[erlang-questions] How to return first {Key, Value} from Map where Predicate is true

[Harit Himanshu]

Thanks Nathan

On Wed, Jan 7, 2015 at 9:20 PM, Nathan Fiedler <nathanfiedler@REDACTED>
wrote:

> The maps module would be the first place to look, but it seems to lack
> a find(Pred, Map) function. And it doesn't look like proplists would
> be of any advantage, either. Nor does Jakob Sievers stdlib2 have
> anything helpful here. The best I can offer is to use [H|_T] =
> lists:filter(Pred, maps:to_list(Map)), which is shorter, but no more
> efficient than what you have so far.
>
> Hopefully someone more knowledgable can help.
>
> n
>
>
> On Wed, Jan 7, 2015 at 7:56 PM, Harit Himanshu
> <harit.subscriptions@REDACTED> wrote:
> > The more formal definition of problem is
> >
> > Write a function map_search_pred(Map, Pred) that returns the first
> element
> > {Key,Value} in the map for which Pred(Key, Value) is true.
> >
> > My attempt, looks like
> >
> > map_search_pred(Map, Pred)  ->
> >   M = [{Key, Value} || {Key, Value} <- maps:to_list(Map), Pred(Key,
> Value)
> > =:= true],
> >   case length(M) of
> >     0 -> {};
> >     _ -> lists:nth(1, M)
> >   end.
> >
> > When I run this, I get correct answer
> >
> > 1> lib_misc:map_search_pred(#{}, fun(X, Y) -> X =:= Y end).
> > {}
> > 2> lib_misc:map_search_pred(#{1 => 1, 2 => 2}, fun(X, Y) -> X =:= Y end).
> > {1,1}
> > 3> lib_misc:map_search_pred(#{1 => 1, 2 => 3}, fun(X, Y) -> X =:= Y end).
> > {1,1}
> > 4> lib_misc:map_search_pred(#{1 => 2, 2 => 2}, fun(X, Y) -> X =:= Y end).
> > {2,2}
> > 5>
> >
> > Problem?
> > The problem is efficiency.
> > It uses list comprehension and runs for all the elements in the list. The
> > better solution would be to return after the first match.
> >
> > As a beginner to language, I do not know a better idiomatic way to solve
> > this, so looking for better ideas and suggestions
> >
> > Thank you
> >
> >
> > _______________________________________________
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> > erlang-questions@REDACTED
> > http://erlang.org/mailman/listinfo/erlang-questions
> >
>
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