Average (LINQ)

Enumerable.Average is extension method from System.Linq namespace. It computes average value of numeric collection.

Average Examples

Computes average of int values. The result type is double.

var list = new List<int> { 1, 8, 3, 2 }; double result = list.Average(); // result: 3.5

Computes average of decimal values. The result type is decimal.

var list = new List<decimal> { 1.0m, 8.1m, 3.3m, 2.0m }; decimal result = list.Average(); // result: 3.6

Computes average of nullable int values. The result type is nullable double. The null values in collection are ignored for calculation.

var list = new List<int?> { 1, 8, null, 3 }; double? result = list.Average(); // result: 4.0 = (1+8+3)/3

If Average method is called on empty collection, it throws the exception.

var list = new List<int>(); // empty list double result = list.Average(); // throws InvalidOperationException

If Average method is called on empty collection of nullable type, it returns null.

var list = new List<int?>(); // empty list of nullable ints double? result = list.Average(); // result: null

Average with Selector

You can calculate average value on collection of any type (IEnumerable<T>), but you have to tranform the type T into any numeric type using selector. It can be done by Select method or using selector as a parameter of the Average method.

This example counts the average length of the strings.

var stringList = new List<string> { "1", "88888888", "333", "22" }; // these two lines do the same double result = stringList.Select(x => x.Length).Average(); // result: 3.5 double result = stringList.Average(x => x.Length); // result: 3.5

Average with Query Syntax

LINQ query expression to calculate average value of all items in the collection.

var list = new List<int> { 1, 8, 3, 2 }; double result = (from x in list select x).Average(); // result: 3.5

LINQ query expression to calculate average value of items which match specified predicate.

var list = new List<int> { 1, 8, 3, 2 }; double result = (from x in list where x < 5 select x).Average(); // result: 2.0

LINQ query expression to calculate average string length using selector.

var list = new List<string> { "1", "88888888", "333", "22" }; double result = (from x in list select x.Length).Average(); // result: 3.5

Average with Group By

This example shows how to calculate average value for each group. Lets have players. Each player belongs to a team and have a score. The result is average score per player in a team.

var players = new List<Player> { new Player { Name = "Alex", Team = "A", Score = 10 }, new Player { Name = "Anna", Team = "A", Score = 20 }, new Player { Name = "Luke", Team = "L", Score = 60 }, new Player { Name = "Lucy", Team = "L", Score = 40 }, }; var teamAverageScores = from player in players group player by player.Team into playerGroup select new { Team = playerGroup.Key, AverageScore = playerGroup.Average(x => x.Score), }; // teamAverageScores is collection of anonymous objects: // { Team = "A", AverageScore = 15.0 } // { Team = "L", AverageScore = 50.0 }

Average Implementation

This is .NET Framework implementation of Enumerable.Average method for collection of numeric types (in this case IEnumerable<int>). Note that it throws the exception for an empty collection on the last line (throw Error.NoElements();)).

public static double Average(this IEnumerable<int> source) { if (source == null) throw Error.ArgumentNull("source"); long sum = 0; long count = 0; checked { foreach (int v in source) { sum += v; count++; } } if (count > 0) return (double)sum / count; throw Error.NoElements(); }

This is .NET Framework implementation of Enumerable.Average method for collection of nullable numeric types (in this case IEnumerable<int?>). Note that it returns null for an empty collection on the last line (return null;). Also notice that it ignores null values (if (v != null) {)

public static double? Average(this IEnumerable<int?> source) { if (source == null) throw Error.ArgumentNull("source"); long sum = 0; long count = 0; checked { foreach (int? v in source) { if (v != null) { sum += v.GetValueOrDefault(); count++; } } } if (count > 0) return (double)sum / count; return null; }

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