read multiple points from matrix

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F Schmid 2021-5-31

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https://ww2.mathworks.cn/matlabcentral/answers/844300-read-multiple-points-from-matrix

回答： Star Strider 2021-5-31

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?

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Answer 1

Star Strider 2021-5-31

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https://ww2.mathworks.cn/matlabcentral/answers/844300-read-multiple-points-from-matrix#answer_713445

在 MATLAB Online 中打开

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