For Loops#

Warning

For those that know Java, the for loop in Python looks quite different than the for loop in Java!


If you have learned for-each loops in Java, the for loop in Python behaves a lot more like the for-each loop (also called the “enhanced for loop”).

In Python, the for loop lets you iterate over a sequence of values. The for loop has a body that runs for each item in a sequence and uses a loop variable to keep track of the current item.

We’ll start by showing an example and then explain the parts.

for i in range(5):
    print('Loop', i)

The for loop has the following components

  • range(5) describes the sequence of values we want to use. In this case, range(5) means the values 0, 1, 2, 3, 4 . We will explain range in the next section.

  • i is the loop variable that can be used in the body. On the first iteration i = 0 , then i = 1 on the next, and so on until the last iteration where i = 4 .

  • print('Loop', i) is the body

The for loop operates very similarly to the while loop, but the key difference is it will loop over the sequence of values specified after the in keyword. Just like the while loop, you put a : at the end of the line containing the keyword for and the body is indented inside the loop.

range#

range is a function in Python provided to make it easy to make sequences of numbers in a range. It turns out, there are three different ways to call range that let you do slightly different types of loops!

  • range(A) is the sequence from 0 (inclusive) to A (exclusive)

    • For example: range(4) is 0, 1, 2, 3

  • range(A, B) is the sequence from A (inclusive) to B (exclusive)

    • For example: range(3, 7) is 3, 4, 5, 6

  • range(A, B, C) is the sequence from A (inclusive) to B (exclusive), using step size C .

    • For example: range(1, 10, 3) is 1, 4, 7 (notice 10 is not included since it is exclusive)

There is no reason that these numbers have to be positive! For example, you can use negative numbers for the start/stop or use a negative step size to make the numbers decrease instead! The semantics are exactly the same!