read multiple points from matrix

I want to read multiple points out of a matrix. Therefore I have the matrix A, and my indices X and Y for the coordinates. I don't need any other values, only those.

A = rand(50,50);
X = [1 2 5];
Y = [1 2 5];
P = A(X,Y)

the code reads all the values, so I only need the diagonal of the matrix. I could use diag but that takes a lot of time, and I need to be time-efficient. Is there a possible way to do that different, to be faster?

Answer 1

[Star Strider]

Use the sub2ind function to create linear indices from the subscripts —

A = rand(50,50);
X = [1 2 5];                    % Assuming These Are The Row Indices
Y = [1 2 5];                    % Assuming These Are The Column Indices
I = sub2ind(size(A), X, Y)

I = 1×3

1 52 205

P = A(I)

P = 1×3

0.8620 0.3396 0.2798

Check = [A(X(1),Y(1)),  A(X(2),Y(2)), A(X(3),Y(3))]

Check = 1×3

0.8620 0.3396 0.2798

If you want a different result, please describe it in some detail.

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