Computers are often used to automate repetitive tasks. Repeating identical or similar tasks without making errors is something that computers do well and people do poorly.
Repeated execution of a sequence of statements is called iteration. Because iteration is so common, Python provides several language features to make it easier. We’ve already seen the for statement in a previous chapter. This is a very common form of iteration in Python. In this chapter we are going to look at the while statement — another way to have your program do iteration.
Recall that the for loop processes each item in a list. Each item in turn is (re-)assigned to the loop variable, and the body of the loop is executed. We saw this example in an earlier chapter.
We have also seen iteration paired with the update idea to form the accumulator pattern. For example, to compute the sum of the first n integers, we could create a for loop using the range to produce the numbers 1 through n. Using the accumulator pattern, we can start with a running total variable initialized to 0 and on each iteration, add the current value of the loop variable. A function to compute this sum is shown below.
To review, the variable theSum is called the accumulator. It is initialized to zero before we start the loop. The loop variable, aNumber will take on the values produced by the range(1, aBound + 1) function call. Note that this produces all the integers from 1 up to the value of aBound. If we had not added 1 toaBound, the range would have stopped one value short since range does not include the upper bound.
The assignment statement, theSum = theSum + aNumber, updates theSum each time through the loop. This accumulates the running total. Finally, we return the value of the accumulator.