In Python, the `numpy.argsort()`

function **returns the indices that would sort an array in ascending order.**

Here is the argument table of the `numpy.argsort()`

function.

If it sounds great to you, please continue reading, and you will fully understand the `numpy.argsort()`

function through Python code snippets and vivid visualization.

This tutorial is about `numpy.argsort()`

function.

- Concretely, I will introduce its syntax and arguments.
- Then, you will learn some basic examples of this function.
- Finally, I will address three top questions about
`numpy.argsort(`

), including np.argsort descending,`np.argsort`

2d array/axis, and`np.argsort`

order.

You can find all codes in this tutorial here.

Table of Contents

## Syntax and Arguments

Here is the syntax of `numpy.argsort()`

:

# syntax numpy.argsort(a[, axis=-1[, kind=None[, order=None]]])

Here is the argument table of `numpy.argsort()`

:

Argument | Accept | Description |

`a` | `array_like` | Array to sort. |

`axis` | `int` or `None` , optional | Axis along which to sort. The default is -1 (the last axis). If `None` , the flattened array is used.For more information about a flattened array, please refer to `numpy.ndarray.flatten()` function. |

`kind` | `{'quicksort` , optional | Sorting algorithm. The default is `'quicksort'` .Note that both ‘stable’ and ‘mergesort’ use timsort under the covers and, in general, the actual implementation will vary with data type. The ‘mergesort’ option is retained for backwards compatibility. |

`order` | `str` or `list` of `str` , optional | This argument specifies the order in which to compare field. |

Generally, you only need to deploy the a and axis arguments. And I will clearly explain the axis and order argument later.

The **output** of `numpy.argsort()`

function is **an array of indices** which sort the input array along with the specified axis.

On top of that, If the input array `'a'`

is 1-D, `a[output]`

yields a sorted `'a'`

. More generally, `np.take_along_axis(a, output, axis=axis)`

always yields the sorted `'a'`

, irrespective of dimensionality. We will see more examples later in this article. 🙂

## Basic Examples

Here is a one-dimensional array code example:

import numpy as np one_dim = np.array([1, 5, 4, 0, 3]) sort_index = np.argsort(one_dim) # axis defaults to -1 (the last axis) print(f'Unsorted array: {one_dim}') print(f'Sorted array index: {sort_index}') print(f'Sorted array: {one_dim[sort_index]}')

Output:

In this basic example, we use the `numpy.argsort()`

function to return the index of the sorted input array (in an ascending order) and then index the sorted array using the output.

## np.argsort() descending

We can also return the index that sorts the input array in a descending order using the `[::-1]`

reverse trick.

Here is a one-dimensional array code example:

import numpy as np one_dim = np.array([1, 2, 3, 4, 5]) # use the [::-1] to reverse the ascending order to descending order. sort_index = np.argsort(one_dim)[::-1] print(f'Unsorted array: {one_dim}') print(f'Sorted array index: {sort_index}') print(f'Sorted array: {one_dim[sort_index]}')

Output:

Yes, it is just like reversing a string. We can simply add the `[::-1]`

to the output of the `np.argsort()`

function to get a descending order sorted index.

## np.argsort() 2d array / axis

So far, we’ve only seen some 1d array examples.

In this part, I will show you how to deploy the axis argument with some 2d array examples!

By the way, you can always use the `np.take_along_axis(input_array, sort_index, axis=axis)`

to get the sorted `'a'`

, irrespective of dimensionality.

