Branching practice and the for loop

When the same job must be done many times, repetition (a loop) is used; if the number of repetitions is known beforehand, for is handy. It is written for i in range(start, stop, step): and the loop body follows on the next line indented by 4 spaces. i is the loop variable and takes the next value in every iteration; start is the starting value (0 if not given), step is the step (1 if not given), and stop is not included in the loop: range(1, 6) gives the numbers 1, 2, 3, 4, 5. For decreasing order the step is negative: range(5, 0, -1) goes from 5 to 1. A loop can be written inside a loop; when the outer loop turns once the inner loop turns completely, so a 3 × 3 loop is run 9 times. To find a sum, s = 0 is written before the loop and s += i is done in the loop.