Array.foldBack2<'T1,'T2,'State> Function (F#)

Article
02/04/2013

Apply a function to pairs of elements drawn from the two collections, right-to-left, threading an accumulator argument through the computation. The two input arrays must have the same lengths, otherwise an ArgumentException is raised.

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

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

// Signature:
Array.foldBack2 : ('T1 -> 'T2 -> 'State -> 'State) -> 'T1 [] -> 'T2 [] -> 'State -> 'State

// Usage:
Array.foldBack2 folder array1 array2 state

Parameters

folder
Type: 'T1 -> 'T2 -> 'State -> 'State

The function to update the state given the input elements.
array1
Type: 'T1 []

The first input array.
array2
Type: 'T2 []

The second input array.
state
Type: 'State

The initial state.

Exceptions

Exception	Condition
ArgumentException	Thrown when the input arrays differ in length.

Return Value

The final state.

Remarks

This function is named FoldBack2 in the .NET assembly. If accessing the member from a .NET language other than F#, or through reflection, use this name.

Example

The following code shows how to use Array.foldBack2.

type Transaction =
    | Deposit
    | Withdrawal

let transactionTypes = [| Deposit; Deposit; Withdrawal |]
let transactionAmounts = [| 100.00; 1000.00; 95.00 |]
let initialBalance = 200.00

let endingBalance = Array.foldBack2 (fun elem1 elem2 acc ->
                        match elem1 with
                        | Deposit -> acc + elem2
                        | Withdrawal -> acc - elem2)
                        transactionTypes
                        transactionAmounts
                        initialBalance
printfn "Ending balance: $%.2f" endingBalance

Output

Ending balance: $1205.00

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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