While Loops

3.7. While Loops#

While loops can be used to repeat code.

The structure of a while loop is:

while condition:
        # code that executes while condition is true

Here is an example:

i = 0
while i < 3:
    print(i)
    i = i + 1 # increment i by 1

0
1
2

Note

Another way you can increment i by 1 is using i += 1. This is equivalent to i = i + 1. Similarly you can decrease i by 1 using i -= 1, which is equivalent to i = i - 1. Actually the += and -= notation will work for any number i += 3 increase i by 3 and i -= 5 decrease i by 5.

What’s happening here?

We can see that the condition we have set for our while loop is i < 3.

At the first iteration i = 0. This means the condition is True. We execute the code block:

print(i)
i = i + 1

This means that we output:

At the second iteration i = 1. This means the condition is True. Again, we execute the code block and we output:

At the third iteration i = 2. This means that the condition is True. Again, we execute the code block and we output:

At the fourth iteration i = 3. This means that the condition is False. We do not execute the code block.

This is how we can represent this code diagrammatically.

Here i = 0 and i = i + 1 are processes that are happening within the program but aren’t being shown to the user. This means we represent these with rectangles in the flowchart. The print statement results in output to the user so we represent it with a parallelogram. Here we colour the processes related to i in green to identify it as the variable related to the loop, but the colouring is non-standard and most of the time these diagrams will be shown in black and white.

Note that we don’t always have to increment by 1 and we don’t always have to start at i = 0. For example, we might want to print out all the odd numbers less than 10. We can do so with the following.

i = 1
while i < 10:
    print(i)
    i = i + 2 # increment i by 2

What’s happening here?

We can see that the condition we have set for our while loop is i < 10.

At the first iteration i = 1. This means the condition is True. We execute the code block:

print(i)
i = i + 2

This means that we output:

At the second iteration i = 3. This means the condition is True. Again, we execute the code block and we output:

At the third iteration i = 5. This means that the condition is True. Again, we execute the code block and we output:

At the fourth iteration i = 7. This means that the condition is True. Again, we execute the code block and we output:

At the fifth iteration i = 9. This means that the condition is True. Again, we execute the code block and we output:

At the sixth iteration i = 11. This means that the condition is False. We do not execute the code block.

This is how we can represent this code diagrammatically.