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How to index the values of an array using a series of rows, column indices?

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Bill Tubbs
Bill Tubbs on 9 Jul 2021
Commented: Bill Tubbs on 10 Jul 2021
Couldn't find an answer about this and it is not mentioned in the Matlab documentation page on array indexing.
I want to know how to read (or assign to) a set of values in an array using a series of positions (row, col).
For example, suppose my array is
rng(0); A = randi(9, 4, 6)
A =
8 6 9 9 4 6
9 1 9 5 9 1
2 3 2 8 8 8
9 5 9 2 9 9
and I want to retrieve three values from some arbitrary positions such as (3, 2), (3, 3), and (4, 3).
I figured out I can do it this way, but is this the correct/best way?
my_pts = [3 2; 3 3; 4 3];
A(sub2ind(size(A),my_pts(:,1),my_pts(:,2)))
ans =
3
2
9
or
A(sub2ind(size(A),my_pts(:,1),my_pts(:,2))) = [0 0 0]
A =
8 6 9 9 4 6
9 1 9 5 9 1
2 0 0 8 8 8
9 5 0 2 9 9
  3 Comments

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Answers (1)

Scott MacKenzie
Scott MacKenzie on 9 Jul 2021
If you want to pull those three values from A and concatenate them, as in your output, then just retrieve the three elements using row-column indexing in A and concatenate the values using brackets:
A = randi(9, 4, 6)
A = 4×6
2 4 3 3 1 1 4 1 7 2 8 4 1 4 6 3 8 4 1 8 2 3 6 4
[A(3,2) A(3,3) A(4,3)]
ans = 1×3
4 6 2
  3 Comments
Bill Tubbs
Bill Tubbs on 10 Jul 2021
By arbitrary points I meant variable. I can see how it could be done with a for loop but I was looking for a MATLAB equivalent of this Python (Numpy) code where my_pts is a variable which could have any values:
In [1]: my_pts = ([2, 2, 3], [1, 2, 2])
In [2]: A[my_pts]
Out[2]: array([4, 6, 2])
I modified the question to make this more clear.

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