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

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#)