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

[Jesper Louis Andersen]

The current (Release 17.x) implementation of maps are deliberately kept
simple. This has some severe performance problems, but it maximizes
implementation flexibility. I think it is a good approach to the problem.

* Make it right
* Make it beautiful
* Make it fast

Of course, having a defined function for this:

-spec maps:find(fun ((B) -> boolean()), map(A, B)) -> {ok, {A, B}} |
not_found.

Could be useful in the long run. But this is an iterative process that
moves ahead one release every year :)

On Thu Jan 08 2015 at 11:42:37 AM Joe Armstrong <erlang@REDACTED> wrote:

> Reply in
>
> http://joearms.github.io/2015/01/08/Some_Performance-
> Measurements-On-Maps.html
>
> /Joe
>
>
> On Thu, Jan 8, 2015 at 8:47 AM, Ulf Wiger <ulf@REDACTED> wrote:
> >
> > On 08 Jan 2015, at 04:56, Harit Himanshu <harit.subscriptions@REDACTED>
> > wrote:
> >
> > 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.
> >
> > ...
> >
> > 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
> >
> >
> > You can use
> >
> > map_search_pred(Map, Pred) ->
> >     case lists:dropwhile(fun(X) -> not Pred(X) end, maps:to_list(Map)) of
> >         [Found | _] -> Found;
> >         [] -> false
> >     end.
> >
> > A few comments.
> >
> > - Using {} as a return value denoting “not found” is unusual. My version
> > mimicks lists:keyfind/2, which returns the matching object or ‘false’.
> >
> > - Testing the length of the list using length/1 will force a count of all
> > elements. In this case, you are only interested in checking if the list
> is
> > non-empty, which is better done with a pattern-match. This also
> eliminates
> > the need for calling lists:nth(1,M), which is also a more expensive
> option
> > than hd(M).
> >
> > BR,
> > Ulf W
> >
> > Ulf Wiger, Co-founder & Developer Advocate, Feuerlabs Inc.
> > http://feuerlabs.com
> >
> >
> >
> >
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> >
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