PreScan Method (source, transformation, identity)

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

Declaration Syntax

Visual Basic

Visual C++

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
)

Generic Template Parameters

TSource: Type of elements in source sequence

Parameters

source (IEnumerable<(Of <(<'TSource>)>)>): Source sequence

transformation (Func<(Of <(<'TSource, TSource, TSource>)>)>): Transformation operation

identity (TSource): Identity element (see remarks)

Return Value

The scanned sequence

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<(Of <(<'TSource>)>)>. When you use instance method syntax to call this method, omit the first parameter.

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 Scan<(Of <<'(TSource>)>>)(IEnumerable<(Of <<'(TSource>)>>), Func<(Of <<'(TSource, TSource, TSource>)>>)). This operator uses deferred execution and streams its result.

Examples

	Copy
Func<int, int, int> plus = (a, b) => a + b; int[] values = { 1, 2, 3, 4 }; IEnumerable<int> prescan = values.PreScan(plus, 0); IEnumerable<int> scan = values.Scan(plus; a + b); IEnumerable<int> result = values.Zip(prescan, plus);

Copy

Func<int, int, int> plus = (a, b) => a + b;
int[] values = { 1, 2, 3, 4 };
IEnumerable<int> prescan = values.PreScan(plus, 0);
IEnumerable<int> scan = values.Scan(plus; a + b);
IEnumerable<int> result = values.Zip(prescan, plus);

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.

Assembly: MoreLinq (Module: MoreLinq.dll) Version: 1.0.16006.0 (1.0.16006.1845)