Loops

Imagine you had to write your name a hundred times. You surely wouldn't want to fill out a hundred slips of paper, you'd rather say: just write this a hundred times. Programs know exactly this trick. Instead of writing the same command over and over, they use a loop.

Repeat n times

The simplest loop is the counting loop: repeat something a fixed number of times. Instead of writing the command print Hello five times in a row, you write: repeat 5 times: print Hello. While running, the program counts along and stops exactly after the fifth repetition. You save writing, and if it suddenly needs to be 500 times, you only change a single number.

The loop body

Everything inside the loop is called the loop body. One single execution of that body is called a pass. In every pass the body is executed completely from top to bottom, then the next pass begins. The body may contain several commands too: repeat 3 times: take a step, turn around. Then the program does both three times in a row, always in the same order.

How many times does it run in total?

Often you want to know how much a loop gets done in total. For that you multiply: the number of passes times what happens per pass. If a loop repeats a body with 3 commands 4 times, a total of 4 times 3, that is 12 commands, gets executed. You work it out the same way when the body changes a variable: if it adds 2 in every pass, then after 4 passes the variable has grown by 4 times 2, that is by 8. This little calculation will also help you later when you want to estimate how long a program takes.