Seq.averageBy - F#

Lets examine Seq.averageBy.

Seq.averageBy: Returns the average of the results generated by applying the function to each element of the sequence. ~ FSharp.Core

Lets say you have an employee type, and a bunch of employees.

type Employee = {
    Name: string;
    StartDate: DateTime;
    Salary: double;
}

let employees = seq {
    { Name = "George"; StartDate = DateTime.Now.AddYears(-3); Salary = 100_000.0};
    { Name = "Bobby"; StartDate = DateTime.Now.AddYears(-1); Salary = 80_000.0};
    { Name = "Susan"; StartDate = DateTime.Now.AddYears(-10); Salary = 1_000_000.0};
    { Name = "Joe"; StartDate = DateTime.Now.AddYears(-2); Salary = 10_000.0};
}

And you wanted to compute the average salary of all your employees. You could use a map:

[<EntryPoint>]
let main argv =
    let salaries = 
        employees 
        |> Seq.map (fun x -> x.Salary)
        |> Seq.average

    printfn "%f" salaries
    0

Prints 297500.0.

Correct answer, but then you’re iterating through your sequence twice. Once to extract the salary, and second to compute the average.

Lets use Seq.averageBy instead:


[<EntryPoint>]
let main argv =
    let salaries = 
        employees 
        |> Seq.averageBy (fun x -> x.Salary)

    printfn "%f" salaries
    0

The same function we were passing into our map function is now just passed into averageBy.

Notes

averageBy does share the same limitations of Seq.average and doesn’t support the int type. (See link for full explanation)