", terminator: "")
	}
    print()
}

// 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 = [[1, 2], [3, 4], [5, 6]]

for i in 0..<arr.count {
	for j in 0..<arr[i].count {
		print(arr[i][j])
	}
}

This outputs each sub-element in arr one at a time. Note that for the inner loop, we are checking the .count of arr[i], since arr[i] is itself an array.

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 = [[1, 2], [3, 4], [5, 6]]

for i in 0..<arr.count {
	for j in 0..<arr[i].count {
		if arr[i][j] == 3 {
			break
		}
		print(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 = [[1, 2], [3, 4], [5, 6]]

for i in 0..<arr.count {
	if arr[i][0] == 3 {
		break
	}
	for j in 0..<arr[i].count {
		print(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.