Combining loops and logic

All of our examples so far have been relatively simple individual logical statements, but there is no reason that we cannot apply logical operations in loops:

for number in range(1, 22):
    if number % 3 == 0:
        print(f'{number} is divisible by 3.')

3 is divisible by 3.
6 is divisible by 3.
9 is divisible by 3.
12 is divisible by 3.
15 is divisible by 3.
18 is divisible by 3.
21 is divisible by 3.

Here we have combined a simple if statement with a for loop to find all of the multiples of \(3\) between \(1\) and \(21\).

In addition to all the logic we have learnt about thus far, there are two useful keywords that are relevant specifically to using logic in loops.

for number in range(1, 101):
    if number > 10:
        break

    print(number)

print('We have broken out of the loop!')

1
2
3
4
5
6
7
8
9
10
We have broken out of the loop!

In the example above, we are looping over numbers between \(1\) and \(100\). The if statement checks if number is greater than \(10\), and if it is, the break keyword is run. As the name implies, the break keyword breaks us out of the loop: it stop the rest of the iterations that would otherwise have run from occurring. You can see this happening above because as soon as 10 is printed, no more numbers are displayed and instead the 'We have broken out of the loop!' message is seen, which should only be printed after the loop has finished.

The other keyword that pairs nicely with the logic we have learnt is continue:

for number in range(1, 101):
    if number % 5 != 0:
        continue

    print(number)

5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100

Here we are testing whether or not number is not divisible by 5. If number is divisible by 5, then the continue keyword does not run and we print the number. If the number is not divisible by 5, then the continue keyword is run and this immediately skips us to the next iteration of the loop. The end result is that print(number) only runs if number is a multiple of 5.

So, to summarise:

break - When this runs, we exit the loop immediately.

continue - When this runs, we skip the rest of the code in the loop and immediately skip to its next iteration.