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Section 4.4 The for Loop

When we drew the square, it was quite tedious. We had to move then turn, move then turn, etc. etc. four times. If we were drawing a hexagon, or an octogon, or a polygon with 42 sides, it would have been a nightmare to duplicate all that code.

A basic building block of all programs is to be able to repeat some code over and over again. In computer science, we refer to this repetitive idea as iteration. In this section, we will explore some mechanisms for basic iteration.

In Python, the for statement allows us to write programs that implement iteration. As a simple example, let's say we have some friends, and we'd like to send them each an email inviting them to our party. We don't quite know how to send email yet, so for the moment we'll just print a message for each friend.

Take a look at the output produced when you press the run button. There is one line printed for each friend. Here's how it works:

  • name in this for statement is called the loop variable.

  • The list of names in the square brackets is called a Python list. Lists are very useful. We will have much more to say about them later.

  • Line 2 is the loop body. The loop body is always indented. The indentation determines exactly what statements are “in the loop”. The loop body is performed one time for each name in the list.

  • On each iteration or pass of the loop, a check is done to see if there are still more items to be processed. If there are none left (this is called the terminating condition of the loop), the loop has finished. Program execution continues at the next statement after the loop body.

  • If there are items still to be processed, the loop variable is updated to refer to the next item in the list. This means, in this case, that the loop body is executed here 7 times, and each time name will refer to a different friend.

  • At the end of each execution of the body of the loop, Python returns to the for statement, to see if there are more items to be handled.

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