Passing values to a sparse matrix in MATLAB

Question

Might sound too simple to you but I need some help in regrad to do all folowings in one shot instead of defining redundant variables ie tmp_x , tmp_y :

X= sparse(numel(find(G==0)),2);
[tmp_x, temp_y] = ind2sub(size(G), find(G == 0));
X(:)=[tmp_x, tmp_y];

(More info: G is a sparse matrix)

I tried:

X(:)=ind2sub(size(G), find(G == 0));

but that threw an error. How can I achieve this without defining tmp_x , tmp_y ?

Answer 1

A couple of comments with your code:

1. numel(find(G == 0)) is probably one of the worst ways to determine how many entries that are zero in your matrix. I would personally do numel(G) - nnz(G) . numel(G) determines how many elements are in G and nnz(G) determines how many non-zero values are in G . Subtracting these both would give you the total number of elements that are zero.
2. What you are doing is first declaring X to be sparse... then when you're doing the final assignment in the last line to X , it reconverts the matrix to double . As such, the first statement is totally redundant.

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If I understand what you are doing, you want to find the row and column locations of what is zero in G and place these into a N x 2 matrix. Currently with what MATLAB has available, this cannot be done without intermediate variables. The functions that you'd typically use ( find , ind2sub , etc.) require intermediate variables if you want to capture the row and column locations. Using one output variable will give you the column locations only.

You don't have a choice but to use intermediate variables. However, if you want to make this more efficient, you don't even need to use ind2sub . Just use find directly:

[I,J] = find(~G);
X = [I,J];