Sorting arrays in NumPy by column

Question

How do I sort a NumPy array by its nth column?

For example, given:

a = array([[9, 2, 3],
           [4, 5, 6],
           [7, 0, 5]])

I want to sort the rows of a by the second column to obtain:

array([[7, 0, 5],
       [9, 2, 3],
       [4, 5, 6]])

score 991 · Answer 1 · edited Apr 02 '21 at 11:24

991

To sort by the second column of a:

a[a[:, 1].argsort()]

edited Apr 02 '21 at 11:24

Mateen Ulhaq

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answered May 13 '10 at 15:39

Steve Tjoa

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If you want the reverse sort, modify this to be `a[a[:,1].argsort()[::-1]]` – Steven C. Howell May 14 '15 at 14:49
1

Looks simple and works! Is it faster than `np.sort` or not? – Václav Pavlík Feb 04 '16 at 12:34
25

I find this easier to read: `ind = np.argsort( a[:,1] ); a = a[ind]` – poppie Feb 13 '17 at 03:40
3

a[a[:,k].argsort()] is the same as a[a[:,k].argsort(),:]. This generalizes to the other dimension (sort cols using a row): a[:,a[j,:].argsort()] (hope i typed that right.) – bean Feb 04 '18 at 17:40
As far as I can see this does not generalize to the case that one would want to sort by multiple columns `cols`, i.e. all columns. For that case I do `a[np.lexsort(a.T[cols])]` where `[cols]` could be left out if sorted by all columns and from right to left. This also allows to define the order of columns by which to sort, i.e `cols = range(a.shape[1])[::-1]`. – Radio Controlled Apr 11 '18 at 11:05
2

needed to use b = a[a[:, 1].argsort()] then b is the sorted one – pippo1980 Jul 06 '21 at 13:19
1

@pippo1980 , I think this comment is important. The answer is a bit misleading as if `a` itself changes to the sorted version only with `a[a[:, 1].argsort()]` – Sangjun Lee Feb 21 '23 at 01:42
@SangjunLee re-tried that just now with online python compiler and its true need to use b = ... to change the indexes. So why its that array.argsort() works diffferent than argsort(array) ?? – pippo1980 Feb 21 '23 at 11:10
@SangjunLee they dont they work exactly the same : Returns the indices that would sort an array. thats why b = .... is needed – pippo1980 Feb 21 '23 at 11:50

score 181 · Accepted Answer · edited Jun 16 '20 at 00:22

181

@steve's answer is actually the most elegant way of doing it.

For the "correct" way see the order keyword argument of numpy.ndarray.sort

However, you'll need to view your array as an array with fields (a structured array).

The "correct" way is quite ugly if you didn't initially define your array with fields...

As a quick example, to sort it and return a copy:

In [1]: import numpy as np

In [2]: a = np.array([[1,2,3],[4,5,6],[0,0,1]])

In [3]: np.sort(a.view('i8,i8,i8'), order=['f1'], axis=0).view(np.int)
Out[3]: 
array([[0, 0, 1],
       [1, 2, 3],
       [4, 5, 6]])

To sort it in-place:

In [6]: a.view('i8,i8,i8').sort(order=['f1'], axis=0) #<-- returns None

In [7]: a
Out[7]: 
array([[0, 0, 1],
       [1, 2, 3],
       [4, 5, 6]])

@Steve's really is the most elegant way to do it, as far as I know...

The only advantage to this method is that the "order" argument is a list of the fields to order the search by. For example, you can sort by the second column, then the third column, then the first column by supplying order=['f1','f2','f0'].

edited Jun 16 '20 at 00:22

Trenton McKinney

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answered May 13 '10 at 16:10

Joe Kington

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In my numpy 1.6.1rc1, it raises `ValueError: new type not compatible with array.` – Clippit Oct 05 '11 at 17:40
12

Would it make sense to file a feature request that the "correct" way be made less ugly? – endolith Aug 21 '13 at 03:15
6

What if the values in the array are `float`? Should I change anything? – Marco Mar 23 '14 at 09:23
If you have transposed ``a`` before getting to this point in the code, you will need to take a full copy so as to get the proper meaning from the ``view`` method. [See this quension](http://stackoverflow.com/a/19826315/2399799). – dan-man Jul 14 '14 at 11:18
1

