Logical indexing with matrix issue

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

Answer 1

[Yu Jiang]

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

[Senaasa]

Why does

A=[1 2 3 4 5]
B=A>2
C=A(B)

return C=[3 4 5] and not C=[3;4;5]?

[Yu Jiang]

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]

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