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

Artikel
02/04/2013

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

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

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

// Signature:
List.scanBack : ('T -> 'State -> 'State) -> 'T list -> 'State -> 'State list

// Usage:
List.scanBack folder list state

Parameters

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

The function to update the state given the input elements.
list
Type: 'T list

The input list.
state
Type: 'State

The initial state.

Return Value

The list of states.

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 example shows how to use List.scanBack, and also contrasts its behavior with List.scan.

// A list of functions that transform 
// integers. (int -> int)
let ops1 =
 [ (fun x -> x + 1), "add 1"
   (fun x -> x + 2), "add 2"
   (fun x -> x - 5), "subtract 5" ]

let ops2 =
 [ (fun x -> x + 1), "add 1"
   (fun x -> x * 5), "multiply by 5"
   (fun x -> x * x), "square" ]

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

    printfn "Operations:"
    opNames |> List.iter (fun opName -> printf "%s  " opName)
    printfn ""

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

compareOpOrder ops1 10
compareOpOrder ops2 10

Output

Operations:
add 1  add 2  subtract 5  
List.scan List.scanBack
        10         10
        11          5
        13          7
         8          8

Operations:
add 1  multiply by 5  square  
List.scan List.scanBack
        10         10
        11        100
        55        500
      3025        501

Platforms

Windows 7, Windows Vista SP2, Windows XP SP3, Windows XP x64 SP2, Windows Server 2008 R2, Windows Server 2008 SP2, Windows Server 2003 SP2

Version Information

F# Runtime

Supported in: 2.0, 4.0

Silverlight

Supported in: 3

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