access the elements in a matrix given the column indexes

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zhang
zhang 2013-8-15
For example x = [1 2 3 4; 5 6 7 8; 9 10 11 12]
Given the row indexes vector y = [1 3 4]; I'd like to get the associated elements : x(1, y(1)) = 1, x(2, y(2)) = 7, x(3, y(3)) = 12. [1 7 12]
Don't use loop.
THanks

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回答（2 个）

dpb
dpb 2013-8-15
编辑：dpb 2013-8-15
iy=[1 3 4];
ix=[1:length(iy)];
y=x(sub2ind(size(x),ix',iy');
Jan
Jan 2013-8-15
how about
diag( x(:, y) )
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zhang
zhang 2013-8-15
Yes, you're right. I'm just curious why does this work? can you give a little bit more explanation?
Jan
Jan 2013-8-15
in x(:, y) the colon operator returns all rows of the matrix x. the second argument 'y' does 'column selection', i.e. it picks only the interesting columns from x:
x(:, [1 2 4])
ans =
1 2 4
5 6 8
9 10 12
the values you are looking for obviously appear as diagonal elements in the result and can be extracted with the diag() function.
the reason why the intersting values are on the diagonal is pretty simple, allthough I admit, that one might must give it a second thought :)
I hope that helped!

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