The for and while Loops

When the same action has to be done many times, a loop is used. Python has two kinds of loops: for is convenient when the number of repetitions is known in advance, and while when you need to repeat as long as a condition stays true. The loop body is written with an indent (4 spaces), just like in the conditional statement.

The for loop usually works with the range() function. If the first line is for i in range(5): and the second line, indented by 4 spaces, is print(i), then i takes the values 0, 1, 2, 3, 4 one after another, and these numbers appear on the screen. range(1, 6) gives the numbers from 1 to 5 (the last number is not included), and range(0, 10, 2) gives 0, 2, 4, 6, 8.

The while loop checks the condition each time and, if it is true, runs the body. For example, with n = 3, the loop while n > 0: with the commands print(n) and n = n - 1 inside prints 3, 2, 1 and stops when n becomes zero. If n is not changed inside the loop, the condition always stays true and the loop runs forever.

Calculating a sum is a classic example of a loop: starting with total = 0, the loop for i in range(1, 6): runs total = total + i inside it (indented by 4 spaces), and in the end total equals 15. To leave a loop early, use break, and to skip the current pass, use continue. If the number of repetitions is known, it is more convenient to choose for; if it is unknown, choose while.

Key takeaways

• A loop repeats a group of commands several times.
• for is used when the number of repetitions is known, and while as long as a condition is true.
• range(5) gives the numbers 0, 1, 2, 3, 4 (5 is not included).
• If nothing inside while affects the condition, the loop becomes endless.
• break stops the loop, and continue moves on to the next pass.