Group totals in F# - easy with sequences, is it possible with lists?

后端 未结 3 2005

Given a sequence of a group id/value tuples, it was easy to calculate group totals (pretty much the same way I would do it with C# and LINQ):

let items = [\"g1\"         


        
3条回答
  •  青春惊慌失措
    2021-02-09 05:37

    There is no built in List.groupBy. A number of F# built in types have functions that are assigned the seq version of said function. e.g. from list.fs

    let inline sumBy f (list : list<_>) = Seq.sumBy f list

    I'm pretty sure the designers of F# had many discussions about what to duplicate for the sake of consistency and what to omit for for sake of DRY. I personally wish they stuck with DRY.

    If you want to make your own "functional" List.groupBy I'd use map and list.

    let groupBy list =
        list 
        |> List.fold (fun group (g, x) -> 
            match group |> Map.tryFind g with
            | Some(s) -> group |> Map.remove g |> Map.add g (x::s)
            | None -> group |> Map.add g [x]
            ) Map.empty
        |> Map.toList 
    
    let groupsums = groupBy >> List.map (snd >> List.sum)
    

    You can skip keeping lists if you only need the sum.

    let groupAndSumBy list =
        list 
        |> List.fold (fun group (g, x) -> 
            match group |> Map.tryFind g with
            | Some(s) -> group |> Map.remove g |> Map.add g (x + s)
            | None -> group |> Map.add g x
            ) Map.empty
        |> Map.toList
        |> List.map snd
    

    Output

    > groupsums items;;
    val it : int list = [25; 10]
    
    > groupAndSumBy items;;
    val it : int list = [25; 10]
    

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