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List.mapi<'T,'U> Function (F#)

Den Delimarsky|Last Updated: 8/18/2017
1 Contributor

Creates a new collection whose elements are the results of applying the given function to each of the elements of the collection. The integer index passed to the function indicates the index (from 0) of element being transformed.

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

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


// Signature:
List.mapi : (int -> 'T -> 'U) -> 'T list -> 'U list

// Usage:
List.mapi mapping list


mapping Type: int-> 'T -> 'U

The function to transform elements and their indices.

list Type: 'Tlist

The input list.

Return Value

The list of transformed elements.


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


let list1 = [1; 2; 3]
let newList = List.mapi (fun i x -> (i, x)) list1
printfn "%A" newList


[(0, 1); (1, 2); (2, 3)]


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.List Module (F#)

Microsoft.FSharp.Collections Namespace (F#)

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