A property like the number of elements or the number of elements that are not missing in a collection is meaningful for all collections regardless of their element type. The result is always an integer. A property like the first element is also universally relevant, but the type of the result depends on the element type. More than that, the type of the result is equal to the element type. These two examples illustrate the two main kinds of aggregators: those where the result is of a specific type, and those where the result type is the same as the element type.

The most common aggregators are defined as static properties or methods of the Aggregators class, and are listed below.

The above properties and methods return an aggregator group. An aggregator group represents an aggregation operation in a way that is independent of the element type. When the aggregator group is applied to a collection, an aggregator function for the element type of the collection is selected to perform the actual operation. AggregatorGroup is an abstract class that represents an aggregator group. It has two descendant classes corresponding to the two kinds of aggregators mentioned above: AggregatorGroupT represents an aggregator group where the generic type argument is the return type, and TypePreservingAggregatorGroup represents an aggregator group where the return type is the same as the element type.

Aggregators that compute numerical descriptive statistics, like Mean and Variance always return a Double value regardless of the element type. To preserve the element type in the result, use the 'exact' variant, like ExactSum.

Some aggregations, such as the correlation between two variables, involve more than one collection. The Aggregators class defines a number of aggregators that operate on more than one collection:

The simplest way to define an aggregator that is not available from the Aggregators class is to use the AggregatorsCreate method. This method takes two arguments: a name for the aggregator, and a delegate that maps a vector to the aggregated value. The following creates an aggregator that computes the geometric mean:

var agg = Aggregators.Create("geo.mean", 
    (Vector<double> x) => Stats.GeometricMean(x));

Dim agg = Aggregators.Create("geo.mean",
    Function(x As Vector(Of Double)) Stats.GeometricMean(x))

let agg = Aggregators.Create("geo.mean", 
            fun (x : Vector<float>) -> Stats.GeometricMean(x))