Finding sums and products:

A very general application of a for loop is to compute sums and products. For illustration, rather than of just printing the integers 1 through 5, we can compute the sum of the integers 1 through 5 (or, in common, 1 through n, here n is any positive integer). Principally, we want to implement

or compute the sum 1 + 2 + 3 + ... + n.

In order to do this, we require to add each value to a running sum. The running sum is a sum which will keep changing; we keep adding to it. At First the sum has to be initialized to 0, then in this situation it will be 1 (0 + 1), then 3 (0 + 1 + 2), then 6 (0 + 1 + 2 + 3), and so on.

In a function to compute the sum, we require a loop or iterator variable i, and also a variable to store the running sum. In this situation we will use the output argument runsum as the running sum. Each time through the loop, the later value of i is added to the value of the runsum. This function will return the end outcome that is the sum of all the integers from 1 to the input argument n stored in the output argument runsum.