Select Armadillo Sub-matrix With Non Contiguous Indices

I am passing a python code to C++ where I find python expressions like this: J11 = dS_dVa[array([pvpq]).T, pvpq].real Here, J11 and dS_dVa are Sparse matrices, and pvpq is an arra

Solution 1:

Armadillo only support submatrix views for contiguous forms. See Caveats section in sp_mat doc.

Solution 2:

After a while, I made my own function. It uses the inner CSC structure.

/**
    * @brief sp_submatrix Function to extract columns and rows from a sparse matrix
    * @param A Sparse matrix pointer
    * @param rows vector of the rown indices to keep (must be sorted)
    * @param cols vector of the clumn indices to keep (must be sorted)
    * @return Sparse matrix of the indicated indices
    */
    arma::sp_mat sp_submatrix(arma::sp_mat *A, std::vector<std::size_t> *rows, std::vector<std::size_t> *cols) {

        std::size_t n_rows = rows->size();
        std::size_t n_cols = cols->size();

        bool found = false;
        std::size_t n = 0;
        std::size_t p = 0;
        std::size_t found_idx = 0;

        arma::vec new_val(A->n_nonzero);
        arma::uvec new_row_ind(A->n_nonzero);
        arma::uvec new_col_ptr(n_cols + 1);

        new_col_ptr(p) = 0;

        for (auto const& j: *cols) { // for every column in the cols vectorfor (std::size_t k = A->col_ptrs[j]; k < A->col_ptrs[j + 1]; k++) {  // k is the index of the "values" and "row_indices" that corresponds to the column j// search row_ind[k] in rows
                found = false;
                found_idx = 0;
                while (!found && found_idx < n_rows) {
                    if (A->row_indices[k] == rows->at(found_idx))
                        found = true;
                    found_idx++;
                }

                // store the values if the row was found in rowsif (found) { // if the row index is in the designated rows...                   
                    new_val(n) = A->values[k]; // store the value
                    new_row_ind(n) = found_idx - 1;  // store the index where the original index was found inside "rows"
                    n++;
                }
            }

            p++;
            new_col_ptr(p) = n;
        }
        new_col_ptr(p) = n ;

        // reshape the vectors to the actual number of elements
        new_val.reshape(n, 1);
        new_row_ind.reshape(n, 1);

        return arma::sp_mat(new_row_ind, new_col_ptr, new_val, n_rows, n_cols);
    }