What determines the shape of a logically indexed array?

Matlab help explains: " Logical Indexing : ... The output is always in the form of a column vector."

In practice, typing

a = ones(3,3);
b = a([true, true]);

gives a row vector b. Can someone tell me why does it behave this way? And is there any other instance (apart from indexing by one row logical vector) that gives a non-column result? Thanks, O.

Answer 1

[Ondrej Budac]

Just to complete the story, the answer seems to be:

If B is a logical array and A is any array, then A(B) is equivalent (but faster) way to produce the same result as A(find(B)), including the shape of the output.

Answer 2

[Azzi Abdelmalek]

Maybe you need

a = ones(3,3);
b = a([true, true],:)

[Ondrej Budac]

I know of several ways how to "enforce" the result to be a column vector. The question is: why the vector in my example isn't?

Since indexing an array is at the very core of every m-file I write, I'd like to understand it.

[Azzi Abdelmalek]

a = ones(3,3);

a is a 3x3 array

your index is idx=[true, true] is a 1x2 array, the result a(idx) is obviously a 1x2 array.

[Ondrej Budac]

If that's so obvious, why taking idx = true(2,2) gives a(idx) of size 4x1 and not 2x2?

[Azzi Abdelmalek]

Ok,

a=[1 2 3 ;4 5 6;7 8 9]
a([4 5;3 1]) % is a 2x2 array
% but if you use logical indexing
a(logical([1 0;1 1]))
% this is equivalent to a([1 2 4])

[Ondrej Budac]

Well, that's not entirely true. In fact

a(logical([1 0;1 1]))

is equivalent to

a([1; 2; 4])

which brought me to a possible answer of my original question:

The expression A(B) with a matrix A and a logical array B computes (in an faster way) the exact same result as A(find(B)) would give.

Can anyone confirm this? If yes, it would be nice to have it in the help files, since the command find and linear indexing of an array are well-explained.

[Azzi Abdelmalek]

Just one question, what are you expecting with a(logical([1 0;1 1])) ? The result contains 3 elements

[Ondrej Budac]

If I now understand logical indexing correctly, I expect a(logical([1 0;1 1])) to give a column vector with values 1, 4, 2, assuming your definition of a.