List.fold2<'T1,'T2,'State> Function (F#)

Applies a function to corresponding elements of two collections, threading an accumulator argument through the computation. The collections must have identical sizes. If the input function is f and the elements are i0...iN and j0...jN then computes f (... (f s i0 j0)...) iN jN.

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

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

// Signature:
List.fold2 : ('State -> 'T1 -> 'T2 -> 'State) -> 'State -> 'T1 list -> 'T2 list -> 'State

// Usage:
List.fold2 folder state list1 list2


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

    The function to update the state given the input elements.

  • state
    Type: 'State

    The initial state.

  • list1
    Type: 'T1 list

    The first input list.

  • list2
    Type: 'T2 list

    The second input list.

Return Value

The final state value.





Thrown when the input lists differ in length.


This function is named Fold2 in compiled assemblies. If you are accessing the function from a .NET language other than F#, or through reflection, use this name.


The following code example illustrates the use of List.fold2.

// Use List.fold2 to perform computations over two lists (of equal size) at the same time.
// Example: Sum the greater element at each list position.
let sumGreatest list1 list2 = List.fold2 (fun acc elem1 elem2 ->
                                              acc + max elem1 elem2) 0 list1 list2

let sum = sumGreatest [1; 2; 3] [3; 2; 1]
printfn "The sum of the greater of each pair of elements in the two lists is %d." sum


The sum of the greater of each pair of elements in the two lists is 8.

The following code example illustrates the use of List.fold2 to compute the ending balance in a bank account after a series of transactions. The two input lists represent the transaction type (deposit or withdrawal) and the transaction amount.

// Discriminated union type that encodes the transaction type.
type Transaction =
    | Deposit
    | Withdrawal

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

// Use fold2 to perform a calculation on the list to update the account balance.
let endingBalance = List.fold2 (fun acc elem1 elem2 ->
                                match elem1 with
                                | Deposit -> acc + elem2
                                | Withdrawal -> acc - elem2)
printfn "%f" endingBalance




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


Supported in: 3

See Also


Collections.List Module (F#)

Microsoft.FSharp.Collections Namespace (F#)