Added support for MEDIAN function

Author: rominator1983

ClosedXML/Excel/CalcEngine/Functions/Statistical.cs

@@ -52,7 +52,7 @@ public static void Register(CalcEngine ce)
             //LOGNORMDIST	Returns the cumulative lognormal distribution
             ce.RegisterFunction("MAX", 1, int.MaxValue, Max);
             ce.RegisterFunction("MAXA", 1, int.MaxValue, MaxA);
-            //MEDIAN	Returns the median of the given numbers
+            ce.RegisterFunction("MEDIAN", 1, int.MaxValue, Median);
             ce.RegisterFunction("MIN", 1, int.MaxValue, Min);
             ce.RegisterFunction("MINA", 1, int.MaxValue, MinA);
             //MODE	Returns the most common value in a data set
@@ -217,6 +217,11 @@ private static object MaxA(List<Expression> p)
             return GetTally(p, false).Max();
         }
 
+        private static object Median(List<Expression> p)
+        {
+            return GetTally(p, false).Median();
+        }
+
         private static object Min(List<Expression> p)
         {
             return GetTally(p, true).Min();

ClosedXML/Excel/CalcEngine/Functions/Tally.cs

@@ -68,6 +68,30 @@ public void AddValue(Object v)
             _numericValues = null;
         }
 
+        public double Median()
+        {
+            var nums = NumericValuesInternal()
+                        .OrderBy(n => n)
+                        .ToArray();
+
+            if (nums.Length == 0) throw new ApplicationException("No values");
+
+            var hasEvenCount = nums.Length % 2 == 0;
+
+            if (hasEvenCount)
+            {
+                var numElementsToSkip = (nums.Length / 2) - 1;
+
+                return nums.Skip(numElementsToSkip)
+                           .Take(2)
+                           .Average();
+            }
+
+            var medianIndex = (nums.Length - 1) / 2;
+
+            return nums[medianIndex];
+        }
+
         public double Count()
         {
             return Count(NumbersOnly);

ClosedXML_Tests/Excel/CalcEngine/StatisticalTests.cs

@@ -260,6 +260,65 @@ public void Max()
             Assert.AreEqual(96, value);
         }
 
+        [Test]
+        public void Median_CellRangeOfNonNumericValues_ThrowsApplicationException()
+        {
+            //Arrange
+            var ws = workbook.Worksheets.First();
+
+            //Act - Assert
+            Assert.Throws<ApplicationException>(() =>
+            {
+                ws.Evaluate("AVERAGE(D3:D45)");
+            });
+        }
+
+        [Test]
+        public void Median_EvenCountOfCellRange_ReturnsAverageOfTwoElementsInMiddleOfSortedList()
+        {
+            //Arrange
+            var ws = workbook.Worksheets.First();
+
+            //Act
+            double value = ws.Evaluate("MEDIAN(I3:I10)").CastTo<double>();
+
+            //Assert
+            Assert.AreEqual(244.225, value, tolerance);
+        }
+
+        [Test]
+        public void Median_EvenCountOfManualNumbers_ReturnsAverageOfTwoElementsInMiddleOfSortedList()
+        {
+            //Act
+            double value = workbook.Evaluate("MEDIAN(-27.5,93.93,64.51,-70.56)").CastTo<double>();
+
+            //Assert
+            Assert.AreEqual(18.505, value, tolerance);
+        }
+
+        [Test]
+        public void Median_OddCountOfCellRange_ReturnsElementInMiddleOfSortedList()
+        {
+            //Arrange
+            var ws = workbook.Worksheets.First();
+
+            //Act
+            double value = ws.Evaluate("MEDIAN(I3:I11)").CastTo<double>();
+
+            //Assert
+            Assert.AreEqual(189.05, value, tolerance);
+        }
+
+        [Test]
+        public void Median_OddCountOfManualNumbers_ReturnsElementInMiddleOfSortedList()
+        {
+            //Act
+            double value = workbook.Evaluate("MEDIAN(-27.5,93.93,64.51,-70.56,101.65)").CastTo<double>();
+
+            //Assert
+            Assert.AreEqual(64.51, value, tolerance);
+        }
+
         [Test]
         public void Min()
         {
@@ -391,7 +450,6 @@ public void VarP()
             value = workbook.Evaluate(@"=VARP(Data!H:H)").CastTo<double>();
             Assert.AreEqual(2189.430863, value, tolerance);
         }
-
         private XLWorkbook SetupWorkbook()
         {
             var wb = new XLWorkbook();