find value of matrix by position
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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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Accepted Answer
Jan
on 21 Jul 2018
Edited: Jan
on 21 Jul 2018
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)
5 Comments
Jan
on 28 May 2019
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. :-)
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