Table of contents
Collapse the table of content
Expand the table of content

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

Den Delimarsky|Last Updated: 6/8/2016
1 Contributor

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

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

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


// Signature:
Array.fold2 : ('State -> 'T1 -> 'T2 -> 'State) -> 'State -> 'T1 [] -> 'T2 [] -> 'State

// Usage:
Array.fold2 folder state array1 array2


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

The function to update the state given the input elements.

state Type: 'State

The initial state.

array1 Type: 'T1[]

The first input array.

array2 Type: 'T2[]

The second input array.

Return Value

The final state.


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


The following code shows how to use Array.fold2.

    // Use Array.fold2 to perform computations over two arrays (of equal size)
    // at the same time.
    // Example: Add the greater element at each array position.
    let sumGreatest array1 array2 =
        Array.fold2 (fun acc elem1 elem2 ->
            acc + max elem1 elem2) 0 array1 array2

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


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


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

Collections.Array Module (F#)

Microsoft.FSharp.Collections Namespace (F#)

© 2016 Microsoft