[erlang-questions] list comprehensions speed

Joe Armstrong <>
Tue Feb 25 18:44:58 CET 2014


On Tue, Feb 25, 2014 at 4:12 PM, Garrett Smith <> wrote:

> On Tue, Feb 25, 2014 at 2:08 PM, Oleg <> wrote:
> >   Hello.
> >
> > I'm erlang newbie. Don't beat me, please :-)
>
> This is list is typically very docile -- but, for example, if you
> propose that Ericsson rename list comprehensions to "enumeration
> generators" to to appeal to Java programmers, well, brace yourself.
>
> > I have a list of key-value pairs:
> >
> > A=[{"name1", 1}, {"name2", 77}, {"name3", 33}, {"name4", 234}].
> >
> > What is faster:
> >
> > [ Value || {Name, Value} <- A, string:equal(Name, "name3")].
> >
> > Or:
> >
> > get_value(Key, []) ->
> >   [].
> > get_value(Key, [H|T]) ->
> >   {Name, Value} = H,
> >   case string:equal(Name, Key) of
> >     true ->
> >       [Value];
> >     false ->
> >       get_value(Key, T)
> >   end.
> >
> > start() ->
> >   get_value("name3", A).
>
> I have questions like this all the time. I use this thing to answer my
> own questions:
>
> https://github.com/gar1t/erlang-bench
>
> As I've stated emphatically before, these stats, like all stats, lie.
> *But* they are at least some attempt to measure and test.
>
> This particular test I think is similar to what you're looking for:
>
> https://github.com/gar1t/erlang-bench/blob/master/name-lookup.escript
>
> As Sean Cribbs pointed out, lists:keyfind/2 has super powers. I don't
> know at under what conditions it makes sense to move from its blazing
> fast O(N) scans to log(N) hash/tree lookups. Of course this can be
> tested.
>
> As Loïc Hoguin has pointed out, with zeal, performance characteristics
> tend to change under system load. I haven't seen what he's seen
> specifically, but he's very emphatic.
>
> And as Richard O'Keefe has pointed out, focus on getting your program
> to work correctly before worrying about performance.
>
> And as Joe Armstrong has pointed out, performance considerations comes
> after correctness *and* maintainability, except for known performance
> bottlenecks where it pays to make a compromise.
>


  Yes - try the following:

   Repeat 10 times before meals, and before going to bed.

      " I must program as inefficiently and beautifully as possible"

  If the symptoms persist and you feel the need to program efficiently take
a cold
  shower and a brisk bracing walk.

  Your job is to write beautiful and clear code.

   It is the compiler writers job to turn clear and beautiful code in
quickly executing code.

   It is the language designers job to design languages that will make it
difficult for
   you to shoot yourself in the foot.

  If you write clear code and you think it should run quickly and it does
not do not
  optimise the code - tell the compiler writer to fix the compiler/run-time.

  We will get 1000+ core machines  within a few years - they are so bloody
fast that we
  are having problems thinking of what we can use the CPU cycles for so
please don't
  write efficient programs - otherwise we won't need the 1K core machines
:-)

/Joe






>
> I'd say, get in the habit of answering your own questions when it
> comes to "speed" -- I suggest playing around with erlang-bench, as
> it's super fun :)
>
> If you come up something interesting as an erlang-bench script, please
> pass it along as a pull request.
>
> Best of luck with your Erlang adventures!
>
> Garrett
> _______________________________________________
> erlang-questions mailing list
> 
> http://erlang.org/mailman/listinfo/erlang-questions
>
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