How do I intersect two Maps by their keys in F#

Question

I want to interesct two F# Maps, which have common keys, into a Map that has the common keys and a tuple of both values as it's value.

i.e the signature is something like:

Map<K, T1> -> Map<K, T2> -> Map<K, T1 * T2>

Any ideas of an easy functional & performant way to do it?

I know I can intersect the sets of keys and then build out a new map, I'd just like to do something that doesn't feel so dirty...

Gus · Accepted Answer · 2015-01-21T21:18:03.880

10

I had a similar problem before and finally solved it like this:

let intersect a b = Map (seq {
    for KeyValue(k, va) in a do
        match Map.tryFind k b with
        | Some vb -> yield k, (va, vb)
        | None    -> () })

edited Jan 21 '15 at 21:18

answered Jan 21 '15 at 18:00

Gus

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I think this is the most readable one yet... I've implemented it much like @Vandroiy below, but this looks much nicer – damageboy Jan 21 '15 at 19:01
Thanks, the other proposed solutions have their interesting point too. I remember first I tried with folds but I didn't like the code so I switched to this one which convinced me. It's a personal decision in the end. – Gus Jan 21 '15 at 21:21

score 3 · Answer 2 · answered Jan 21 '15 at 17:43

3

One approach is to implement this in terms of Map.fold:

module Map =
    let intersect (m1:Map<'K, 'V1>) (m2:Map<'K, 'V2>) =
        (Map.empty, m1) ||> Map.fold (fun acc k v1 ->
            (acc, Map.tryFind k m2) ||> Option.fold (fun acc v2 ->
                acc |> Map.add k (v1, v2)))

answered Jan 21 '15 at 17:43

ildjarn

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Assuming `Map.Count` is a constant-time operation, it would be more efficient to fold over the smaller map. – Lee Jan 21 '15 at 17:49
@Lee : Agreed; I just checked ILSpy and unfortunately it's O(n) as I suspected. :-[ Of course, if performance were a real concern, I suspect `Dictionary` would be in play here rather than `Map` to begin with... That said, if the caller knows the relative map sizes in advance, they can document the function that it's more efficient to pass the smaller map as the first argument. – ildjarn Jan 21 '15 at 17:59
Nice solution, but `match m2.TryFind k with...` reads a bit better than using `Option.fold` IMO and avoids some allocations. – Daniel Jan 21 '15 at 17:59
@Daniel : Agreed on the allocations, but readability is in the eye of the beholder – I prefer higher-order functions, personally. ;-] – ildjarn Jan 21 '15 at 18:00

Vandroiy · Answer 3 · 2015-01-21T18:23:52.150

I posted this prematurely and deleted it, as I'm unsure whether this counts as just intersecting the keys... but I guess it can't hurt to undelete it, since it's fairly short:

let intersect otherMap =
    Map.filter (fun k _ -> Map.containsKey k otherMap)
    >> Map.map (fun k v1 -> v1, otherMap.[k])

Edit Without intermediate map, via a sequence:

let intersect otherMap =
    Map.toSeq >> Seq.choose (fun (k, v) ->
        Map.tryFind k otherMap |> Option.map (fun v2 -> v, v2))
    >> Map.ofSeq

How do I intersect two Maps by their keys in F#

3 Answers3