While loops

The while loop is rather like the for loop (see: Print, If and For).

Here is a simple while loop where I add up all the numbers between 1 and 21 inclusive:

>>> # We use a variable to keep track of the total
>>> total = 0
>>> # We use a variable to keep track of what number we're on
>>> current_number = 0
>>> while current_number < 22:
...     total = total + current_number
...     current_number = current_number + 1
...
>>> print(total)
231

The while loop, like the if statement, tests a condition, here current_number < 22. Like the if statement, there are indented statements after the condition test, that only run if the condition test is True. The difference is that a while loop keeps running the indented statements until the condition is True. So, the first time through, when current_number is set to 0, the condition will be True (0 is less than 22), and the body of the while loop executes. But, when the while loop finishes running current_number = current_number + 1 at the end of the block, it goes back and checks the conditional again. Now current_number is 1, but this is still less than 22, and so we proceed to run total = total + current_number and the rest of the statements in the body. We do this until the condition is False, and then the while loop stops and the program continues after the end of the while loop.

Exercises:

• Use a version of the while loop above to add up all the numbers between 10 and 21, inclusive.
• Use a version of the while loop above to add up all the even numbers between 0 and 20, inclusive.
• Make an empty list, and use a while loop to go through all the numbers between 5 and 30, inclusive, appending them to the list.