Array.fold<'T,'State> Function (F#)

Article
04/28/2015

Applies a function to each element of the collection, threading an accumulator argument through the computation. If the input function is f and the elements are i0...iN then computes f (... (f s i0)...) iN.

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

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

// Signature:
Array.fold : ('State -> 'T -> 'State) -> 'State -> 'T [] -> 'State

// Usage:
Array.fold folder state array

Parameters

folder
Type: 'State -> 'T -> 'State

The function to update the state given the input elements.
state
Type: 'State

The initial state.
array
Type: 'T[]

The input array.

Return Value

The final state.

Remarks

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

Example

The following code illustrates the use of Array.fold.

let sumArray array = Array.fold (fun acc elem -> acc + elem) 0 array
printfn "Sum of the elements of array %A is %d." [ 1 .. 3 ] (sumArray [| 1 .. 3 |])

// The following example computes the average of a array. 
let averageArray array = (Array.fold (fun acc elem -> acc + float elem) 0.0 array / float array.Length)

// The following example computes the standard deviation of a array. 
// The standard deviation is computed by taking the square root of the 
// sum of the variances, which are the differences between each value 
// and the average. 
let stdDevArray array =
    let avg = averageArray array
    sqrt (Array.fold (fun acc elem -> acc + (float elem - avg) ** 2.0 ) 0.0 array / float array.Length)

let testArray arrayTest =
    printfn "Array %A average: %f stddev: %f" arrayTest (averageArray arrayTest) (stdDevArray arrayTest)

testArray [|1; 1; 1|]
testArray [|1; 2; 1|]
testArray [|1; 2; 3|]

// Array.fold is the same as to Array.iter when the accumulator is not used. 
let printArray array = Array.fold (fun acc elem -> printfn "%A" elem) () array
printArray [|0.0; 1.0; 2.5; 5.1 |]

Output

Sum of the elements of array [1; 2; 3] is 6.
Array [|1; 1; 1|] average: 1.000000 stddev: 0.000000
Array [|1; 2; 1|] average: 1.333333 stddev: 0.471405
Array [|1; 2; 3|] average: 2.000000 stddev: 0.816497
0.0
1.0
2.5
5.1

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