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

Question

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\"         


        

Answer 1

Although I would use kvb's suggestion, if you're going to roll your own, I suggest using Dictionary instead of Map. In my testing it was at least 400% faster.

let groupBy f (list:list<_>) =
  let dict = Dictionary()
  for v in list do
    let k = f v
    match dict.TryGetValue(k) with
    | true, l -> dict.[k] <- v :: l
    | _ -> dict.Add(k, [v])
  dict |> Seq.map (|KeyValue|) |> Seq.toList

Or:

let groupSumBy (list:list<_>) =
  let dict = Dictionary()
  for k, v in list do
    match dict.TryGetValue(k) with
    | true, n -> dict.[k] <- v + n
    | _ -> dict.Add(k, v)
  dict |> Seq.map (|KeyValue|) |> Seq.toList

By ref version:

let groupSumBy (list:list<_>) =
  let dict = Dictionary()
  let mutable n = 0
  for k, v in list do
    match dict.TryGetValue(k, &n) with
    | true -> dict.[k] <- v + n
    | false -> dict.Add(k, v)
  dict |> Seq.map (|KeyValue|) |> Seq.toList