SB The while Statement

Section 7.2 The `while` Statement

Let’s look at our first Python statement that can be used to build an iteration. It is called the while statement. When used with other code it can be used to repeat code in a while loop. Similar to the if statement, it uses a boolean expression to control the flow of execution. The body of while (code indented one space in) will be repeated as long as the controlling boolean expression evaluates to True.

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Here is a simple example that counts down from 10 to 0.

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count is a normal variable here, but since it is governing the while loop it is also called the loop variable.

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Line 2 here is the loop condition. It must always be a boolean expression that will evaluate to False or True.

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Lines 3 and 4 are the loop body. The loop body is always indented. The indentation determines exactly what statements are "in the loop". The loop body is run each time the loop is repeated.

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On each iteration or pass of the loop, a check is done to see if the loop condition is True (if count is greater than zero). If it is not (this is called the terminating condition of the loop), the loop has finished. Program execution continues at the next statement after the loop body.

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If count is greater than zero, the loop body is executed again.

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At the end of each execution of the body of the loop, Python returns to the while statement, to see if the loop should repeat.

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Notice that if the condition is False the first time through the loop, the statements inside the loop are never executed.

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Warning 7.2.1.

Though Python’s while is very close to the English “while”, there is an important difference: In English “while X, do Y”, we usually assume that immediately after X becomes false, we stop with Y. In Python there is not an immediate stop: After the initial test, any following tests come only after the execution of the whole body, even if the condition becomes false in the middle of the loop body.

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The body of the loop should change the value of one or more variables so that eventually the condition becomes False and the loop terminates. Otherwise the loop will repeat forever. This is called an infinite loop. An endless source of amusement for computer scientists is the observation that the directions written on the back of the shampoo bottle (lather, rinse, repeat) create an infinite loop.

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We can use the while loop to create any type of iteration we wish, making it more general-purpose than the for loop we’ll learn next week. For example, let us consider a program that adds all numbers from 1 to n. To do this, we will create a variable called aNumber and initialize it to 1, the first number in the summation. Every iteration will add aNumber to the running total until all the values have been used. In order to control the iteration, we must create a boolean expression that evaluates to True as long as we want to keep adding values to our running total. In this case, as long as aNumber is less than or equal to the bound, we should keep going.

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Here is the summation program that uses a while statement.

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You can almost read the while statement as if it were in natural language. It means, while aNumber is less than or equal to aBound, continue executing the body of the loop. Within the body, each time, update theSum using the accumulator pattern and increment aNumber. After the body of the loop, we go back up to the condition of the while and reevaluate it. When aNumber becomes greater than aBound, the condition fails and flow of control continues to the return statement.

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In the case shown above, we can prove that the loop terminates because we know that the value of aBound is finite, and we can see that the value of aNumber increments each time through the loop, so eventually it will have to exceed aBound. In other cases, it is not so easy to tell.

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Note 7.2.2.

This workspace is provided for your convenience. You can use this activecode window to try out anything you like.

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Check your understanding

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Checkpoint 7.2.3.

True or False: You can rewrite any for-loop as a while-loop.

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True
Although the while loop uses a different syntax, it 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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Checkpoint 7.2.4.

The following code contains an infinite loop. Which is the best explanation for why the loop does not terminate?

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n = 10
answer = 1
while n > 0:
    answer = answer + n
    n = n + 1
print(answer)

n starts at 10 and is incremented by 1 each time through the loop, so it will always be positive
The loop will run as long as n is positive. In this case, we can see that n will never become non-positive.
answer starts at 1 and is incremented by n each time, so it will always be positive
While it is true that answer will always be positive, answer is not considered in the loop condition.
You cannot compare n to 0 in while loop. You must compare it to another variable.
It is perfectly valid to compare n to 0. Though indirectly, this is what causes the infinite loop.
In the while loop body, we must set n to False, and this code does not do that.
The loop condition must become False for the loop to terminate, but n by itself is not the condition in this case.

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Checkpoint 7.2.5.

What is printed by this code?

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n = 1
x = 2
while n < 5:
    n = n + 1
    x = x + 1
    n = n + 2
    x = x + n
print(n, x)

4 7
Setting a variable so the loop condition would be false in the middle of the loop body does not keep the variable from actually being set.
5 7
Setting a variable so the loop condition would be false in the middle of the loop body does not stop execution of statements in the rest of the loop body.
7 15
After n becomes 5 and the test would be False, but the test does not actually come until after the end of the loop - only then stopping execution of the repetition of the loop.

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