Numpy insert matrix values with matrix index

Question

I have the following code which creates a 4D grid matrix and I am looking to insert the rolled 2D vals matrix into this grid.

import numpy as np

k = 100
x = 20
y = 10
z = 3

grid = np.zeros((y, k, x, z))
insert_map = np.random.randint(low=0, high=y, size=(5, k, x))
vals = np.random.random((5, k))

for i in range(x):
    grid[insert_map[:, :, i], i, 0] = np.roll(vals, i)

If vals would be a 1D array and I would use a 1D insert_map array as a reference it would work, however using it in multiple dimensions seems to be an issue and it raises error:

ValueError: shape mismatch: value array of shape (5,100)  could not be broadcast to indexing result of shape (5,100,3)

I'm confused as to why it's saying that error as grid[insert_map[:, :, i], i, 0] should in my mind give a (5, 100) insert location for the y and k portion of the grid array and then fixes the x and z portion with i and 0?

Is there any way to insert the 2D (5, 100) rolled vals matrix into the 4D (10, 100, 20, 3) grid matrix by 2D indexing?

hpaulj · Accepted Answer · 2021-02-05 17:41:26Z

1

grid is (y, k, x, z)

insert_map is (5, k, x). insert_map[:, :, i] is then (5,k).

grid[insert_map[:, :, i], i, 0] will then be (5,k,z). insert_map[] only indexes the first y dimension.

vals is (5,k); roll doesn't change that.

np.roll(vals, i)[...,None] can broadcast to fill the z dimension, if that's what you want.

Your insert_map can't select values along the k dimension. It's values, created by the randint are valid the y dimension.

If the i and 0 are supposed to apply to the last two dimensions, you still need an index for the k dimension. Possibilities are:

grid[insert_map[:, j, i], j, i, 0]
grid[insert_map[:, :, i], 0, i, 0]
grid[insert_map[:, :, i], :, i, 0]
grid[insert_map[:, :, i], np.arange(k), i, 0]

edited Feb 5, 2021 at 17:41

answered Feb 5, 2021 at 17:32

hpaulj

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