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# NumPy | lexsort method

schedule Aug 10, 2023
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Numpy's `lexsort(~)` method returns the sorted integer indices of multiple input arrays that correspond to column data. See the examples below for clarification.

# Parameters

1. `keys` | `array_like` of `sequences`

The keys you want sorted.

2. `axis` | `int` | `optional`

The axis along which to sort the input array. For 2D arrays, the allowed values are as follows:

Value

Meaning

`None`

Flattens the array and sorts it

`0`

Sorts column-wise

`1`

Sorts row-wise

By default, `axis=-1` which means that sorting is performed only on the last axis. For 2D arrays, this means that the default sorting behaviour is row-wise.

# Return value

A Numpy array that holds the integer indices of the sorted columns.

# Examples

Suppose we have the following data about 3 people:

First name

Last name

Bob

Marley

Alex

Davis

Cathy

Watson

In code, this translates to the following:

``` first_names = np.array(["Bob", "Alex", "Cathy"])last_names = np.array(["Marley", "Davis", "Watson"]) ```

What is important here is that our data is split by columns - we don't have a single array that houses all our data.

## Sorting by a single column

To sort by a single column, say first name:

``` first_names = np.array(["Bob", "Alex", "Cathy"])last_names = np.array(["Marley", "Davis", "Watson"])sorted_indices = np.lexsort([first_names])sorted_indices array([1, 0, 2]) ```

What is returned here is a list of indices of the sorted data - `["Alex Davis", "Bob Marley", "Cathy Watson"]` have original indices of 1, 0 and 2 respectively.

To see the sorted data:

``` for i in sorted_indices: print(first_names[i] + " " + last_names[i]) Alex DavisBob MarleyCathy Watson ```

## Sorting by multiple columns

To sort by multiple columns:

``` first_names = np.array(["Bob", "Alex", "Alex"])last_names = np.array(["Marley", "Davis", "Beck"])sorted_indices = np.lexsort([first_names, last_names])sorted_indices array([2, 1, 0]) ```

Here, our first key (i.e. first_names) have duplicate values, so for those values, they are sorted by their last name - this is why Alex Beck comes before Alex Davis.

To see the sorted data:

``` for i in sorted_indices: print(first_names[i] + " " + last_names[i]) Alex BeckAlex DavisBob Marley ```
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