Break and Continue

break and continue are keywords in python that allow you to stop the loop or skip the current iteration.

Many times programmers use break and continue when they don't need to. Generally break and continue can lead to funny flows in your program and bugs. If you can avoid using them then do!

1. Break

The break keyword allows us to exit ("break") out of a loop early. We can do this with both while and for loops.

Example 1.1

The following program will stop short of printing the numbers 0,1,2,3,4,5,6,7,8,9 and only print 0,1,2,3,4.

If you need to convince yourself, copy, paste and run this code in Python.

i = 0
while i < 10:
    if i == 5:
        break
    print(i)
    i += 1

The program tests each value of i and if it is equal to 5 the program exits the loop.

We can write this without the break.

i = 0
while i < 5:
    print(i)
    i += 1

Example 1.2

Here is the same program with a for loop

for i in range(0,10):
    if i == 5:
        break
    print(i)

We can write this without the break.

for i in range(0,5):
    if i < 5:
        print(i)

2. Continue

The continue keyword allows us to skip the current iteration of a loop. We can do this with both while and for loops.

Example 2.1

The following program will not print out the number 5, but all other numbers.

So the output will be the numbers 0,1,2,3,4,6,7,8,9. Notice that 5 is missing.

If you need to convince yourself, copy, paste and run this in Python.

i = -1
while i < 9:
    i += 1
    if i == 5:
        continue
    print(i)

The program tests each value of i and if it is equal to 5 the program exits the loop.

We can write this without the continue.

i = 0
while i < 10:
    if not i == 5:
        print(i)
    i += 1

Example 2.2

Here is the same program with a for loop

for i in range(0,10):
    if i == 5:
        continue
    print(i)

We can write this without continue.

for i in range(0,10):
    if not i == 5:
        print(i)

3. No Do...While

To date in Python, we have seen a while loop that will check a condition and execute a code block until the condition becomes False.

while <condition>
    # Do something

In many languages it is possible to do the following:

do
  run a block of code
while <condition>

This guarantees that the code block runs at least once as the while is checked after the code block.

One of the major reasons Python does not support such a statement is that it breaks the indentation rule that a code block should be indented. Here it isn't!

3.1. Try Not To Use Break

It is quite common to see the following in Python.

while True:
    # do something
    if <condition>:
        break

The following essentially emulates a do...while loop. Here is an example.

# The Number Guessing Game 
import random

# randomly generate a number between 1 and 10
secret_guess = random.randint(1,10)

print("Welcome to the Number Guessing Game!")
print("You need to try and guess the number between 1 and 10...")

while True:
    guess = int(input("Please enter a guess:\n"))
    
    print("That is not correct, please try again.")
    
    if guess == secret_guess:
        break
    
print(f"Well done the correct answer is {secret_guess}")

However, the use of a break statement here relies on the person reading this to implicitly understand that the programmer is intending to use a do...while.

There are also other reasons we would not want to do this in other languages which are compiled to do with the fetch-decode-execute cycle.

We can easily rewrite this with an additional boolean.

# The Number Guessing Game 
import random

# randomly generate a number between 1 and 10
secret_guess = random.randint(1,10)

print("Welcome to the Number Guessing Game!")
print("You need to try and guess the number between 1 and 10...")

game_over = False

while not game_over:
    guess = int(input("Please enter a guess:\n"))
    
    print("That is not correct, please try again.")
    
    if guess == secret_guess:
        game_over = True
    
print(f"Well done the correct answer is {secret_guess}")

Now, these are almost identical, however, it is now more clear that the while will end when gameover is set to True. This conveys the programmers' intention.

=== TASK ===

Print out the numbers 1 to 100 but leave out the meaning of life 42.

Use a continue statement.

Programming in Python