Sparse instantiation from dense with simultaneous dropzeros

General Usage
6

That’s good, thanks! I’ve taken that and amended it by what fkeep! does
to

function to_sparse(A, f)
    colptr = zeros(Int, size(A, 2) + 1)
    nzval = Float64[]
    rowval = Int[]
    colptr[1] = 1
    cnt = 1
    for i in 1:size(A, 1)
        for j in 1:size(A,2)
            v = A[i, j]
            if f(i,j,v)
                push!(rowval, j)
                push!(nzval, v)
                cnt += 1
            end
        end
        colptr[i+1] = cnt
    end
    return SparseMatrixCSC(size(A, 1), size(A, 2), colptr, rowval, nzval)
end

in case I want to filter by other criteria during instantiation.

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