PreScanExtensionPreScanTSource Method

Performs a pre-scan (exclusive prefix sum) on a sequence of elements.

Namespace: MoreLinq.Extensions
Assembly: MoreLinq (in MoreLinq.dll) Version: 3.0.0

Syntax

public static IEnumerable<TSource> PreScan<TSource>(
	this IEnumerable<TSource> source,
	Func<TSource, TSource, TSource> transformation,
	TSource identity
)

<ExtensionAttribute>
Public Shared Function PreScan(Of TSource) ( 
	source As IEnumerable(Of TSource),
	transformation As Func(Of TSource, TSource, TSource),
	identity As TSource
) As IEnumerable(Of TSource)

public:
[ExtensionAttribute]
generic<typename TSource>
static IEnumerable<TSource>^ PreScan(
	IEnumerable<TSource>^ source, 
	Func<TSource, TSource, TSource>^ transformation, 
	TSource identity
)

[<ExtensionAttribute>]
static member PreScan : 
        source : IEnumerable<'TSource> * 
        transformation : Func<'TSource, 'TSource, 'TSource> * 
        identity : 'TSource -> IEnumerable<'TSource>

Parameters

source: Type: System.Collections.GenericIEnumerableTSource
Source sequence
transformation: Type: SystemFuncTSource, TSource, TSource
Transformation operation
identity: Type: TSource
Identity element (see remarks)

Type Parameters

TSource: Type of elements in source sequence

Return Value

Type: IEnumerableTSource
The scanned sequence

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerableTSource. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

Remarks

An exclusive prefix sum returns an equal-length sequence where the N-th element is the sum of the first N-1 input elements (the first element is a special case, it is set to the identity). More generally, the pre-scan allows any commutative binary operation, not just a sum. The inclusive version of PreScan is ScanTSource(IEnumerableTSource, FuncTSource, TSource, TSource). This operator uses deferred execution and streams its result.

Examples

Copy

int[] values = { 1, 2, 3, 4 };
var prescan = values.PreScan((a, b) => a + b, 0);
var scan = values.Scan((a, b) => a + b);
var result = values.EquiZip(prescan, ValueTuple.Create);

prescan will yield { 0, 1, 3, 6 }, while scan and result will both yield { 1, 3, 6, 10 }. This shows the relationship between the inclusive and exclusive prefix sum.

Reference

PreScanExtension Class