code: public Double[] exportReal()
      {
          Double[] ad = new Double[indexSet.size()];
          int i = 0;
          for (IndexMN index: indexSet) {
              if (real.containsKey(index)) {
                  ad[i] = real.get(index);
              } else {
                  ad[i] = 0.0;
              }
              i++;
          }
          return ad;
      }
docs: Returns the real part (PR) array. PR has length number-of-nonzero-values.
  
  @return real part