[SciPy-User] Selecting elements in a numpy array by their indices contained in two arrays

[Alexander Barth]

Dear Eric,
I think you can use numpy.ravel_multi_index for this.
https://docs.scipy.org/doc/numpy/reference/generated/numpy.ravel_multi_index.html

Here is an example:

In [10]: A = np.random.randn(4,4)

In [11]: A
Out[11]:
array([[ 1.9158152 , -0.69065018,  0.71567087, -1.31872821],
       [ 1.00126163,  0.34461176,  0.30298258, -1.19507848],
       [-0.14355935,  0.18827163,  0.78383867, -1.01362843],
       [ 0.33303015, -1.49289243, -1.02713378,  0.24379566]])

In [12]: i = np.array([2,2,1,1])

In [13]: j = np.array([3,3,3,3])

In [16]: ij = np.ravel_multi_index((i,j),A.shape)

In [17]: ij
Out[17]: array([11, 11,  7,  7])

In [21]: A.flatten()[ij]
Out[21]: array([-1.01362843, -1.01362843, -1.19507848, -1.19507848])

In [23]: [A[i[l],j[l]] for l in range(len(i))]
Out[23]:
[-1.0136284267170597,
 -1.0136284267170597,
 -1.195078482610719,
 -1.195078482610719]

I hope this helps,
Cheers,
Alex