Here is the argument table for reference:

Here is the 2d array example with `axis=0`

:

import numpy as np # Here is the 2d array example with axis=0: two_dim = np.array([[1, 2, 3], [3, 2, 1]]) sort_index = np.argsort(two_dim, axis=0) print(f"Unsorted array: {two_dim}") print(f"Sorted array index: {sort_index}") print(f"Sorted array: {np.take_along_axis(two_dim, sort_index, axis=0)}")

Output:

Here is the 2d array example with `axis=1`

:

# Here is the 2d array example with axis=1: import numpy as np two_dim = np.array([[1, 2, 3], [3, 2, 1]]) sort_index = np.argsort(two_dim, axis=1) print(f"Unsorted array: {two_dim}") print(f"Sorted array index: {sort_index}") print(f"Sorted array: {np.take_along_axis(two_dim, sort_index, axis=1)}")

Output:

Here is the 2d array example with `axis=None`

:

# Here is the 2d array example with axis=None: import numpy as np two_dim = np.array([[1, 2, 3], [3, 2, 1]]) sort_index = np.argsort(two_dim, axis=None) print(f"Unsorted array: {two_dim}") print(f"Sorted array index: {sort_index}") print(f"Sorted array: {np.take_along_axis(two_dim, sort_index, axis=None)}")

Output:

## np.argsort() order

Although I said that you would probably only need to deploy the `a`

and `axis`

argument, I think the `order`

argument is probably confusing to you. So I will give you an example to help you understand what it means!

In the official documentation, the `order`

argument goes,

*“When a is an array with fields defined, this argument specifies which fields to compare first, second, etc. A single field can be specified as a string, and not all fields need be specified, but unspecified fields will still be used, in the order in which they come up in the dtype, to break ties.”*

Intuitively, the `order`

argument sets the sorting order for an input array with defined fields. You will have a better sense of what it means after seeing the next code examples.

Here are the 1d array examples for `order`

argument:

import numpy as np # order = x -> y one_dim = np.array([(1, 2), (1, 1), (2, 2), (2, 1)], dtype=np.dtype([('x', int), ('y', int)])) sort_index = np.argsort(one_dim) # or np.argsort(x, order=('x', 'y')) print(f'Unsorted array: {one_dim}') print(f'Sorted array index: {sort_index}') print(f'Sorted array: {one_dim[sort_index]}') print('-' * 85) # order = y -> x one_dim = np.array([(1, 2), (1, 1), (2, 2), (2, 1)], dtype=np.dtype([('x', int), ('y', int)])) sort_index = np.argsort(one_dim, order=('y', 'x')) print(f'Unsorted array: {one_dim}') print(f'Sorted array index: {sort_index}') print(f'Sorted array: {one_dim[sort_index]}')

Output:

## Summary

That’s it for our `np.argsort()`

article.

We learned about its syntax, arguments, and basic examples.

We also worked on the top three questions about the `np.argsort()`

function, ranging from `np.argsort()`

`descending`

, `np.argsort()`

2d array/axis, and `np.argsort() order`

.

Actually, Python provides quite a lot sorting related functions aside from the `numpy.argsort()`

function.

- If you want to have more flexibility with ties situation, check out the
`scipy.stats.rankdata()`

function. - If you want to indirectly sort an array with multiple keys, check out the
`numpy.lexsort()`

function. - If you want to directly sort an array, check out the
`numpy.sort()`

and`numpy.ndarray.sort()`

function.

Of course, if you want me to explain more hard-to-understand function, just let me know. 🙂

Hope you enjoy all this and happy coding!

Anqi Wu is an aspiring Data Scientist and enthusiastic Python Freelancer. She is an incoming student for a Master’s program in Analytics and builds her Python Freelancer profile on Upwork.

Anqi is passionate about machine learning, statistics, data mining, programming, and many other data science related fields. She has proven her expertise during her undergraduate years, including multiple winning and top placements in mathematical modeling contests. She loves supporting and enabling data-driven decision-making, developing data services, and teaching.

She is skilled at programming languages like Python, R, and SQL, actively delving into the world of Machine Learning and Deep Learning and traveling along her data science journey with joy. Data sensitivity and business acumen are her advantages to march towards the career path as a data scientist 🙂

Here is a link to the author’s website: https://www.anqiwu.one/. She uploads data science blogs weekly to document her data science learning and practicing for the past week, along with some best learning resources and inspirational thoughts.

I hope you enjoy this article! Cheers!