While Loops

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If you want to repeat some computation, a programming language usually provides a construct called a loop that lets you repeat code some number of times. For those that know Java, Python has very similar constructs to the for loop and the while loop.

While Loop

A while loop has a condition and a body. The while loop proceeds in iterations, each iteration executes the body only if the condition is True , otherwise the loop ends. In general, a loop while loop looks like

while condition:
    # Loop body
    statement
    statement
    statement

For example, the code below shows an example of a very simple loop.

x = 1
while x < 100:
    print(x)
    x = x * 2

print('After loop', x)

Notice that the condition here is x < 100 so the loop keeps executing the body ( print(x) and x = x * 2 ) until the next iteration it is false (when x = 128 ). After the loop ends, it continues on to the code after the loop ( print('After loop', x) )

Warning

It was probably easy to miss, but you need a : at the end of the first line of a while loop! This is how Python knows you are saying that this is where the condition ends and the body of the loop begins!

A Very Important Point About Python

Notice in the example above, Python is somehow able to differentiate that the print statement at the end of the program is outside the loop! How does it decide what is inside or outside a loop? Indentation. Unlike other languages that use explicit delimiters (like {} in Java) to determine what goes inside a loop, Python only uses indentation.

In Python, getting your indentation right is very important. A very common error for beginning Python programmers comes from incorrectly indenting your code. If you see the error

IndentationError: unexpected indent

It means there is an error with your indentation!