list.countby['t,'key]-function-[fsharp].md

title	List.countBy<'T,'Key> Function (F#)
description	List.countBy<'T,'Key> Function (F#)
keywords	visual f#, f#, functional programming
author	liboz
manager	danielfe
ms.date	05/16/2016
ms.topic	language-reference
ms.prod	visual-studio-dev14
ms.technology	devlang-fsharp
ms.assetid	a0b6e7e0-c30a-4e5a-a9dd-f7f9c4761870

List.countBy<'T,'Key> Function (F#)

Applies a key-generating function to each element of a list and returns a list yielding unique keys and their number of occurrences in the original list.

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

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

Syntax

// Signature:
List.countBy : ('T -> 'Key) -> 'T list -> ('Key * int) list ('Key requires equality)

// Usage:
List.countBy projection source

Parameters

projection Type: 'T -> 'Key

A function transforming each item of input list into a key to be compared against the others.

source Type: 'Tlist

The input list.

Exceptions

Exception	Condition
ArgumentNullException	Thrown when the input list is null

Return Value

A list of unique keys and their number of occurrences in the original list.

Remarks

Note that this function returns a list that traverses the whole initial list. As a result this function should not be used with large lists. The function makes no assumption on the ordering of the original list.

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

Example

The following example demonstrates the use of List.countBy to determine the number of elements in a list that are odd or even.

[!code-fsharpMain]

Output

(1, 50) (0, 50)

Platforms

Windows 8, Windows 7, Windows Server 2012, Windows Server 2008 R2

Version Information

F# Core Library Versions

Supported in: 4.0, Portable

See Also

Collections.List Module (F#)

Microsoft.FSharp.Collections Namespace (F#)