遍歷數組一次求得數組的平均數、標准差、方差

• 理論根據：DX^2=EX^2-(EX)^2
• 實現:
  

  static Tuple<double,double,double> ComputeVariance2(double[] a)
  {
      double variance = 0;    //方差
      double sum = 0;         //總和
      double sum2 = 0;        //平方和
      int len = a.Length;
      for (int i = 0; i < len; i++)
      {
          sum += a[i];
          sum2 += a[i] * a[i];
      }
      variance = sum2 / len - (sum / len) * (sum / len);
      //                  平均數   標准差               方差
      return Tuple.Create(sum/len, Math.Sqrt(variance), variance);
  }