Logical indexing with matrix issue

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Senaasa
Senaasa 2014-8-27
评论： Senaasa 2014-8-27
I ran into this issue writing a program using logical indexing. Suppose you have
A=ones(32,10);
M=zeros(32,10);
N=ones(1,10);
for i=1:32;
M(i,:)=N;
end
M=logical(M);
C=A(M);
Why is C a 320x1? Shouldn't it be equal to A since the logical is all 1's? Thanks, Charles

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Yu Jiang
Yu Jiang 2014-8-27
编辑：Yu Jiang 2014-8-27
This is the intended behavior of Logical indexing. For example,
A = [1 2; 3 4];
Then, A(logical([1,1;1,1])) will return A(:), which is equal to
[1;
3;
2;
4].
And A(logical([1,0;0,1])) will return
[1;
4]
You can find more examples via the following link
http://www.mathworks.com/help/matlab/math/matrix-indexing.html#bq7egb6-1
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Yu Jiang
Yu Jiang 2014-8-27
Senaasa
You are right. It seems for row vectors, the results are kept as rows. However, if A is a matrix (with more than one row), the result will always be a column vector.
Senaasa
Senaasa 2014-8-27
Thanks for your quick replies. I guess I've only ever used 1 row when using logicals, so I never noticed this issue.
Thanks for your help

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