[erlang-questions] perform a function on a fixed time interval

Hynek Vychodil <>
Tue Aug 24 12:34:32 CEST 2010


use timer:send_interval/2,3

On Tue, Aug 24, 2010 at 12:15 PM, Roberto Ostinelli <> wrote:
> dear list,
>
> i have a loop process which needs to perform a computation every 1000
> ms. this process needs also to receive messages from external
> processes.
>
> the issue arises from the fact that in every normal erlang loop, the
> 'after' statement is called only if no messages are receive by the
> process for a specific timeout.
>
> this is more or less how i could write a process which executes a
> computation every 1000 ms.
>
> the send_loop/2 emulates the external sending of messages, main_loop/1
> is the process which must receive external processes messages and
> ensure to perform a computation every 1000 ms. you may run the code
> from the console with test:start/0.
>
> -----------------------------------
>
> -module(test).
> -compile(export_all).
>
> start() ->
>        % start main loop
>        Pid = spawn(?MODULE, main_loop, [now()]),
>        % start sender loop
>        send_loop(Pid, 20),
>        Pid ! shutdown.
>
> % send 20 messages every 500 ms
> send_loop(_Pid, 0) -> ok;
> send_loop(Pid, Count) ->
>        receive
>        after 500 ->
>                Pid ! test,
>                send_loop(Pid, Count - 1)
>        end.
>
> main_loop(LastUpdate) ->
>        receive
>                shutdown ->
>                        shutdown;
>                test ->
>                        io:format("received test message~n"),
>                        Now = now(),
>                        case timer:now_diff(Now, LastUpdate) / 1000 >= 1000 of
>                                true ->
>                                        compute(),
>                                        main_loop(Now);
>                                false ->
>                                        main_loop(LastUpdate)
>                        end
>        after 1000 ->
>                compute(),
>                main_loop(now())
>        end.
>
> compute() ->
>        io:format("ping~n").
>
> -----------------------------------
>
> another solution would imply running a timer process which would
> signal the main_loop when to perform the computation. you may run the
> code from the console with test:start/0.
>
> -----------------------------------
>
> -module(test).
> -compile(export_all).
>
> start() ->
>        % start main loop
>        Pid = spawn(?MODULE, main_loop, []),
>        % start sender loop
>        send_loop(Pid, 20),
>        Pid ! shutdown.
>
> % send 20 messages every 500 ms
> send_loop(_Pid, 0) -> ok;
> send_loop(Pid, Count) ->
>        receive
>        after 500 ->
>                Pid ! test,
>                send_loop(Pid, Count - 1)
>        end.
>
> main_loop() ->
>        % start timer loop
>        TimerPid = spawn(?MODULE, timer, [self()]),
>        main_loop(TimerPid).
> main_loop(TimerPid) ->
>        receive
>                shutdown ->
>                        TimerPid ! shutdown;
>                test ->
>                        io:format("received test message~n"),
>                        main_loop(TimerPid);
>                compute ->
>                        compute(),
>                        main_loop(TimerPid)
>        end.
>
> % send a ping every 1000 ms to the main_loop
> timer(MainPid) ->
>        receive
>                shutdown ->
>                        shutdown
>        after 1000 ->
>                MainPid ! compute,
>                timer(MainPid)
>        end.
>
> compute() ->
>        io:format("ping~n").
>
> -----------------------------------
>
> which one would you choose, or is there any other way to perform this better?
>
> thank you,
>
> r.
>
> ________________________________________________________________
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>



-- 
--Hynek (Pichi) Vychodil

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