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

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: 'T list

    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.


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

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 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#)