list - F# System.InvalidOperationException: Collection was modified; enumeration operation may not execute -

i encountering problem in f# [not c# there similar post similar answer]

i understand not possible modify dictionary while enumerating in loop how should go around ?

let edgelist1 = [(1,2,3.0f);(1,2,4.0f);(5,6,7.0f);(5,6,8.0f)] let dict_edges = new dictionary<int*int,(int*int*float32) list>() x in edgelist1 dict_edges.add ((fun (a,b,c)-> (a,b)) x, x) k in dict_edges.keys dict_edges.[k] <- (dict_edges.[k] |> list.rev) 

system.invalidoperationexception: collection modified; enumeration operation may not execute.

at system.throwhelper.throwinvalidoperationexception(exceptionresource resource) @ system.collections.generic.dictionary`2.keycollection.enumerator.movenext() @ .$fsi_0101.main@()

individually working

dict_edges.[(1,2)] <- dict_edges.[(1,2)] |> list.rev;; 

in loop need change dictionary values, not keys.

thanks

the code posted not syntactically correct, it's not clear precisely trying achieve (compiler screams @ ((fun (a,b,c)-> (a,b)) x, x) saying expects second x list)

i guess after is: have list of weighted edges, there can multiple edges between nodes. you'd collapse them canonical form, have edges grouped connect pair of nodes (i,j). use of groupby library functions, , you're good:

let map_edges =     edgelist1     |> list.groupby (fun (a, b, _) -> (a, b))     |> map.oflist 

in current code using ((fun (a,b,c)-> (a,b)) x, x) extract members of tuple. instead, use patterns right in for expression:

for (a, b, c) in edgelist1 dict_edges.add ((a, b), [(a, b, c)]) 

(i've added [] make @ least compile)

note duplicating information: store node tuple in keys , in values of list, making data structure possibly inconsistent , larger. consider following:

let map_edges =     edgelist1     |> list.map (fun (a, b, c) -> (a, b), c)     |> list.groupby fst     |> list.map (fun (nodetuple, edgelist) ->          nodetuple, (edgelist |> list.map snd))     |> map.oflist   map_edges |> map.iter (fun (nodei, nodej) edgelist ->     edgelist     |> seq.map string     |> string.concat "; "     |> printfn "nodes (%i, %i): weights %s" nodei nodej ) 

(you may want use sequences intermediate representation rather list)