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

Den Delimarsky|Last Updated: 8/18/2017
|
2 Contributors

Returns the sum of the results generated by applying the function to each element of the list.

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

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

## Syntax

``````// Signature:
List.sumBy : ('T -> ^U) -> 'T list -> ^U (requires ^U with static member (+) and ^U with static member Zero)

// Usage:
List.sumBy projection list
``````

#### Parameters

projection Type: 'T -> ^U

A function that transforms the list elements into the type to be summed.

list Type: 'Tlist

The input list.

## Return Value

The sum of the results of applying the projection function to each element of the list.

## Exceptions

May throw an OverflowException due to arithmetic overflows.

## Remarks

The results are summed using the `checked + operator` and `Zero` property associated with the summed type.

This function is named `SumBy` 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 example illustrates the use of List.sum, List.sumBy, and List.average:

``````// Compute the sum of the first 10 integers by using List.sum.
let sum1 = List.sum [1 .. 10]

// Compute the sum of the squares of the elements of a list by using List.sumBy.
let sum2 = List.sumBy (fun elem -> elem*elem) [1 .. 10]

// Compute the average of the elements of a list by using List.average.
let avg1 = List.average [0.0; 1.0; 1.0; 2.0]

printfn "%f" avg1
``````

Output

``````1.000000
``````

## 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