2) The while Statement

There is another Python statement that can also be used to build an iteration. It is called the whilestatement. The while statement provides a much more general mechanism for iterating. Similar to theif statement, it uses a boolean expression to control the flow of execution. The body of while will be repeated as long as the controlling boolean expression evaluates to True.

The following figure shows the flow of control.

We can use the while loop to create any type of iteration we wish, including anything that we have previously done with a for loop. For example, the program in the previous section could be rewritten using while. Instead of relying on the range function to produce the numbers for our summation, we will need to produce them ourselves. To to 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 asaNumber is less than or equal to the bound, we should keep going.

Here is a new version of the summation program that uses a while statement.

Note

The names of the variables have been chosen to help readability.

More formally, here is the flow of execution for a while statement:

The body consists of all of the statements below the header with the same indentation.

This type of flow is called a loop because the third step loops back around to the top. Notice that if the condition is False the first time through the loop, the statements inside the loop are never executed.

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.

In the case shown above, we can prove that the loop terminates because we know that the value ofaBound 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.

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.

So why have two kinds of loop if for looks easier? This next example shows an indefinite iteration where we need the extra power that we get from the while loop.