Array.scanBack<'T,'State> Function (F#)

Article
07/11/2012

Like Array.foldBack, but returns both the intermediate and final results.

Namespace/Module Path: Microsoft.FSharp.Collections.Array

Assembly: FSharp.Core (in FSharp.Core.dll)

// Signature:
Array.scanBack : ('T -> 'State -> 'State) -> 'T [] -> 'State -> 'State []

// Usage:
Array.scanBack folder array state

Parameters

folder
Type: 'T -> 'State -> 'State

The function to update the state given the input elements.
array
Type: 'T[]

The input array.
state
Type: 'State

The initial state.

Return Value

The array of state values.

Remarks

This function is named ScanBack in compiled assemblies. If you are accessing the function from a language other than F#, or through reflection, use this name.

Example

The following code contrasts the behavior of Array.scan and Array.scanBack.

// An array of functions that transform  
// integers. (int -> int) 
let ops1 =
 [| fun x -> x + 1
    fun x -> x + 2
    fun x -> x - 5 |]

let ops2 =
 [| fun x -> x + 1
    fun x -> x * 5
    fun x -> x * x |]

// Compare scan and scanBack, which apply the 
// operations in the opposite order. 
let compareOpOrder ops x0 =
    let xs1 = Array.scan (fun x op -> op x) x0 ops
    let xs2 = Array.scanBack (fun op x -> op x) ops x0

    // Print the intermediate results 
    let xs = Array.zip xs1 (Array.rev xs2)
    for (x1, x2) in xs do
        printfn "%10d %10d" x1 x2
    printfn ""

compareOpOrder ops1 10
compareOpOrder ops2 10

Output

        10         10
        11          5
        13          7
         8          8

        10         10
        11        100
        55        500
      3025        501

Platforms

Windows 8, Windows 7, Windows Server 2012, Windows Server 2008 R2

Version Information

F# Core Library Versions

Supported in: 2.0, 4.0, Portable

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