/**
  * Row-copies the given matrix to this sparse implementation.
  *
  * @param mat the matrix to copy.
  */
 public SparseDoubleRowMatrix(DoubleMatrix mat) {
   this(mat.getRowCount(), mat.getColumnCount());
   for (int i = 0; i < numColumns; i++) {
     setRowVector(i, mat.getRowVector(i));
   }
 }