Indexing one array by another in numpy

Question

Suppose I have a matrix A with some arbitrary values:

array([[ 2, 4, 5, 3],
       [ 1, 6, 8, 9],
       [ 8, 7, 0, 2]])

A

Answer 1

You can use NumPy's advanced indexing -

A[np.arange(A.shape[0])[:,None],B]

One can also use linear indexing -

m,n = A.shape
out = np.take(A,B + n*np.arange(m)[:,None])

Sample run -

In [40]: A
Out[40]: 
array([[2, 4, 5, 3],
       [1, 6, 8, 9],
       [8, 7, 0, 2]])

In [41]: B
Out[41]: 
array([[0, 0, 1, 2],
       [0, 3, 2, 1],
       [3, 2, 1, 0]])

In [42]: A[np.arange(A.shape[0])[:,None],B]
Out[42]: 
array([[2, 2, 4, 5],
       [1, 9, 8, 6],
       [2, 0, 7, 8]])

In [43]: m,n = A.shape

In [44]: np.take(A,B + n*np.arange(m)[:,None])
Out[44]: 
array([[2, 2, 4, 5],
       [1, 9, 8, 6],
       [2, 0, 7, 8]])