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

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

Version Information

F# Core Library Versions

Supported in: 2.0, 4.0, Portable

See Also

Reference

Collections.Array Module (F#)

Microsoft.FSharp.Collections Namespace (F#)