Python - construct a matrix by matrix of index

Question

Suppose I have an array, I want to have a matrix from that array by a matrix of index.

import numpy as np
arr = np.array([1,5])
mtxidx = np.array([[0,1,0],[0,1,1],[0,0,0]])

How can I get a matrix [[1,5,1],[1,5,5],[1,1,1]] ?

An initial thought is simply say

arr(mtxidx)

however it doesn't work

Is there any function/method that do this elegantly?

Answer 1

"Fancy" indexing works for me (NB in your question you are trying to call the array object (round brackets) but NumPy "ndarray" objects are not callable):

In [61]: arr[mtxidx]
Out[61]: 
array([[1, 5, 1],
       [1, 5, 5],
       [1, 1, 1]])

Answer 2

Your initial thought was pretty close, simply replacing the parenthesis with [] would make it work. arr[mtxidx]

A list comprehension would work as well.

>>> np.array([arr[row] for row in mtxidx])
array([[1, 5, 1],
       [1, 5, 5],
       [1, 1, 1]])

Answer 3

I upvote the fancy indexing proposed by @xnx but if you would have done something in same range but involving an operation (or ..anything else) you can also try this :

arr = np.array([1,5])
mtxidx = np.array([[0,1,0],[0,1,1],[0,0,0]])

def func(v):
    return arr[v]

vfunc = np.vectorize(func)
vfunc(mtxidx)
# array([[1, 5, 1],
#        [1, 5, 5],
#        [1, 1, 1]])