# Use the Python range() Function to Generate Sequences of Numbers

John (304)
5 minutes

In Python, `range` is an immutable sequence type, meaning it's a class that generates a sequence of numbers that cannot be modified. The main advantage to the `range` class over other data types is that it is memory efficient. No matter how large a sequence you want to iterate over, the `range` class only stores the `start`, `stop`, and `step` values (we'll cover these later), not the entire sequence of numbers.

In this guide we'll cover various was to use the `range` class.

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## Basic usage

While it looks more like a built-in function, `range` is actually a class. So when you use range, you're actually passing arguments into the `range` constructor.

When given a single argument, `range` will use this value as the stop value. Stop refers to the end of the sequence. Keep in mind that `range` sequences are not inclusive, meaning the sequence will contain numbers up to but not including the stop value.

``range(stop)``

Example:

``list(range(10))``

Output:

``[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]``

Note: In order to see the numbers in the sequence, we must convert the sequence to a list.

Notice how `range` assumes a start value of `0` and that the sequence contains numbers up to but not including the stop value.

## Specifying start and stop values

As one might guess, you can also specify the start value. When two arguments are passed to the `range` constructor, the first is start and the second is stop.

``range(start, stop)``

Example:

``list(range(10, 20))``

Output:

``[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]``

## Incrementing by something other than one

The default step value is `1`, but sometimes we'll want to increment by something other than 1. If provided a third argument, it will be used as the step value.

``range(start, stop, step)``

Example:

``list(range(10, 20, 2))``

Output:

``[10, 12, 14, 16, 18]``

## Iterating over a range

Iterating over a range of numbers is easy using the Python for loop.

Example:

``````for x in range(5):
print(x)``````

Output:

``````0
1
2
3
4``````

## Iterating a specific number of times

You can use the `range` function to iterate a specific number of times, even when you don't need to access each element of the sequence.

Imagine we want to build a string that contains ten of a certain character.

Example:

``````result = ''

for x in range(10):
result += '#'

print(result)``````

Output:

``##########``

In this example, `range` provides a memory efficient way of iterating exactly ten times.

## Reversing a range

Python provides a built-in function for reversing sequences called `reversed`.

Example:

``list(reversed(range(10)))``

Output:

``[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]``

## Using slice notation on a range

You can access indexes of a range as well as use slice notation to get a subsection of the sequence.

Example:

``list(range(100)[10:20])``

Output:

``[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]``

## Conclusion

At this point, you should have a solid understanding of what the `range` class is and how it's used. Do you have any questions or feedback? Let me know in the comments below.

John (304)
2 minutes

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John
Joined in 2015 304 guides
Software Engineer and creator of howchoo.
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