code: public static void swapRow(Matrix A, int j, int k, int start, int to)
      {
          double t;
          for(int i = start; i < to; i++)
          {
              t = A.get(j, i);
              A.set(j, i, A.get(k, i));
              A.set(k, i, t);
          }
      }
docs: Swaps the rows <tt>j</tt> and <tt>k</tt> in the given matrix.
  @param A the matrix to perform he update on
  @param j the first row to swap
  @param k the second row to swap
  @param start the first column that will be included in the swap (inclusive)
  @param to the last column to be included in the swap (exclusive)