Aggregate

Learn custom accumulation with Aggregate.

Folding with Aggregate

Aggregate folds a sequence into a single value. It takes an accumulator and an update function; for each item it updates the accumulator and finally returns one value.

Example: numbers.Aggregate((acc, n) => acc + n) sums all numbers. On the first call acc is the first item and n the second. Through the loop acc carries the running total.

The seed value

Aggregate can also take a seed value: numbers.Aggregate(0, (acc, n) => acc + n). The seed is the initial accumulator; it does not mix with the first item. On an empty sequence the seed becomes the result.

Using a seed also lets the accumulator type differ from the item type. For example, join a string list with Aggregate("", (acc, s) => acc + s) into a single string.

Exercise Sum numbers with Aggregate

Compute the sum of the numbers list below with Aggregate and print it with Console.WriteLine.

Show example solution

Compute the sum with var sum = numbers.Aggregate((acc, n) => acc + n); then print with Console.WriteLine(sum);.
Example solution:
using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        var numbers = new List<int> { 1, 2, 3, 4 };
        var sum = numbers.Aggregate((acc, n) => acc + n);
        Console.WriteLine(sum);
    }
}

Test yourself

What does Aggregate do?
- Filters the sequence
- Sorts the sequence
- Groups the sequence
- Folds the sequence to one value
Show answer

Folds the sequence to one value — Correct. Aggregate reduces a sequence to a single value with an accumulator.

Complete the Aggregate call

var sum = numbers.____((acc, n) => acc + n);

Show answer

var sum = numbers.Aggregate((acc, n) => acc + n);

Match the part to its role

acc

Accumulated value
n

Current item
seed

Starting value
Aggregate

Fold method

Next lesson: Zip

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