# How do use itertools in Python to build permutation or combination

There is a python module dedicated to permutations and combinations called itertools.

`import itertools`

The permutation function allows you to get permutation of N values within a list, where order matters. For instance, selecting N=2 values with [1,2,3] is done as follows:

```>>> print list(itertools.permutations([1,2,3], 2)) [(1, 2), (1, 3), (2, 1), (2, 3), (3, 1), (3, 2)]```

If order is not important, you can use combinations:

```>>> print list(itertools.combinations([1,2,3], 2)) [(1, 2), (1, 3), (2, 3)]```

Then, you may want to build all possible arrays of N values taken from a list of possible values. For instance, you may want to build a vector of length N=3 made of 0 and 1. This can be done with the cartesian product function:

```>>> print list(itertools.product([0,1], repeat=3)) [(0, 0, 0), (0, 0, 1), (0, 1, 0), (0, 1, 1), (1, 0, 0), (1, 0, 1), (1, 1, 0), (1, 1, 1)]```
This entry was posted in Python and tagged . Bookmark the permalink.