How to avoid linear indexing in operations involving matrices of different sizes

Question

deathtime 2023-5-5

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https://ww2.mathworks.cn/matlabcentral/answers/1958584-how-to-avoid-linear-indexing-in-operations-involving-matrices-of-different-sizes

编辑： Matt J 2023-5-5

If I carry out an operation of matrices with different sizes using indexing, the end result tends to be a column matrix with linear indexing. For example:

A = rand(3,3);
B = rand(3,2);
idx = logical([0 1 1; 0 1 1; 0 1 1]);

If I want to add A with idx indices to B, the only way I can seem to make this work is if I do:

C = A(idx) + B(:)
C = 6×1
5283
5581
0464
4741
5704
2374

Is there any way to carry out the above operation and end up with a matrix the same shape as B? My initial attempt was to simply do C = A(idx) + B.

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Answer 1

Stephen23 2023-5-5

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https://ww2.mathworks.cn/matlabcentral/answers/1958584-how-to-avoid-linear-indexing-in-operations-involving-matrices-of-different-sizes#answer_1229849

在 MATLAB Online 中打开

RESHAPE is very efficient, because no data gets moved in memory:

A = rand(3,3);
B = rand(3,2);
idx = logical([0,1,1;0,1,1;0,1,1]);
C = reshape(A(idx),size(B)) + B
C = 3×2
    1.6889    0.4419
    0.7161    0.1917
    1.4049    0.7527

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Answer 2

Matt J 2023-5-5

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https://ww2.mathworks.cn/matlabcentral/answers/1958584-how-to-avoid-linear-indexing-in-operations-involving-matrices-of-different-sizes#answer_1229899

编辑：Matt J 2023-5-5

在 MATLAB Online 中打开

A = rand(3,3);
B = rand(3,2);
idx = logical([0 1 1; 0 1 1; 0 1 1]);
C=B;
C(:)=A(idx)+B(:)
C = 3×2
    1.1539    0.5087
    1.1585    1.0807
    1.4266    0.7590