PreScanExtensionPreScanTSource Method

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

Definition

Namespace: MoreLinq.Extensions
Assembly: MoreLinq (in MoreLinq.dll) Version: 3.4.0+b99a6a8cc504caf2d48372fe54a2f8116c59cd0c

C#

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

VB

<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)

C++

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

F#

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

Parameters

source IEnumerableTSource: Source sequence
transformation FuncTSource, TSource, TSource: Transformation operation
identity TSource: Identity element (see remarks)

Type Parameters

TSource: Type of elements in source sequence

Return Value

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.

Example

C#

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.