find value of matrix by position

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Tiki Tiki 2018-7-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/411407-find-value-of-matrix-by-position

编辑： Tiki Tiki 2019-5-29

采纳的回答： Jan

在 MATLAB Online 中打开

Hi everyone

Can you help me to optimize code?

this is my code:

 aa =
     1     4     5     7
     9     2     3     8
bb =
     1     3
     2     4

I want to get value of each row aa by position of row bb. Like this

cc = [1 5; 2 8]

Thank

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Answer 1

Jan 2018-7-21

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/411407-find-value-of-matrix-by-position#answer_329730

编辑：Jan 2018-7-21

在 MATLAB Online 中打开

I guess, that the elements of bb are the row indices related to aa, and the column index is the corresponding column of bb:

aa = [1     4     5     7; ...
      9     2     3     8; ...
      13   21    33    44];
bb = [1     3; ... 
      2     4; ...
      2     3];
s = size(bb);
c = aa(sub2ind(size(aa), repmat((1:s(1)).', 1, s(2)), bb))

Or faster with a direct call, but without exhaustive error checking:

sa = size(aa, 1);
sb = size(bb, 1);
index = (1:sb).' + (bb - 1) .* sa;
aa(index)

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Jan 2019-5-28

I've learned it by using sub2ind at first, and then reading the code of sub2ind.m. I've removed all parts for the error checking and simplified the code such that it works for 2 dimensions only. During implementing this as code, I understood the underlying logic: With linear indexing, the formula is almost trivial. :-)

Tiki Tiki 2019-5-29