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Computer Science for STEM: The PreTeXt Interactive Edition

Section 7.9 The for loop

The while statement is a general-purpose tool for iteration, and is necessary for any instance of iteration where we don’t know how many repetitions will be needed. However, if we do know how many are needed, there is a more efficient approach: the for statement.
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 is the loop variable in this case.
  • Frustratingly, in here is not the same as the keyword used to check for membership. This just a necessary part of every for loop construction and not an operator.
  • The list of names in the square brackets is a regular list. Later we’ll see that other types besides lists can be put in this spot.
  • Line 2 is the loop body. Like with while, the loop body is always indented. 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.

Note 7.9.1.

Introduction of the for statement causes us to think about the types of iteration we have seen. The for statement will always iterate through a sequence of values like the list of names for the party.
Since we know that it will iterate once for each value in the collection, it is often said that a for loop creates a definite iteration because we definitely know how many times we are going to iterate. On the other hand, the while statement is dependent on a condition that needs to evaluate to False in order for the loop to terminate. Since we do not necessarily know when this will happen, it creates what we call indefinite iteration. Indefinite iteration simply means that we don’t know how many times we will repeat but eventually the condition controlling the iteration will fail and the iteration will stop. (Unless we have an infinite loop which is of course a problem.)

Note 7.9.2. Choosing between for and while.

Use a for loop if you know the maximum number of times that you’ll need to execute the body. For example, if you’re traversing a list of elements, or can formulate a suitable call to range, then choose the for loop.
So any problem like "iterate this weather model run for 1000 cycles", or "search this list of words", "check all integers up to 10000 to see which are prime" suggest that a for loop is best.
By contrast, if you are required to repeat some computation until some condition is met, as we did in the 3n + 1 problem last week, you’ll need a while loop.
What you will notice here is that the while loop is more work for you --- the programmer --- than the equivalent for loop. When using a while loop you have to control the loop variable yourself. You give it an initial value, test for completion, and then make sure you change something in the body so that the loop terminates.
Check your understanding

Checkpoint 7.9.3.

    True or False: You can rewrite any for-loop as a while-loop.
  • True
  • The syntax for a for-loop can make it easier and more appealing, but a while loop is just as powerful as a for-loop and often more flexible.
  • False
  • Often a for-loop is more natural and convenient for a task, but that same task can always be expressed using a while loop.
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