Test and Fix Median and Mean

Author: JakeRadMSFT

src/Microsoft.Data.Analysis/PrimitiveDataFrameColumn.cs

@@ -228,7 +228,7 @@ public override double Median()
             PrimitiveDataFrameColumn<long> sortIndices = GetAscendingSortIndices(out Int64DataFrameColumn _);
             long middle = sortIndices.Length / 2;
             double middleValue = (double)Convert.ChangeType(this[sortIndices[middle].Value].Value, typeof(double));
-            if (Length % 2 == 0)
+            if (sortIndices.Length % 2 == 0)
             {
                 double otherMiddleValue = (double)Convert.ChangeType(this[sortIndices[middle - 1].Value].Value, typeof(double));
                 return (middleValue + otherMiddleValue) / 2;
@@ -243,7 +243,7 @@ public override double Mean()
         {
             if (Length == 0)
                 return 0;
-            return (double)Convert.ChangeType((T)Sum(), typeof(double)) / Length;
+            return (double)Convert.ChangeType((T)Sum(), typeof(double)) / (Length - NullCount);
         }
 
         protected internal override void Resize(long length)

test/Microsoft.Data.Analysis.Tests/DataFrameTests.cs

@@ -3323,5 +3323,15 @@ public void GetColumnTests()
             Assert.Throws<ArgumentException>(() => dataFrame.Columns.GetSingleColumn("Ushort"));
 
         }
+
+        [Fact]
+        public void TestMeanMedian()
+        {
+            DataFrame df = MakeDataFrameWithNumericColumns(10, true, 0);
+
+            Assert.Equal(40.0 / 9.0, df["Decimal"].Mean());
+            Assert.Equal(4, df["Decimal"].Median());
+
+        }
     }
 }