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

Article
04/28/2015

Applies a function to each element of the array, threading an accumulator argument through the computation. If the input function is f and the elements are i0...iN then computes f i0 (...(f iN s)).

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

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

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

// Usage:
Array.foldBack 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 final state.

Remarks

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

Example

The following code example shows the difference between Array.fold and Array.foldBack.

// This computes 3 - 2 - 1, which evalates to -6. 
let subtractArray array1 = Array.fold (fun acc elem -> acc - elem) 0 array1
printfn "%d" (subtractArray [| 1; 2; 3 |])

// This computes 3 - (2 - (0 - 1)), which evaluates to 2. 
let subtractArrayBack array1 = Array.foldBack (fun elem acc -> elem - acc) array1 0
printfn "%d" (subtractArrayBack [| 1; 2; 3 |])

Output

-6
2

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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