For example, to count the number of items in a list, we would write the following for loop:
count = 0for itervar in [3, 41, 12, 9, 74, 15]: count = count + 1print 'Count: ', count
We set the variable count to zero before the loop s tarts, then we write a for loop to run through the list of numbers. Our iteration variable is named itervar and while we do not use itervar in the loop, it does control the loop and cause the loop body to be executed once for each of the values in the list.
In the body of the loop, we add one to the current value of count for each of the values in the list. While the loop is executing, the value of count is the number of values we have seen “so far”.
Once the loop completes, the value of count is the total number of items. The total number “falls in our lap” at the end of the loop. We construct the loop so that we have what we want when the loop finishes.
Another similar loop that computes the total of a set of numbers is as follows:
total = 0 for itervar in [3, 41, 12, 9, 74, 15]: total = total + itervarprint 'Total: ', total
In this loop we do use the iteration variable. Instead of simply adding one to the count as in the previous loop, we add the actual number (3, 41, 12, etc.) to the running total during each loop iteration. If you think about the variable total, it contains the “running total of the values so far”. So before the loop starts total is zero because we have not yet seen any values, during the loop total is the running total, and at the end of the loop total is the overall total of all the values in the list.
As the loop executes, total accumulates the sum of the elements; a variable used this way is sometimes called an accumulator.
Neither the counting loop nor the summing loop are particularly useful in practice because there are built-in functions len() and sum() that compute the number of items in a list and the total of the items in the list respectively.