C# Language Tutorial => Aggregate

Example

Aggregate Applies an accumulator function over a sequence.

int[] intList = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
int sum = intList.Aggregate((prevSum, current) => prevSum + current);
// sum = 55

At the first step prevSum = 1
At the second prevSum = prevSum(at the first step) + 2
At the i-th step prevSum = prevSum(at the (i-1) step) + i-th element of the array

string[] stringList = { "Hello", "World", "!" };
string joinedString = stringList.Aggregate((prev, current) => prev + " " + current);
// joinedString = "Hello World !"

A second overload of Aggregate also receives an seed parameter which is the initial accumulator value. This can be used to calculate multiple conditions on a collection without iterating it more than once.

List<int> items = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 };

For the collection of items we want to calculate

The total .Count
The amount of even numbers
Collect each forth item

Using Aggregate it can be done like this:

var result = items.Aggregate(new { Total = 0, Even = 0, FourthItems = new List<int>() },
                (accumelative,item) =>
                new {
                    Total = accumelative.Total + 1,
                    Even = accumelative.Even + (item % 2 == 0 ? 1 : 0),
                    FourthItems = (accumelative.Total + 1)%4 == 0 ? 
                        new List<int>(accumelative.FourthItems) { item } : 
                        accumelative.FourthItems 
                });
// Result:
// Total = 12
// Even = 6
// FourthItems = [4, 8, 12]

Note that using an anonymous type as the seed one has to instantiate a new object each item because the properties are read only. Using a custom class one can simply assign the information and no new is needed (only when giving the initial seed parameter

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