code: public double at(int r, int c) throws MatrixException
      {
          if ((r < 0) || (r >= nRows) || (c < 0) || (c >= nCols)) {
              throw new MatrixException(MatrixException.INVALID_INDEX);
          }
  
          return values[r][c];
      }
docs: Get the value of element [r,c] in the matrix.
  @param r the row index
  @param c the column index
  @return the value
  @throws numbercruncher.MatrixException for an invalid index