MyMath 添加 double Sigma(IEnumerable<double> buf, int first, int last)

Project.FLY.Misc/MISC/MyMath.cs

@@ -274,7 +274,35 @@ namespace Misc
             else
                 return 0;
         }
+        /// <summary>
+        /// 排除 NULL_VALUE, 求Sigma
+        /// </summary>
+        /// <param name="buf"></param>
+        /// <param name="first">开始序号</param>
+        /// <param name="last">结束序号</param>
+        /// <returns></returns>
+        public static double Sigma(IEnumerable<double> buf, int first, int last)
+        {
+            double avg = Avg(buf, first, last);
+            if (double.IsNaN(avg))
+                return double.NaN;
+
+            double sum = 0;
+            int cnt = 0;
 
+            for (int i = first; i <= last; i++)
+            {
+                if (!double.IsNaN(buf.ElementAt(i)))
+                {
+                    sum += (buf.ElementAt(i) - avg) * (buf.ElementAt(i) - avg);
+                    cnt++;
+                }
+            }
+            if (cnt >= 3)
+                return (Math.Sqrt(sum / (cnt - 1)));
+            else
+                return 0;
+        }
         /// <summary>
         /// 排除 NULL_VALUE, 求Sigma
         /// </summary>