Loop control: break, continue, pass

Loop control statements are nested inside loops and designed to change their typical behavior. In this topic, we'll find out how they work and what they are used for.

Break

The break statement is used to terminate a loop of any type (i. e. for and while loops). It may be said that break "jumps out" of the loop where it was placed. Let’s examine a tiny example:

pets = ['dog', 'cat', 'parrot']
for pet in pets:
    print(pet)
    if pet == 'cat':
        break

We wanted to stop the loop before it iterated for the last time. For that purpose, we introduced a condition when the loop should be stopped. The output is as follows:

dog
cat

Be careful where you put print(). If you put it at the loop’s end, the output will return only the first value – ‘dog’. This happens because break exits from the loop immediately.

Often enough, break is used to stop endless while loops like this one:

count = 0
while True:
    print("I am Infinite Loop")
    count += 1
    if count == 13:
        break

Let's consider how break works in for...else or while...else statements. else statements will be executed if no break is encountered during the execution of the loop. Look how it works in the examples below!

numbers = [1, 2, 3]
for num in numbers:
    if num == 5:
        print("It's 5!")
        break
else:
    print('5 was not listed')
# 5 was not listed

In this case, the body of the else statement is executed because no break was met. Let's change the number list.

numbers = [1, 2, 3, 4, 5, 6]
for num in numbers:
    if num == 5:
        print("It's 5!")
        break
else:
    print('5 was not listed')
# It's 5!

break was met, so the print() command after else wasn't executed.

Continue

The continue operator is commonly used, too. You can stop the iteration if your condition is true and return to the beginning of the loop (that is, jump to the loop's top and continue execution with the next value). Look at the following example:

pets = ['dog', 'cat', 'parrot']
for pet in pets:
    if pet == 'dog':
        continue
    print(pet)

The output will contain all values except the first one ('dog') since it fulfills the condition:

cat
parrot

Thus, the loop just skips one value and goes on running.

One nuance is worth mentioning: the continue operator should be used moderately. Sometimes you can shorten the code by simply using an if statement with the reversed condition:

pets = ['dog', 'cat', 'parrot']
for pet in pets:
    if pet != 'dog':
        print(pet)

In this case, the output will remain the same:

cat
parrot

Pass

When no action is required (e.g. some condition is met, you need to take it into account in a loop, but do nothing if that's the case), in Python, you can use pass statement which does exactly what you need – nothing. Here the program is just waiting to be manually interrupted:

while True:
    pass

Summary

To sum up, loop control statements represent a useful tool to alter the way a loop works. You can introduce extra conditions using the break, continue, and pass operators. In addition, they allow you to skip a beforehand selected set of values, terminate an endless loop, or even do nothing. Use them wisely and they'll work wonders.