mapping between arrays in numpy

Question

I have some data in one array that I want to map into another array, given an array of correspondences:

• originaldata is a numpy 2D array,
• targetdata is another numpy 2D array,
• mapping is an array that maps between positions, so mapping[x,y] gives me a pair of coordinates of where the data of targetdata[x,y] comes from in originaldata.

So far I do something like this:

for (x,y) in ALLTHEPOINTS:
    targetdata[x,y]=originaldata[mapping[x,y][0],mapping[x,y][1]]

...which I suspect is very inefficient.

Is there any way to vectorize this? Or is there any numpy function that addresses this type of operation?

Answer 1

This is what fancy indexing is there for:

targetdata = originaldata[mapping[..., 0], mapping[..., 1]]

As a simple example:

>>> original_data = np.arange(6).reshape(2, 3)
>>> original_data
array([[0, 1, 2],
       [3, 4, 5]])
>>> mapping = np.array([[[1,0], [1, 1], [1, 2]],   # swap rows and reverse
...                     [[0, 2], [0, 1], [0, 0]]]) # the second one
>>> original_data[mapping[..., 0], mapping[..., 1]]
array([[3, 4, 5],
       [2, 1, 0]])