How to return values from one array by comparing values with another array in Python?

Question

I have two arrays A and B as shown below:

A = [[ 1  2  3  4]
     [ 5  6  7  8]
     [ 9 10 11 12]
     [13 14 15 16]]

B = [3, 11]

Now I need the values in the 4th column of A for rows having values in B and 3rd column of A matching. For this particular case the output array C should be:

C = [4, 12]

I tried this, using the following code, but it returns an empty array:

import numpy as np
A = np.array([[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13, 14, 15, 16]])
B = np.array([3, 11])
C = A[np.where(B[:] == A[:][2]), 3]
print(C)

Ashwini Chaudhary · Accepted Answer · 2015-04-03 19:29:21Z

1

You can use numpy.in1d to check which of the items from B match with items in A's third column.

>>> np.in1d(A[:, 2], B)
array([ True, False,  True, False], dtype=bool)

Now using this boolean array simply index A's fourth column:

>>> A[:,3][np.in1d(A[:, 2], B)]
array([ 4, 12])

answered Apr 3, 2015 at 19:29

Ashwini Chaudhary

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TigerhawkT3 · Accepted Answer · 2015-04-03 19:29:31Z

1

Here's a pure Python implementation, if that's okay:

>>> A = [[ 1,  2,  3,  4], [ 5,  6,  7,  8], [ 9, 10, 11, 12], [13, 14, 15, 16]]
>>> B = [3,11]
>>> C = [l[3] for l in A if l[2] in B]
>>> C
[4, 12]

answered Apr 3, 2015 at 19:29

TigerhawkT3

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