In reply to @Marco's question, you can use ``a.dtype.str`` to get the ``'i8'`` or ``'f8'`` string...which you then need to build into the full comma-delimited string. – dan-man Jul 14 '14 at 11:24
1

And for hybrid type like `a = np.array([['a',1,2,3],['b',4,5,6],['c',0,0,1]])` what approach should I follow? – ePascoal May 09 '15 at 16:50
1

@MrMartin - Based on this and some of your other comments, it sounds like you have data with a common type in each column (e.g. spreadsheet-like data). If so, have a look at `pandas`. It will simplify a lot of the type of operations you're wanting to do. – Joe Kington May 09 '15 at 20:16
14

One major advantage of this method over Steve's is that it allows very large arrays to be sorted in place. For a sufficiently large array, the indices returned by `np.argsort` may themselve take up quite a lot of memory, and on top of that, indexing with an array will also generate a copy of the array that is being sorted. – ali_m Jul 11 '15 at 23:38
@Clippit `new type not compatible with array.` can also be caused by NaN values. Use `np.nan_to_num(a)` to fix. – jaycode Dec 19 '15 at 11:25
@dan-man: Thanks for the link (two years later...). If `array.flags.c_contiguous` is true, is that a guarantee that the view method will work? – Linuxios Nov 29 '16 at 06:36
6

Can someone explain the `'i8,i8,i8'`? This is for each column or each row? What should change if sorting a different dtype? How do I find out how many bits are being used? Thank you – evn Nov 28 '20 at 23:52
This sorts `[[2,4], [1, 3], [2, 2], [1, 1]]` into `[[1,1], [2, 2], [1, 3], [2, 4]]` instead of `[[2,1], [1, 2], [2, 3], [1, 4]]` if sorted based on 2nd col. – gargoylebident Apr 05 '21 at 19:02
Sorry bad example. This sorts `[[2, 3], [1, 2], [2, 1]]` into `[[1, 1], [2, 3], [2, 2]]` instead of `[[2, 3], [1, 1], [2, 2]]` if sorted based on 1st col. – gargoylebident Apr 05 '21 at 19:09
@JoeKingtonwhat does (order=['f1'] stand for ? field 1 ? cant figure out from https://numpy.org/doc/stable/user/basics.rec.html – pippo1980 Jul 07 '21 at 18:13
N.B. don't forget a floating point precision. I got burned by this. My multi-column sort "wasn't working" as expected and I realized it was due to precision errors. – roshambo Oct 23 '22 at 01:54

J.J · Answer 3 · 2017-02-25T22:37:00.443

58

You can sort on multiple columns as per Steve Tjoa's method by using a stable sort like mergesort and sorting the indices from the least significant to the most significant columns:

a = a[a[:,2].argsort()] # First sort doesn't need to be stable.
a = a[a[:,1].argsort(kind='mergesort')]
a = a[a[:,0].argsort(kind='mergesort')]

This sorts by column 0, then 1, then 2.

edited Feb 25 '17 at 22:37

answered Jul 05 '16 at 01:42

J.J

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Why does First Sort not need to be stable? – Little Bobby Tables Oct 26 '16 at 20:59
14

Good question - stable means that when there's a tie you maintain the original order, and the original order of the unsorted file is irrelevant. – J.J Oct 27 '16 at 13:06
This seems like a really super important point. having a list that silently doesn’t sort would be bad. – Clumsy cat May 21 '18 at 09:07

score 25 · Answer 4 · answered Feb 25 '16 at 10:37

In case someone wants to make use of sorting at a critical part of their programs here's a performance comparison for the different proposals:

import numpy as np
table = np.random.rand(5000, 10)

%timeit table.view('f8,f8,f8,f8,f8,f8,f8,f8,f8,f8').sort(order=['f9'], axis=0)
1000 loops, best of 3: 1.88 ms per loop

%timeit table[table[:,9].argsort()]
10000 loops, best of 3: 180 µs per loop

import pandas as pd
df = pd.DataFrame(table)
%timeit df.sort_values(9, ascending=True)
1000 loops, best of 3: 400 µs per loop

So, it looks like indexing with argsort is the quickest method so far...

score 24 · Answer 5 · edited May 26 '17 at 10:00

24

From the NumPy mailing list, here's another solution:

