double[] x = { 31.01, 31, 31.09 };
                double[] new_array = x.Select(a=>a+3).ToArray();

------解决方案--------------------
   double[] x = { 31.01, 31, 31.09 };
           double[] z = { 0, 0, 0 };
           double[] t = new double[x.Length];

           for (int i = 0; i <= x.Length-1 ; i++)
           {
               z[i] = x[i] + 3.0;
               t[i] = x[i] + 4.0;
           }

------解决方案--------------------
探讨

double [] x = {31.01, 31, 31.09};
简单点，我想一维数组分别加个固定值3，怎么得到的新数组？