Subsetting a 2D numpy array

Question

I have looked into documentations and also other questions here, but it seems I have not got the hang of subsetting in numpy arrays yet.

I have a numpy array, and

Answer 1

You could use np.meshgrid to give the n1, n2 arrays the proper shape to perform the desired indexing:

In [104]: a[np.meshgrid(n1,n2, sparse=True, indexing='ij')]
Out[104]: 
array([[ 0,  1,  2,  3,  4],
       [10, 11, 12, 13, 14],
       [20, 21, 22, 23, 24],
       [30, 31, 32, 33, 34],
       [40, 41, 42, 43, 44]])

---

Or, without meshgrid:

In [117]: a[np.array(n1)[:,np.newaxis], np.array(n2)[np.newaxis,:]]
Out[117]: 
array([[ 0,  1,  2,  3,  4],
       [10, 11, 12, 13, 14],
       [20, 21, 22, 23, 24],
       [30, 31, 32, 33, 34],
       [40, 41, 42, 43, 44]])

There is a similar example with an explanation of how this integer array indexing works in the docs.

See also the Cookbook recipe Picking out rows and columns.