How to print out a value periodically in Erlang

Question

Im trying to simply get this Erlang program to print out a value periodically and I'm using this module to do so:

   -module(hot).
   -export([start/0,handle_info/2,  add/1]).

   start() ->
           Num = 0,
           timer:send_interval(1000, interval),
           {ok, Num}.

   handle_info(interval, Num) ->
          erlang:display(0),
          {noreply, Num}.

   add(X) ->
          X+2.

I'm referencing one of the answers from this post

I also tried using a modification from the top answer as follows:

init([]) ->
  Timer = erlang:send_after(1, self(), check),
  {ok, Timer}.

handle_info(check, OldTimer) ->
  erlang:cancel_timer(OldTimer),
  erlang:display(0),
  Timer = erlang:send_after(1000, self(), check),
  {noreply, Timer}.

Nothing prints out in either case. It just tells me something like {ok,<0.43.0>}. It seems that handle_info is never called at all.

of course. It would be better if you write what you are doing (which functions call) and what you expect to see on the display. — , Apr 17 '16 at 23:08
erlang:display(0). Just display 0. At this point it doesnt matter what it is, its just an experiment. I just call init() or start(). — Khaines0625, Apr 17 '16 at 23:16
You misunderstood me, I want to know what you want to get as a result. handle_info - is used for the handling of spontaneous messages in gen_server OTP. You have to process the message received from the timer. — , Apr 17 '16 at 23:23
I'm brand new to Erlang and I must just not understand whats going on here. Ultimately all I want is for a value to be printed onto the screen every second. Perhaps this is the entirely wrong approach? The message itself isn't something I really care about, it will always be the same message--display a value to the screen. — Khaines0625, Apr 17 '16 at 23:29
I just badly know English, so I want to make sure that the correctly understood the problem before answering. — , Apr 17 '16 at 23:34

score 4 · Accepted Answer · answered Apr 17 '16 at 23:35

Show the value of every second you can, for example, as follows:

-module(test).
-export([start/1]).

start(Num) when not is_number(Num)-> {error,not_a_number};
start(Num)->spawn(fun()->print(Num) end).

print(Num)->
    erlang:start_timer(1000, self(), []),
    receive
          {timeout, Tref, _} ->
            erlang:cancel_timer(Tref),
            io:format("~p~n",[Num]),
            if
                Num =:=0 -> io:format("Stop~n");
                true -> print(Num-1)
            end
    end.

For information about this function and its arguments, you can read the documentation. Such a method is more effecient than the use of the timer module.

How to print out a value periodically in Erlang

1 Answers1