Statistics: mean of empty set should be NaN

src/Numerics/Statistics/Statistics.cs

@@ -40,6 +40,53 @@ namespace MathNet.Numerics.Statistics
     /// </summary>
     public static class Statistics
     {
+        /// <summary>
+        /// Returns the minimum value in the sample data.
+        /// </summary>
+        /// <param name="data">The sample data.</param>
+        /// <returns>The minimum value in the sample data.</returns>
+        public static double Minimum(this IEnumerable<double> data)
+        {
+            var array = data as double[];
+            return array != null
+                ? ArrayStatistics.Minimum(array)
+                : StreamingStatistics.Minimum(data);
+        }
+        /// <summary>
+        /// Returns the minimum value in the sample data.
+        /// </summary>
+        /// <param name="data">The sample data.</param>
+        /// <returns>The minimum value in the sample data.</returns>
+        public static double Minimum(this IEnumerable<double?> data)
+        {
+            if (data == null) throw new ArgumentNullException("data");
+            return StreamingStatistics.Minimum(data.Where(d => d.HasValue).Select(d => d.Value));
+        }
+
+        /// <summary>
+        /// Returns the maximum value in the sample data.
+        /// </summary>
+        /// <param name="data">The sample data.</param>
+        /// <returns>The maximum value in the sample data.</returns>
+        public static double Maximum(this IEnumerable<double> data)
+        {
+            var array = data as double[];
+            return array != null
+                ? ArrayStatistics.Maximum(array)
+                : StreamingStatistics.Maximum(data);
+        }
+
+        /// <summary>
+        /// Returns the maximum value in the sample data.
+        /// </summary>
+        /// <param name="data">The sample data.</param>
+        /// <returns>The maximum value in the sample data.</returns>
+        public static double Maximum(this IEnumerable<double?> data)
+        {
+            if (data == null) throw new ArgumentNullException("data");
+            return StreamingStatistics.Maximum(data.Where(d => d.HasValue).Select(d => d.Value));
+        }
+
         /// <summary>
         /// Calculates the sample mean.
         /// </summary>
@@ -47,19 +94,10 @@ namespace MathNet.Numerics.Statistics
         /// <returns>The mean of the sample.</returns>
         public static double Mean(this IEnumerable<double> data)
         {
-            if (data == null)
+            var array = data as double[];
-            {
+            return array != null
-                throw new ArgumentNullException("data");
+                ? ArrayStatistics.Mean(array)
-            }
+                : StreamingStatistics.Mean(data);
-
-            double mean = 0;
-            ulong m = 0;
-            foreach (var item in data)
-            {
-                mean += (item - mean) / ++m;
-            }
-
-            return mean;
         }
 
         /// <summary>
@@ -69,22 +107,8 @@ namespace MathNet.Numerics.Statistics
         /// <returns>The mean of the sample.</returns>
         public static double Mean(this IEnumerable<double?> data)
         {
-            if (data == null)
+            if (data == null) throw new ArgumentNullException("data");
-            {
+            return StreamingStatistics.Mean(data.Where(d => d.HasValue).Select(d => d.Value));
-                throw new ArgumentNullException("data");
-            }
-
-            double mean = 0;
-            ulong m = 0;
-            foreach (var item in data)
-            {
-                if (item.HasValue)
-                {
-                    mean += (item.Value - mean) / ++m;
-                }
-            }
-
-            return mean;
         }
 
         /// <summary>
@@ -321,104 +345,6 @@ namespace MathNet.Numerics.Statistics
             return Math.Sqrt(PopulationVariance(data));
         }
 
-        /// <summary>
-        /// Returns the minimum value in the sample data.
-        /// </summary>
-        /// <param name="data">The sample data.</param>
-        /// <returns>The minimum value in the sample data.</returns>
-        public static double Minimum(this IEnumerable<double?> data)
-        {
-            if (data == null)
-            {
-                throw new ArgumentNullException("data");
-            }
-
-            double min = double.MaxValue;
-            ulong count = 0;
-            foreach (double? d in data)
-            {
-                if (d.HasValue)
-                {
-                    min = Math.Min(min, d.Value);
-                    count++;
-                }
-            }
-
-            return count == 0 ? double.NaN : min;
-        }
-
-        /// <summary>
-        /// Returns the maximum value in the sample data.
-        /// </summary>
-        /// <param name="data">The sample data.</param>
-        /// <returns>The maximum value in the sample data.</returns>
-        public static double Maximum(this IEnumerable<double?> data)
-        {
-            if (data == null)
-            {
-                throw new ArgumentNullException("data");
-            }
-
-            double max = double.MinValue;
-            ulong count = 0;
-            foreach (double? d in data)
-            {
-                if (d.HasValue)
-                {
-                    max = Math.Max(max, d.Value);
-                    count++;
-                }
-            }
-
-            return count == 0 ? double.NaN : max;
-        }
-
-        /// <summary>
-        /// Returns the minimum value in the sample data.
-        /// </summary>
-        /// <param name="data">The sample data.</param>
-        /// <returns>The minimum value in the sample data.</returns>
-        public static double Minimum(this IEnumerable<double> data)
-        {
-            if (data == null)
-            {
-                throw new ArgumentNullException("data");
-            }
-
-            double min = double.MaxValue;
-            ulong count = 0;
-            foreach (double d in data)
-            {
-                min = Math.Min(min, d);
-                count++;
-            }
-
-            return count == 0 ? double.NaN : min;
-        }
-
-        /// <summary>
-        /// Returns the maximum value in the sample data.
-        /// </summary>
-        /// <param name="data">The sample data.</param>
-        /// <returns>The maximum value in the sample data.</returns>
-        public static double Maximum(this IEnumerable<double> data)
-        {
-            if (data == null)
-            {
-                throw new ArgumentNullException("data");
-            }
-
-            double max = double.MinValue;
-            ulong count = 0;
-            foreach (double d in data)
-            {
-                max = Math.Max(max, d);
-                count++;
-            }
-
-            return count == 0 ? double.NaN : max;
-        }
-
         /// <summary>
         /// Calculates the sample median.
         /// </summary>

src/Numerics/Statistics/StreamingStatistics.cs

@@ -90,11 +90,13 @@ namespace MathNet.Numerics.Statistics
 
             double mean = 0;
             ulong m = 0;
+            bool any = false;
             foreach (var d in stream)
             {
                 mean += (d - mean) / ++m;
+                any = true;
             }
-            return mean;
+            return any ? mean : double.NaN;
         }
     }
 }

src/UnitTests/StatisticsTests/StatisticsTests.cs

@@ -261,6 +261,17 @@ namespace MathNet.Numerics.UnitTests.StatisticsTests
             Assert.That(StreamingStatistics.Maximum(new[] { 2d }), Is.Not.NaN);
         }
 
+        [Test]
+        public void MeanOfEmptyMustBeNaN()
+        {
+            Assert.That(Statistics.Mean(new double[0]), Is.NaN);
+            Assert.That(Statistics.Mean(new[] { 2d }), Is.Not.NaN);
+            Assert.That(ArrayStatistics.Mean(new double[0]), Is.NaN);
+            Assert.That(ArrayStatistics.Mean(new[] { 2d }), Is.Not.NaN);
+            Assert.That(StreamingStatistics.Mean(new double[0]), Is.NaN);
+            Assert.That(StreamingStatistics.Mean(new[] { 2d }), Is.Not.NaN);
+        }
+
         [Test]
         public void SampleVarianceOfEmptyAndSingleMustBeNaN()
         {