code: public static void changeSign(DMatrixD1 input , DMatrixD1 output)
      {
          output.reshape(input.numRows,input.numCols);
  
          final int size = input.getNumElements();
  
          for( int i = 0; i < size; i++ ) {
              output.data[i] = -input.data[i];
          }
      }
docs: <p>
  Changes the sign of every element in the matrix.<br>
  <br>
  output<sub>ij</sub> = -input<sub>ij</sub>
  </p>
  
  @param input A matrix. Modified.