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

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

[Bill Tubbs]

I found a similar question here and the answer was the one I provided above using sub2ind.

[Stephen23]

SUB2IND is the best way.

[Bill Tubbs]

Thank you.

Answer 1

[Scott MacKenzie]

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

[Bill Tubbs]

Thanks but I want to index arbitrary points programmatically. I.e. using a variable list of (row, column) tuples.

[Scott MacKenzie]

It's a bit unclear what you might mean by index arbitrary points programmatically. If you know A is a 4x6 matrix, then here's an indexed arbitrary point from A which is generated programmatically:

arbitraryRow = randi(4,1);
aribtraryCol = randi(6,1);
arbitraryPoint = A(arbitraryRow, arbitraryCol);

If you want points -- plural -- then put this in loop and add the points to a vector, or something like that. Good luck.

[Bill Tubbs]

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.