>>> a
array([[1, 2],
       [0, 0],
       [1, 0],
       [0, 2],
       [2, 1],
       [1, 0],
       [1, 0],
       [0, 0],
       [1, 0],
      [2, 2]])
>>> a[np.lexsort(np.fliplr(a).T)]
array([[0, 0],
       [0, 0],
       [0, 2],
       [1, 0],
       [1, 0],
       [1, 0],
       [1, 0],
       [1, 2],
       [2, 1],
       [2, 2]])

edited May 26 '17 at 10:00

Peter Mortensen

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answered Jun 03 '15 at 15:03

fgregg

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The correct generalization is `a[np.lexsort(a.T[cols])]`. where `cols=[1]` in the original question. – Radio Controlled Apr 11 '18 at 13:12

score 24 · Answer 6 · edited Jun 20 '22 at 03:05

24

As the Python documentation wiki suggests:

a = ([[1, 2, 3], [4, 5, 6], [0, 0, 1]]); 
a = sorted(a, key=lambda a_entry: a_entry[1]) 
print a

Output:

[[[0, 0, 1], [1, 2, 3], [4, 5, 6]]]

edited Jun 20 '22 at 03:05

Mateen Ulhaq

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answered Sep 28 '11 at 20:05

user541064

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With this solution, one gets a list instead of a NumPy array, so this might not always be convenient (takes more memory, is probably slower, etc.). – Eric O. Lebigot Sep 28 '11 at 20:13
this "solution" is slower by the most-upvoted answer by a factor of ... well, close to infinity actually – Jivan Jun 18 '20 at 12:03
1

@Jivan Actually, this solution is faster than the most-upvoted answer by a factor of 5 https://imgur.com/a/IbqtPBL – Antony Hatchkins Nov 26 '20 at 16:43
@AntonyHatchkins But this doesn't do the whole job. Produces a list instead of an array. I get similar times as yours (3.02 ms vs 549 μs), but if I finish this by applying `np.array` to the result, it goes up to 4.3 ms. – Kelly Bundy Feb 03 '23 at 03:16

score 8 · Answer 7 · edited May 26 '17 at 10:09

I had a similar problem.

My Problem:

I want to calculate an SVD and need to sort my eigenvalues in descending order. But I want to keep the mapping between eigenvalues and eigenvectors. My eigenvalues were in the first row and the corresponding eigenvector below it in the same column.

So I want to sort a two-dimensional array column-wise by the first row in descending order.

My Solution

a = a[::, a[0,].argsort()[::-1]]

So how does this work?

a[0,] is just the first row I want to sort by.

Now I use argsort to get the order of indices.

I use [::-1] because I need descending order.

Lastly I use a[::, ...] to get a view with the columns in the right order.

score 4 · Answer 8 · edited Jun 27 '20 at 09:14

import numpy as np
a=np.array([[21,20,19,18,17],[16,15,14,13,12],[11,10,9,8,7],[6,5,4,3,2]])
y=np.argsort(a[:,2],kind='mergesort')# a[:,2]=[19,14,9,4]
a=a[y]
print(a)

Desired output is [[6,5,4,3,2],[11,10,9,8,7],[16,15,14,13,12],[21,20,19,18,17]]

note that argsort(numArray) returns the indices of an numArray as it was supposed to be arranged in a sorted manner.

example

x=np.array([8,1,5]) 
z=np.argsort(x) #[1,3,0] are the **indices of the predicted sorted array**
print(x[z]) #boolean indexing which sorts the array on basis of indices saved in z

answer would be [1,5,8]

You sure its not [1,2,0]? – adir abargil Dec 20 '20 at 06:04 — adir abargil, Dec 20 '20 at 06:04

score 3 · Answer 9 · answered Aug 07 '16 at 16:33

A little more complicated lexsort example - descending on the 1st column, secondarily ascending on the 2nd. The tricks with lexsort are that it sorts on rows (hence the .T), and gives priority to the last.

