code: private SparseDoubleVector updateRow(int row) {
          SparseDoubleVector rowVec = rowToColumns.get(row);
          if (rowVec == null) {
              rowVec = emptyVector.instanceCopy();
              rowToColumns.put(row, rowVec);
          }
  
          return rowVec;
      }
docs: Returns the {@link SparseDoubleVector} at index {@code row}.  If one does
  not exist, a new vector will be created and stored in the mapping.