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matlaboctavevectorizationsparse-matrixmultiplication

Take the product of all nonzero elements in each column of a sparse matrix

Is there a well-vectorized way to take the product of all the nonzero elements in each column of a sparse matrix in octave (or matlab) (returning a row-vector of products)?

Solution

  • I'd combine find with accumarray:

    %# create a random sparse array
s = sprand(4,4,0.6);

%# find the nonzero values
[rowIdx,colIdx,values] = find(s);

%# calculate product
product = accumarray(colIdx,values,[],@prod)

    Some alternatives (that might be less efficient; you may want to profile them)

    %# simply set the zero-elements to 1, then apply prod
%# may lead to memory issues
s(s==0) = 1;
product = prod(s,1);

    .

    %# do "manual" accumarray
[rowIdx,colIdx,values] = find(s);

product = zeros(1,size(s,2));
uCols = unique(colIdx);

for col = uCols(:)'
    product(col) = prod(values(colIdx==col));
end