[erlang-questions] What's wrong with this code?

Edward Wang <>
Sun Feb 20 09:01:12 CET 2011


 {parent, Pid} ->
   io:format("---> ~p found parent @ ~p~n", [Id, Pid]),
   loop(#state{id=Id, parentId=ParentId, ppid=Pid});

should be:

 {parent, Pid} ->
   io:format("---> ~p found parent @ ~p~n", [Id, Pid]),
   loop(S#state{id=Id, parentId=ParentId, ppid=Pid});

On Sun, Feb 20, 2011 at 3:15 PM, Marcel Meyer <>wrote:

> Oh sorry, that I did in the shell, to test.
>
> Here is the output:
>
> 100> node1:test().
> I am 1 (<0.329.0>) and my parent Id is 0
> I am 2 (<0.330.0>) and my parent Id is 1
> I am 3 (<0.331.0>) and my parent Id is 2
> started
> Query: am I 1's parent? false. New state:{state,1,<0.329.0>,0,undefined,[]}
> Query: am I 1's parent? false. New state:{state,2,<0.330.0>,1,undefined,[]}
> Query: am I 1's parent? false. New state:{state,3,<0.331.0>,2,undefined,[]}
> Query: am I 2's parent? true. New state:{state,1,<0.329.0>,0,undefined,
>                                          [{2,<0.330.0>}]}
> Query: am I 2's parent? false. New state:{state,2,<0.330.0>,1,undefined,[]}
> Query: am I 2's parent? false. New state:{state,3,<0.331.0>,2,undefined,[]}
> Query: am I 3's parent? false. New state:{state,3,<0.331.0>,2,undefined,[]}
> Query: am I 3's parent? false. New state:{state,1,<0.329.0>,0,undefined,
>                                           [{2,<0.330.0>}]}
> Query: am I 3's parent? true. New state:{state,2,<0.330.0>,1,undefined,
>                                          [{3,<0.331.0>}]}   % <------------
> this shows that the record's children value got updated with {3, <Pid of 3>}
> ---> 3 found parent @ <0.330.0>
> ---> 2 found parent @ <0.329.0>
> 101> pid(0,329,0) ! get_children.
> {state,1,<0.329.0>,0,undefined,[{2,<0.330.0>}]}   %<-------- this is
> correct: Node 2 found his parent, Node 1
> get_children
> 102> pid(0,330,0) ! get_children.
> {state,2,undefined,1,<0.329.0>,[]}     %<------- this is incorrect: Node 2
> does not know of Node 3
> get_children
>
>
>
> On Sun, Feb 20, 2011 at 2:01 AM, Edward Wang <> wrote:
>
>> But I didn't see where you call (node2 ! get_children).
>>
>> -Ed
>>
>>
>> On Sun, Feb 20, 2011 at 2:33 PM, Marcel Meyer <>wrote:
>>
>>> Dear Community,
>>> I am having a hard time debugging a certain behaviour in the following
>>> code.
>>> The general premise of this code is that a bunch of these 'nodes' get
>>> spawned, and then
>>> they have to figure out how they are related. (So here in the test()
>>> func,
>>> node 1 is the parent of 2, and 2 the parent of 3.)
>>> When you start a node, you give it an Id and tell it who it's Parent is.
>>> Using io:formats, I confirm that everyone finds a parent (except for the
>>> root).
>>> But when I ask node 2 for his children (node2 ! get_children), he has
>>> none,
>>> and this is incorrect.
>>> The current io:format of the state acknowledges that of a moment there,
>>> node2 did have children.
>>>
>>> Thank you in advance,
>>> Marcel
>>>
>>>
>>> -module(node).
>>> -record(state, {id, pid, parentId, ppid, children=[]}).
>>> -compile([export_all]).
>>>
>>> test() ->
>>>    Nodes = [{1, 0}, {2, 1}, {3, 2}],
>>>    Pids = [spawn(?MODULE, start, [Id, ParentId]) || {Id, ParentId} <-
>>> Nodes],
>>>    [Pid ! {connect, Pids} || Pid <- Pids],
>>>    started.
>>>
>>> start(Id, ParentId) ->
>>>    io:format("I am ~p (~p) and my parent Id is ~p~n", [Id, self(),
>>> ParentId]),
>>>    loop(#state{id=Id, pid=self(), parentId=ParentId}).
>>>
>>> loop(#state{id=Id, parentId=ParentId}=S) ->
>>>    receive
>>> {connect, Pids} ->
>>>    find_parent(S, Pids),
>>>    loop(S);
>>>  {parent, Pid} ->
>>>    io:format("---> ~p found parent @ ~p~n", [Id, Pid]),
>>>    loop(#state{id=Id, parentId=ParentId, ppid=Pid});
>>>  {id, TestId, ChildId, Pid1}  ->
>>>
>>>    NewState = case TestId == Id of
>>>   true ->
>>>       Pid1 ! {parent, self()},
>>>       Children = S#state.children,
>>>       S#state{children=[{ChildId, Pid1} | Children]};
>>>   _ ->
>>>       S
>>>       end,
>>>    io:format("Query: am I ~p's parent? ~p. New state:~p~n", [ChildId,
>>> TestId==Id, NewState]),
>>>    loop(NewState);
>>>
>>> get_children ->
>>>    io:format("~p~n", [S]),
>>>    loop(S);
>>>  _ ->
>>>    loop(S)
>>>    end.
>>>
>>> find_parent(#state{parentId=ParentId, id=Id}, Pids) ->
>>>    [Pid ! {id, ParentId, Id, self()} || Pid <- Pids].
>>>
>>
>>
>


More information about the erlang-questions mailing list