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How to create and update Map<int,[int]>
f#

Hey guys! I have two question that I want to ask how to do in F#:

1. If I have a list of this obj:

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type Stuff = {
    int key
    int value
}

where in that list, the same key might list more than once... something like (1,2), (1,5),(2,3),(3,4),(4,3),(3,[9])

How would I convert that into a Map?

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(1, [2, 5])
(2, [3])
(3, [4,9])
(4, [3])

My second question is, if I have two Map with that kind of structure how can I merge those two Map into 1 Map? so...

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Map1
(1,[1,2,3]) 
(2,[1,4,5])
+
Map2
(1,[5])
(2,[3])
= Map3
(1,[1,2,3,5]) 
(2,[1,4,5,3])

I tried searching for help online and other place, and couldn't find any real solutions...

Thanks!

.

Another quick answer (a little more work will be needed if you want to handle duplicate values or have specific requirements regarding order) :

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let toMap kvs =
    let addPair map (k, v) =
        match Map.tryFind k map with
        | None -> map |> Map.add k [v]
        | Some vs -> map |> Map.remove k |> Map.add k (v :: vs)
    kvs |> List.fold addPair Map.empty

let pairs = [(1,2); (1,5); (2,3); (3,4); (4,3); (3,9)]

toMap pairs

let merge map1 map2 =
    let addPair map k vs =
        match Map.tryFind k map with
        | None -> map |> Map.add k vs
        | Some vs' -> map |> Map.remove k |> Map.add k (vs @ vs')
    map1 |> Map.fold addPair map2

merge (Map.ofList [(1,[1;2;3]); (2,[1;4;5])]) (Map.ofList [(1,[5]); (2,[3])])
By on 2/2/2011 10:54 AM ()

Thanks guys! These helped me out a lot :)

By on 2/3/2011 8:03 AM ()

Hi,

My answer may not elegant. You can try it anyway.

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// first part of the question
type Stuff = {
     key : int
     value : int
    } 

let  things = [{key = 1; value = 2}; { key = 1; value =5}; {key =2; value =3}; {key =3; value =4}; {key =4; value =3}; {key =3; value =9}]

let result = things 
                |> Seq.groupBy (fun x -> x.key) 
                |> Seq.map (function a,b -> (a,[for i in b -> i.value]))

printfn "%A" result

// second part of the question
let map1 = [(1,[1;2;3]); (2,[1;4;5]) ]
let map2 = [(1,[5]); (2,[3])]

let merge = map1 @ map2 
            |> Seq.groupBy (function x,_ -> x)
            |> Seq.map (function x,y -> (x, y |> Seq.map (function _,z -> z)))
            |> Seq.map (function x,y -> (x, (y |> Seq.collect (fun x -> [ for i in x -> i])) |> Set.ofSeq |> Set.toList))

printfn "%A" merge
By on 2/2/2011 10:28 AM ()

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