Choose between for and while loops

  • 10 minutes

Programs become more useful when they can automate repetitive tasks. Instead of writing the same code over and over, you can use a loop to repeat a block of code automatically.

Python has two types of loops: for loops and while loops. Choosing the right one depends on whether you know how many times to repeat ahead of time.

The for loop (known number of repetitions)

Use a for loop when you want to repeat an action a known number of times, or when you want to go through each item in a collection.

Looping with range()

The built-in range() function generates a sequence of numbers to loop over:

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

Output:

0
1
2
3
4

Notice that range(5) produces the numbers 0 through 4. The loop runs 5 times, and the variable i takes on each value in the sequence. The range() function starts at 0 by default and stops before the number you specify (5 in this case).

You can customize the sequence by providing up to three settings: range(start, stop, step). The loop will stop before it hits the stop target:

for number in range(1, 10, 2):  # Start at 1, stop before 10, step by 2
    print(number)

Output:

1
3
5
7
9

The while loop

Use a while loop when you want to repeat something as long as a condition remains True. You can use this when you don't know how many times you'll need to repeat the action ahead of time.

health = 3

while health > 0:
    print(f"Player is alive. Health: {health}")
    health -= 1  # Modify the condition so the loop will eventually stop

Output:

Player is alive. Health: 3
Player is alive. Health: 2
Player is alive. Health: 1

Avoid infinite loops

A while loop inspects its condition before each iteration. If the condition is always True, the loop will never stop. This behavior is called an infinite loop and will freeze your program.

Always ensure your code updates the condition so that it will eventually become False. In the example above, we subtracted 1 from health each time through the loop. When health reached 0, the condition became False, and the loop stopped.

Choosing the right loop

Choose a for loop when:

  • You know how many times you want to repeat the action.
  • You want an automatic exit after a certain number of repetitions.
  • You want a counter variable to keep track of how many times the loop has run.

Choose a while loop when:

  • The total number of repetitions depends on a variable state.
  • You're waiting for something specific to happen before stopping the loop (e.g., user input).
  • You need control over when the loop stops, rather than a fixed number of repetitions.