MATLAB: Is this sparse multiplication failing

large matricessparse

Hi,
Why does Matlab run out of memory on the multiplication of these two sparse matrices?
The first one is of size 17249876309 x 29 but only has 7 non-zeros elements that are all on different rows and on different (albeit adjacent) columns.
The second one is of size 29 x 82 and has 432 non-zeros elements.
I can certainly exploit the nature of the sparsity in these two matrices, do the multiplication and reconstruct the big matrix.
Why does Matlab fail at doing this?
Thanks

Best Answer

You could use the relevant indices to perform the multiplication:
b = sparse(17249876309, 29);
b(430: 490) = 1;
c = sparse(ones(29, 82));
[row, col] = find(b);
d = sparse(size(b, 1), size(c, 2));
d(row, 1:end) = b(row, :)*c; 
spy(d)
h=gcf; 
set(h.Children,'Ylim',[400 500]);
However it's probably not even necessary to construct the whole matrix again. Note that you have to use 1:end instead of : in the assignment, otherwise you get a memory error.
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