用元组进行花式索引

Question

In the numpy tutorial from Scipy , while teaching fancy indexing of numpy arrays, I got the following explanation diagram.

As no other explanation about this particular array is given there, I have created the array using

a = np.array([[j+i for i in range(0,6)] for j in range(0, 60, 10)])

If I run a[(0,1,2,3,4),(1,2,3,4,5)] I am getting array([ 1, 12, 23, 34, 45]) , consistent with the picture. But I can not understand how the tuples are getting unpacked to a[0,1] and so on.

I am trying to understand the mechanism of this. An in-depth answer will be much appreciated.

Answer 1

All fancy indexing does is essentially give you a list of co-ordinates in the larger array. So think of that picture as a big grid and your two tuples as x-coordinates and y-coordinates (it generalizes to higher dimensions too). So if you zip them together you get:

(x=0, y=1), (x=1, y=2), (x=2, y=3), (x=3, y=4), (x=4, y=5)

which if you read off the image you will see gives you:

(1, 12, 23, 34, 45)

as expected