In [120]: b=np.array([[1,2,1],[3,1,2],[1,1,3],[2,3,4],[3,2,5],[2,1,6]])
In [121]: b
Out[121]: 
array([[1, 2, 1],
       [3, 1, 2],
       [1, 1, 3],
       [2, 3, 4],
       [3, 2, 5],
       [2, 1, 6]])
In [122]: b[np.lexsort(([1,-1]*b[:,[1,0]]).T)]
Out[122]: 
array([[3, 1, 2],
       [3, 2, 5],
       [2, 1, 6],
       [2, 3, 4],
       [1, 1, 3],
       [1, 2, 1]])

score 1 · Answer 10 · answered Jan 30 '18 at 19:36

Here is another solution considering all columns (more compact way of J.J's answer);

ar=np.array([[0, 0, 0, 1],
             [1, 0, 1, 0],
             [0, 1, 0, 0],
             [1, 0, 0, 1],
             [0, 0, 1, 0],
             [1, 1, 0, 0]])

Sort with lexsort,

ar[np.lexsort(([ar[:, i] for i in range(ar.shape[1]-1, -1, -1)]))]

Output:

array([[0, 0, 0, 1],
       [0, 0, 1, 0],
       [0, 1, 0, 0],
       [1, 0, 0, 1],
       [1, 0, 1, 0],
       [1, 1, 0, 0]])

score 1 · Answer 11 · answered Mar 04 '22 at 23:09

Pandas Approach Just For Completeness:

a = np.array([[9, 2, 3],
              [4, 5, 6],
              [7, 0, 5]])              
a = pd.DataFrame(a) 

             
a.sort_values(1, ascending=True).to_numpy()
array([[7, 0, 5], # '1' means sort by second column
       [9, 2, 3],
       [4, 5, 6]])

prl900 Did the Benchmark, comparing with the accepted answer:

%timeit pandas_df.sort_values(9, ascending=True)
1000 loops, best of 3: 400 µs per loop

%timeit numpy_table[numpy_table[:,9].argsort()]
10000 loops, best of 3: 180 µs per loop

score 0 · Answer 12 · edited Mar 05 '22 at 08:17

0

Simply using sort, use column number based on which you want to sort.

a = np.array([1,1], [1,-1], [-1,1], [-1,-1]])
print (a)
a = a.tolist() 
a = np.array(sorted(a, key=lambda a_entry: a_entry[0]))
print (a)

edited Mar 05 '22 at 08:17

marc_s

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answered Apr 19 '20 at 17:13

Jerin Antony

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score 0 · Answer 13 · answered Apr 27 '20 at 04:59

It is an old question but if you need to generalize this to a higher than 2 dimension arrays, here is the solution than can be easily generalized:

np.einsum('ij->ij', a[a[:,1].argsort(),:])

This is an overkill for two dimensions and a[a[:,1].argsort()] would be enough per @steve's answer, however that answer cannot be generalized to higher dimensions. You can find an example of 3D array in this question.

Output:

[[7 0 5]
 [9 2 3]
 [4 5 6]]

score 0 · Answer 14 · answered Aug 15 '20 at 08:45

0

#for sorting along column 1

indexofsort=np.argsort(dataset[:,0],axis=-1,kind='stable') 
dataset   = dataset[indexofsort,:]

answered Aug 15 '20 at 08:45

umair ali

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Arkady · Answer 15 · 2021-01-31T14:58:57.603

def sort_np_array(x, column=None, flip=False):
    x = x[np.argsort(x[:, column])]
    if flip:
        x = np.flip(x, axis=0)
    return x

Array in the original question:

a = np.array([[9, 2, 3],
              [4, 5, 6],
              [7, 0, 5]])

The result of the sort_np_array function as expected by the author of the question:

sort_np_array(a, column=1, flip=False)

[2]: array([[7, 0, 5],
            [9, 2, 3],
            [4, 5, 6]])

score 0 · Answer 16 · answered Jun 01 '21 at 12:12

Thanks to this post: https://stackoverflow.com/a/5204280/13890678

I found a more "generic" answer using structured array. I think one advantage of this method is that the code is easier to read.

import numpy as np
a = np.array([[9, 2, 3],
           [4, 5, 6],
           [7, 0, 5]])

struct_a = np.core.records.fromarrays(
    a.transpose(), names="col1, col2, col3", formats="i8, i8, i8"
)
struct_a.sort(order="col2")

print(struct_a)

[(7, 0, 5) (9, 2, 3) (4, 5, 6)]

Sorting arrays in NumPy by column

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