Get a list of values from 3D array using 2D index array in Matlab

Question

I have a 3D array that looks like this:

edges = round(rand(20,20,20)));

I then create a random list of indices as follows:

indices = floor(rand(10000,3)*(19))+1;

So if I try to use the first row of the indices 2D array to access an element in the edges array, I do:

>>> edges(indices(1,1),indices(1,2),indices(1,3))
>>> ans = 1

I figured, if I wanted to get the value of edges at all of the index positions, I could do

>>> edges(indices)

but that returns an 10000 by 3 matrix. I would expect a 10000 by 1 matrix with values of edges at positions specified by each row of indices. What is going on here, and is there a way to get the values I want without using any for loops?

read about sub2ind and ind2sub

bla
– bla

2020-05-05 00:27:20 +00:00
Commented May 5, 2020 at 0:27 — bla
– bla, Commented May 5, 2020 at 0:27

David · Accepted Answer · 2020-05-05 00:28:29Z

1

Yes. Use a single index into edges instead of 3:

edges = round(rand(20,20,20));
indices = floor(rand(10000,1)*(20^3-1))+1;
edges(indices)

You should use randi() instead of round(rand()) as well, because round(5*rand())+1 will give you fewer 1s and 6s than 2s, 3s, 4s, 5s.

edges = randi(2,20,20,20)-1;
indices = randi(20^3,10000,1);
edges(indices)

answered May 5, 2020 at 0:28

David

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