From index list to logical index vector

Hello, If I have a specific list of indexes, I can acces a subset of the element of a vector:

N=10;
a = rand(2*N,1);
period = 7;
list = 1:period:N;
a(list)

In my code I want that subset to be addressed with logical indexing (for various reasons). How do I convert that list to logical indexing?

The only solution I can think of requires building the list of all the indexes:

all_idx = (1:length(a));
logical_idx = ismember(all_idx, list);
a(logical_idx)

Is there shorter more readable way of doing this? Please do not answer:

a(ismember(1:length(a),list)) % ;-)

Thanks

Answer 1

[Haris K.]

I changed the variable names for brevity.

function lgc = unfind(idx, N)
    %Go from indicies into logical (for vectors only)
    lgc = false(N,1);
    lgc(idx) = true;
end

And you can check it:

X = [10 20 15 16 19];
N = length(X);
idx = find(X>=19);
lgc = unfind(idx, N);
X(lgc)
X(idx)

The two last rows are equivalent.

Answer 2

[Star Strider]

See if this does what you want:

N=10;
a = rand(2*N,1);
period = 7;
list = 1:period:N;
logical_idx(list) = logical(1)
a(logical_idx)

logical_idx =

       1     0     0     0     0     0     0     1

ans =

        0.52399
        0.16543

[Massimiliano Salsi]

Thanks, that made me think. You can also write true instead of logical(1), and false instead of logical(0). Nevertheless for my code I also rely on the fact that the logical_idx vector has the same size of a. This is important because at some point I need to count the zeros. But I could count the ones and subtract them from length(a). Thanks again for your answer.

[Star Strider]

My pleasure.

It’s also an efficient, one-line solution.

Using true is the same as using logical(1).

It would be easier to use sum or nnz to count the 1 values, then subtract that from length(a) to get the number of zeros.

[Rich Gerbino]

Thanks, exactly what I was looking for. If you want to preserve the original vector length (in my case I also want to use ~logical_idx), you can modify the above code slightly with:

logical_idx = false(2*N,1);
logical_idx(list) = logical(1);