List.findIndex<'T> Function (F#)

Returns the index of the first element in the list that satisfies the given predicate. Raises KeyNotFoundException if no such element exists.

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

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

// Signature:
List.findIndex : ('T -> bool) -> 'T list -> int

// Usage:
List.findIndex predicate list

Parameters

• predicate
  Type: 'T ->bool

  The function to test the input elements.

• list
  Type: 'Tlist

  The input list.

Exceptions

Return Value

The index of the first element that satisfies the predicate.

Remarks

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

Example

The following code shows how to use List.findIndex and compares its behavior to that of List.find.

let list1 = [ 2 .. 100 ]
let delta = 1.0e-10
let isPerfectSquare (x:int) =
    let y = sqrt (float x)
    abs(y - round y) < delta
let isPerfectCube (x:int) =
    let y = System.Math.Pow(float x, 1.0/3.0)
    abs(y - round y) < delta
let element = List.find (fun elem -> isPerfectSquare elem && isPerfectCube elem) list1
let index = List.findIndex (fun elem -> isPerfectSquare elem && isPerfectCube elem) list1
printfn "The first element that is both a square and a cube is %d and its index is %d." element index

Output

The first element that is both a square and a cube is 64 and its index is 62.

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

See Also

Reference

Collections.List Module (F#)

Microsoft.FSharp.Collections Namespace (F#)