Loops with while

The for loop knows from the start how many rounds it makes: as many as the list has elements or range() gives numbers. The basic ideas, however, had a second kind of loop, one that runs as long as a condition holds, like stirring dough, which stops only when there are no lumps left. Nobody knows beforehand how many rounds that takes, and this is exactly what Python has the while loop for.

As long as a condition holds

count = 3

while count > 0:
    print(count)
    count = count - 1

print("Go!")

After while comes a condition as you know it from if (Comparisons and truth values), followed by a colon and the indented body. Before every round Python checks the condition: if it is true, the body runs once, and then it goes back up to the next check; if it is false, the program carries on after the loop. Here the body counts count down by one in every round, after the third round count equals 0, the condition count > 0 no longer holds, and the program continues with print("Go!").

If the condition is already false at the first check, the body does not run a single time. To try it, put count = 0 at the start.

When nobody knows how many rounds it takes

For a countdown you could also have used range(). A while loop pays off where the number of rounds only emerges from the calculation, for instance for the question of how many years it takes for savings to double at 5 per cent interest:

amount = 1000
years = 0

while amount < 2000:
    amount = amount * 1.05
    years += 1

print(f"After {years} years it is {amount:.2f} euros.")

The loop credits the interest year by year, counts along and stops in the year in which the amount first reaches 2000 euros. Change the factor to 1.02 or 1.10, and without any further change the loop makes as many rounds as it takes.

The same pattern answers the question of how often you can halve a number until it is smaller than 1:

number = 100
halvings = 0

while number >= 1:
    number = number / 2
    halvings += 1
    print(number)

print(f"Halved {halvings} times.")

The endless loop

Some shampoo bottles say "lather, rinse, repeat", and anyone who takes that literally never gets out of the shower again. A while loop fares just the same when its body changes nothing that eventually makes the condition false. Delete the line count = count - 1 in the countdown above and run the code: count stays 3, count > 0 holds for ever, and the output fills up with threes.

You halt a program that does not stop here with the Stop button, which takes the place of Run while a program is running. Afterwards, Reset brings back the original code. On your own computer, the key combination Ctrl+C stops such a program in the terminal.

Not every endless loop is this easy to see. In the savings example, amount does change in every round, but with the factor 1.00, that is 0 per cent interest, the amount still stays at 1000, and the goal is never reached. Before you write a while loop, therefore, ask yourself what in the body brings the condition closer to its end and whether that happens in every case.

Asking until the answer fits

A common job for while is an input that may only have certain values. The program should keep asking until someone types a positive number, and it does not know beforehand how often that will be. You write this most clearly with while True and break:

while True:
    number = int(input("Enter a positive number: "))
    if number > 0:
        break
    print("The number must be greater than 0, please try again.")

print(f"Thank you, we go on with {number}.")

while True is a loop whose condition always holds, that is an intended endless loop. It is ended from inside: break leaves the loop at once, and the program carries on after it. This way the question stands only once in the program, at the start of the body, with the check right below it. Type -3 first and then 0 before you choose a number that fits. By the way, break also ends a for loop before its list is finished.

for or while?

Both loops repeat their body, and every for loop could also be written with while:

for number in range(1, 4):
    print(f"for: {number}")

number = 1
while number < 4:
    print(f"while: {number}")
    number += 1

Both print 1, 2 and 3, but the while version has to take care of three things itself that for handles on its own: a starting value, a condition and a line that counts on, and anyone who forgets the last one has an endless loop. This gives a simple rule: if you go through a list, a text or a known number of rounds, use for; if the end depends on something that only shows up in the loop, such as a growing amount or an input, use while.

What is still missing

The program that asks for a positive number still has a gap: if someone types twelve instead of 12, the loop does not even get to the check, because int("twelve") already stops the program with a ValueError. Try it in the box above. How to read such a message is shown in Reading error messages, and how a program catches the error and simply asks again follows with Catching errors with try and except in part 4 of this path.

Takeaways

• while condition: repeats the body as long as the condition holds and checks it before every round; if it is false from the start, the body does not run at all.
• while fits when the number of rounds only emerges from the calculation or from inputs.
• If the body changes nothing that eventually makes the condition false, the loop runs endlessly; on this page Stop halts it, in the terminal Ctrl+C.
• while True with break keeps asking until an input is valid.
• For lists, texts and a fixed number of rounds, for is simpler and safer.