Loops: for, while, break, and continue

You repeat instructions with for and while, and control your loops with range, break, and continue.

8 min · 3 questions

A loop repeats a block of instructions several times, without you having to copy it out again. The for loop repeats a number of times known in advance. It's often used with range, a function that produces a sequence of numbers: range(5) gives 0, 1, 2, 3, 4 (you start from 0 and the end bound is excluded). As with if, the line ends with : and the repeated block is indented by 4 spaces.

for number in range(5):
    print(number)

range also accepts a start: range(1, 6) gives 1, 2, 3, 4, 5. The while loop repeats its block as long as a condition is true. You use it when you don't know in advance how many rounds there will be. Careful: the block must make the condition evolve, otherwise the loop never stops.

countdown = 3
while countdown > 0:
    print(countdown)
    countdown = countdown - 1
print("Go!")

Two keywords let you control a loop from the inside: break exits immediately from the loop, and continue skips to the next round without running the rest of the block. In the example below, % gives the remainder of a division: number % 2 == 0 therefore tests whether a number is even.

for number in range(10):
    if number == 5:
        break
    if number % 2 == 0:
        continue
    print(number)

Knowledge check

Make sure you remember the key points of this lesson.

1. How many times does the block of for i in range(3): run?
   • 2 times
   • 3 times
   • 4 times
2. What does the break keyword do in a loop?
   • It pauses the loop
   • It exits the loop immediately
   • It restarts the loop from the beginning
   • It moves to the next round
3. What happens if a while condition stays true forever?
   • The loop never stops
   • Python fixes the program automatically
   • The loop runs only once