Control Flow Ranges

Learning objective: By the end of this lesson, students will be able to identify and use ranges in Python.

Purpose of ranges

Python ranges are a sequence type, similar to a list.

The range type represents an immutable sequence of numbers and is commonly used for looping a specific number of times in for loops.

Ranges have their own class (type): range.

Ranges can only be created by invoking the range() class:

for num in range(5):
    print(num)
    # prints the integers: 0, 1, 2, 3, 4

By default, the sequence starts at 0 and goes up to, but does not include, the integer passed in.

Ranges can also generate more complex sequences when passed a start and a step:

for even in range(4, 12, 2):
    print(even)
    # prints the integers: 4, 6, 8, 10

The start (the first argument above - 4) indicates the starting integer.

The step (the last argument above - 2) is an integer added to the current value after each iteration to determine the next value.

When not passed in, the start value defaults to 0, and the step defaults to 1.

Ranges can also be used to create lists:

nums = list(range(10))
print(nums)
# prints: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

If the step is a negative integer, the sequence counts down:

for num in range(5, 0, -1):
    print(num)
    # prints the integers: 5, 4, 3, 2, 1

For more detail on ranges, check out the Python docs.