How to make Processes Run Parallel in Erlang? - erlang

Question

startTrains() ->
TotalDist = 100,
Trains = [trainA,trainB ],
PID = spawn(fun() ->
train(1,length(Trains)) end),
[ PID ! {self(),TrainData,TotalDist} || TrainData <- Trains],
receive
{_From, Mesg} ->
error_logger:info_msg("~n Mesg ~p ~n",[Mesg])
after 10500 ->
refresh
end.
so, I created Two Processes named trainA, trainB. I want to increment these process by 5 till it gets 100. I made different processes to make each of the train (process) increments its position parallely. But I was surprised to get the output sequentially i.e process trainA ends then process trainB starts. But I want to increment themselves at simultaneously.
I want to run processes like this
trainA 10 trainB 0
trainA 15 trainB 5
....
trainA 100 trainB 100
but I m getting
trainA 0
....
trainA 90
trainA 95
trainA 100
trainA ends
trainB 0
trainB 5
trainB 10
.....
trainB 100
How to make the processes run parallel/simultaneously? Hope you get my Q's. Please help me.

Answer 1

You spawn only one process initialized by function train/2. Your presented code is incomplete so I can only guess but I think your code is wrong because you have only one train process. For inspiration:
-module(trains).
-export([startTrains/0]).
startTrains() ->
TotalDist = 100,
Names = [trainA,trainB ],
Self = self(),
Trains = [spawn_link(fun() ->
train(Name, Self) end) || Name <- Names],
[ Train ! {start, Self, 0, TotalDist} || Train <- Trains],
ok = collectResults(Names).
collectResults([]) -> ok;
collectResults(Trains) ->
receive
{stop, Name, Pos, Length} ->
io:format("~p stops at ~p (~p)~n", [Name, Pos, Length]),
collectResults(Trains -- [Name]);
Msg ->
io:format("Supervisor received unexpected message ~p~n", [Msg]),
collectResults(Trains)
after 10500 -> timeout
end.
train(Name, Sup) ->
receive
{start, Sup, Pos, Length} -> run_train(Name, Sup, Pos, Length);
Msg ->
io:format("~p received unexpected message ~p~n", [Name, Msg]),
train(Name, Sup)
end.
run_train(Name, Sup, Pos, Length)
when Pos < Length ->
receive after 500 ->
NewPos = Pos + 5,
io:format("~p ~p~n", [Name, Pos]),
run_train(Name, Sup, NewPos, Length)
end;
run_train(Name, Sup, Pos, Length) ->
Sup ! {stop, Name, Pos, Length}.
But if I would think it seriously I should look to gen_fsm and OTP principles. But in your current stage keep play with erlang primitives to take better feeling first.