Nested loops in Python

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 range(3):
	for col in range(5):
		print("*", end='')
    print()

# This code prints a rectangle with 3 rows and 5 columns, filled with '*'
# We use print("*", end='') because print() prints with a trailing newline by default

Iterating through multidimensional arrays

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

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

for i in range(len(arr)):
	for j in range(len(arr[i])):
		print(arr[i][j])

This outputs each sub-element in arr one at a time. Note that for the inner loop, we are using len(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:

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

for i in range(len(arr)):
	for j in range(len(arr[i])):
		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:

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


for i in range(len(arr)):
	if arr[i][0] == 3:
		break
	for j in range(len(arr[i])):
		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.