code: public static void subtract(DMatrixD1 a, DMatrixD1 b, DMatrixD1 c)
      {
          if( a.numCols != b.numCols || a.numRows != b.numRows ) {
              throw new MatrixDimensionException("The 'a' and 'b' matrices do not have compatible dimensions");
          }
          c.reshape(a.numRows,a.numCols);
  
          final int length = a.getNumElements();
  
          for( int i = 0; i < length; i++ ) {
              c.data[i] = a.data[i] - b.data[i];
          }
      }
docs: <p>Performs the following subtraction operation:<br>
  <br>
  c = a - b  <br>
  c<sub>ij</sub> = a<sub>ij</sub> - b<sub>ij</sub>
  </p>
  <p>
  Matrix C can be the same instance as Matrix A and/or B.
  </p>
  
  @param a A Matrix. Not modified.
  @param b A Matrix. Not modified.
  @param c A Matrix. Modified.