How to delete columns in numpy.array


Question

I would like to delete selected columns in a numpy.array . This is what I do:

n [397]: a = array([[ NaN,   2.,   3., NaN],
   .....:        [  1.,   2.,   3., 9]])

In [398]: print a
[[ NaN   2.   3.  NaN]
 [  1.   2.   3.   9.]]

In [399]: z = any(isnan(a), axis=0)

In [400]: print z
[ True False False  True]

In [401]: delete(a, z, axis = 1)
Out[401]:
 array([[  3.,  NaN],
       [  3.,   9.]])

In this example my goal is to delete all the columns that contain NaN's. I expect the last command to result in:

array([[2., 3.],
       [2., 3.]])

How can I do that?

1
57
2/17/2011 8:57:57 PM

Accepted Answer

Given its name, I think the standard way should be delete:

import numpy as np

A = np.delete(A, 1, 0)  # delete second row of A
B = np.delete(B, 2, 0)  # delete third row of B
C = np.delete(C, 1, 1)  # delete second column of C

According to numpy's documentation page, the parameters for numpy.delete are as follow:

numpy.delete(arr, obj, axis=None)

  • arr refers to the input array,
  • obj refers to which sub-arrays (e.g. column/row no. or slice of the array) and
  • axis refers to either column wise (axis = 1) or row-wise (axis = 0) delete operation.
75
4/10/2019 7:30:55 AM

Example from the numpy documentation:

>>> a = numpy.array([[ 0,  1,  2,  3],
               [ 4,  5,  6,  7],
               [ 8,  9, 10, 11],
               [12, 13, 14, 15]])

>>> numpy.delete(a, numpy.s_[1:3], axis=0)                       # remove rows 1 and 2

array([[ 0,  1,  2,  3],
       [12, 13, 14, 15]])

>>> numpy.delete(a, numpy.s_[1:3], axis=1)                       # remove columns 1 and 2

array([[ 0,  3],
       [ 4,  7],
       [ 8, 11],
       [12, 15]])

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