indexing sparse gpuArrays?

I have a sparse gpuArray (adj) and gpuArray A. Trying to execute

for i =1:length(A)
  [~,c] = find(adj(A(i,1),:));
  .
  .
  .
end

i get an error about matlab not allowing this way of indexing in sparse gpuArrays. After doing some search i understand that operator : inside adj(A(i,1),:) is causing the problem. Any suggestions of a solution or at least a way around this?

[Matt J]

Hmmm...I'd like to see what the rest of the loop is doing.

[Charis L]

the loop is supposed to find every triangle the edges stored in A, participate in, and adj is the adjacency matrix, but it never moves beyond the fist line.

Answer 1

[Joss Knight]

Generally if you are indexing scalar values in a sparse gpuArray then you're trying to do something very inefficient. Still, if you want the values in the j'th column of a sparse matrix you could go:

   [I,J,V] = find(A);
   j = 1;
   ind = find(J==j);
   Ij = I(ind);
   Vj = V(ind);
   Aj = zeros(size(A,1),1);
   Aj(Ij) = Vj;

However, I get the impression that your A matrix is a column vector, in which case storing it as a sparse array is completely pointless since it saves you no space whatsoever. You may as well convert it to full.