How to extract all columns but one from an array (or matrix) in python?

Question

Given a numpy 2d array (or a matrix), I would like to extract all the columns but the i-th.

E. g. from

1 2 3 4
2 4 6 8
3 6 9 12

I would like to have, e.g.

1 2 3
2 4 6
3 6 9

or

1 2 4
2 4 8
3 6 12

I cannot find a pythonic way to do this. I now that you can extract given columns by simply

a[:,n]

or

a[:,[n,n+1,n+5]]

But what about extracting all of them but one?

Asked By: Ferdinando Randisi

||

Source

Answer 1

Take a look at numpy’s advanced slicing

>>> import numpy as np
>>> a = np.array([[1,2,3,4], [2,4,6,8], [3,6,9,12]])
>>> a[:,np.array([True, True, False, True])]
array([[ 1,  2,  4],
       [ 2,  4,  8],
       [ 3,  6, 12]])

Answered By: Peter Gibson

Answer 2

Use a slice that excludes the last element.

In [19]: a[:,:-1]
Out[19]: 
array([[1, 2, 3],
       [2, 4, 6],
       [3, 6, 9]])

If you want something other than the last element I’d just build a list to select with.

In [20]: selector = [x for x in range(a.shape[1]) if x != 2]
In [21]: a[:, selector]
Out[21]: 
array([[ 1,  2,  4],
       [ 2,  4,  8],
       [ 3,  6, 12]])

http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html

Answered By: chrisb

Answer 3

Since for the general case you are going to be returning a copy anyway, you may find yourself producing more readable code by using np.delete:

>>> a = np.arange(12).reshape(3, 4)
>>> np.delete(a, 2, axis=1)
array([[ 0,  1,  3],
       [ 4,  5,  7],
       [ 8,  9, 11]])

Answered By: Jaime

Answer 4

The answers given already can easily be adapted to selecting all but a list of columns, but here are a couple of explicit examples:

In [1]: import numpy as np
In [2]: a = np.arange(12).reshape(3, 4)
In [3]: a
Out[3]:
array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [ 8,  9, 10, 11]])
In [4]: drop_cols = [0, 3]

# option 1: delete the columns you don't want (like @Jaime)
# (this is really the most straightforward)

In [5]: np.delete(a, drop_cols, axis=1)
Out[5]:
array([[ 1,  2],
       [ 5,  6],
       [ 9, 10]])

# option 2: pass the indices of columns to keep (like @chrisb)

In [6]: a[:, [i for i in range(a.shape[1]) if i not in drop_cols]]
Out[6]:
array([[ 1,  2],
       [ 5,  6],
       [ 9, 10]])

# option 3: use an array of T/F for each col (like @Peter Gibson)

In [7]: a[:, [i not in drop_cols for i in range(a.shape[1])]]
Out[7]:
array([[ 1,  2],
       [ 5,  6],
       [ 9, 10]])

Answered By: Nathan

How to extract all columns but one from an array (or matrix) in python?

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