Selecting elements of logically pruned vector (concatenating indices)

Hi

Given a vector b and a logical vector v_log, is there a straightforward way to extract particular indices of the partial vector b(v_log) without saving it?

In code:

b = randn(20,1);         % a vector
v_log = (randn(20,1)>0); % logical vector to select parts of b
ind = [ 2 ; 3 ];         % the elements I'm interested in

c = b(v_log)(ind);   % This does not work, but is there a way to do this?

Thanks!

Answer 1

[Kelly Kearney]

I think you'd need to save the indices.

idx = find(v_log);
c = b(idx(ind));

Depending on the size/sparsity of v_log, you may be able to save some time and storage space by only saving the necessary number of indices:

idx = find(vlog, max(ind), 'first');
c = b(idx(ind));

Answer 2

[Ced]

@Star Strider: Sure thing, thanks. Just for clarity: b is an arbitrary vector. v_log is a logical vector, hence b(v_log) returns a part of b. My question was how to extract certain elements with indices ind of that partial vector b(v_log) without having to save them.

@Kelly Kearney: Thanks, I'll accept that answer and give that a try. The size of v_log in my case is pretty (very) large and is computed in an iterative loop, which is why I was hoping to do this without saving.