Nested Loops

3.16. Nested Loops#

You can make the block of code inside the loops as complex as you want. This means you can also put loops inside other loops, whether they be while loops or for loops.

Example: multiplication tables with nested for loops.

for i in range(1, 13):
    for j in range(1, 13):
        print("{} x {} = {}".format(i, j, i * j))

x 1 = 1
x 2 = 2
x 3 = 3
...
x 11 = 132
x 12 = 144

What’s happening here?

We can see that at the first iteration of the outer loop, i = 1. Then we run the code block inside this for loop, which is

for j in range(1, 13):
    print("{} x {} = {}".format(i, j, i * j))

This is another for loop, this time with j looping from 1, …, 12, each time printing '{} x {} = {}'.format(i, j, i*j). Note that for the entire duration of this loop running i is still 1. This means we get 1 x 1 = 1, …, 1 x 12 = 12.

We can see that at the second iteration of the outer loop, i = 2. Again we run the code block inside this for loop, which is

for j in range(1, 13):
    print("{} x {} = {}".format(i, j, i * j))

This loop will again run in its entirety with j looping from 1, …, 12, each time printing '{} x {} = {}'.format(i, j, i*j). For the entire duration of this loop running i is still 2. This means we get 2 x 1 = 2, …, 2 x 12 = 24.

We keep running through the outer loop for i = 1, …, 12, each time running the inner loop. This gives us all of the standard multiplication tables.

Note that the iteration variables you use in your outer and inner loops need to have different variable names, otherwise you will overwrite the value of the iteration variable used in your outer loop. In this example our outer iteration variable is i and our inner iteration variable is j.

Example: multiplication tables with nested while loops.

i = 1
while i < 13:
    j = 1
    while j < 13:
        print("{} x {} = {}".format(i, j, i * j))
        j = j + 1
    i = i + 1

Example: multiplication tables with a while loop inside a for loop.

for i in range(1, 13):
    j = 1
    while j < 13:
        print("{} x {} = {}".format(i, j, i * j))
        j = j + 1