Loops

You’ll learn about loops—how to run a block of code multiple times. You’ll use two types: while loops (run while a condition is true) and for loops (run once for each item in a sequence).

In the previous lesson you used if, elif, and else to control whether a block of code runs. Loops control how many times it runs.

What are loops?

A loop repeats a block of code. Sometimes you repeat until a condition becomes false; sometimes you repeat once for each item in a list or range. Python has two main kinds of loops: while and for.

While loops

A while loop runs its block of code over and over as long as a condition is true. When the condition is false, the loop stops and the program continues after it.

Here’s a while loop that prints the numbers 1 through 5:

count = 1
while count <= 5:
    print(count)
    count += 1  # Add 1 to count so we eventually reach 6 and stop

The loop runs as long as count is less than or equal to 5. Each time through, you print the current value of the variable count, then add 1 to it. When count becomes 6, the condition is false and the loop ends.

When you run this code, it will output:

1
2
3
4
5

For loops

A for loop runs a block of code once for each item in a sequence—for example, each number in a range or each item in a list. You don’t have to manage a counter or check a condition yourself; the loop does that for you.

Here’s a for loop that prints the numbers 1 through 5 using the range() function:

for i in range(1, 6):
    print(i)

The range(1, 6) function gives you a sequence of numbers from 1 up to (but not including) 6, so 1, 2, 3, 4, 5. The loop iterates over that sequence: each time through, it assigns the next number to the variable i and runs the indented code. You get the same output as the while example, but without writing a condition or incrementing a counter—the for loop handles it.

When you run this code, the output is:

1
2
3
4
5

You’ll use for loops a lot when you have a list (or any sequence) and want to do something with each item. In Python, the for loop is designed to be simple and readable; other languages often use different syntax for the same idea.