Fun with Linq Aggregate

Say we’ve got a CSV file:

private const string csv = 
@"1,2,3,4,5
6,7,8,9,10
11,12,13,14,15
16,17,18,19,20";

We want to parse it into nested List<int>, and then print out all the numbers on a single line. We might do it like this:

public void ParseCsvAndOutputAsString()
{
    var data = new List<List<int>>();

    foreach (var line in csv.Split('\n'))
    {
        var innerList = new List<int>();
        foreach (var item in line.Split(','))
        {
            innerList.Add(int.Parse(item));
        }
        data.Add(innerList);
    }

    string output = "";
    foreach (var innerList in data)
    {
        foreach (var item in innerList)
        {
            output = string.Format("{0} {1}", output, item);
        }
    }

    Console.WriteLine(output);
}

Yes, I know I should use a StringBuilder and AddRange, but ignore that for the moment, I’m trying to make a point here.

Taking a collection of values and reducing them down to a single value is a very common task in programming. Here we’re doing it twice; first we’re taking a string, splitting it apart and then reducing it down to a single reference to a List<List<int>>; then we’re taking the may items of data and reducing them to a string.

This is so common in fact, that many programming languages have some kind of ‘reduce’ functionality built in. It’s especially common with functional languages. Did you know that C# also has a reduce function? It’s the Aggregate extension method. Here’s the same method written in two statements with it:

[Test]
public void ParseCsvAndOutputAsStringUsingAgregate()
{
    var data = csv
        .Split('\n')
        .Aggregate(
            new List<List<int>>(),
            (list, line) => list.Append(line
                .Split(',')
                .Select(str => int.Parse(str))
                .Aggregate(
                    new List<int>(),
                    (innerList, item) => innerList.Append(item))));

    Console.WriteLine(data
        .SelectMany(innerList => innerList)
        .Aggregate("", (output, item) => string.Format("{0} {1}", output, item)));
}

Aggregate takes two parameters; the first sets up the initial value, in our case we create new instances of List<List<int>>, List<int> and an empty string, this is known as the ‘accumulator’; the second is the function that does the accumulating.

Aggregate works really well with fluent interfaces, where methods return their instance. I’ve added a fluent ‘Append’ extension method to List<int> to help me here:

public static List<T> Append<T>(this List<T> list, T item)
{
    list.Add(item);
    return list;
}

So any time you’ve got a collection of stuff that you want to ‘reduce’ to a single item, remember Aggregate.