[Lemon-user] is there a way to write a subgraph?

[John Lagerquist]

My graphs are quite small, 10 nodes on average.

On Fri, Dec 30, 2016 at 1:47 PM, Kovács Péter <kpeter at inf.elte.hu> wrote:

> Hi All,
>
> Thanks for the corrections. I'm terribly sorry for the stupid mistake. So
> I suggest passing references, but of course, not for local variables. :)
>
> Péter
>
>
>
>     FilterNodes<ListDigraph>& get_subgraph(void)
>>     {
>>         FilterNodes<ListDigraph> subgraph;
>>         ...
>>         return g;
>>     }
>>
>> This is a bad idea as it creates a reference to a temporary variable. I
>> think the real problem is that the FilterNodes is initiated with a local
>> map, which is destroyed when the execution leaves the function scope.
>>
>> Balazs
>>
>> On Fri, Dec 30, 2016 at 9:17 PM, Kovács Péter <kpeter at inf.elte.hu
>> <mailto:kpeter at inf.elte.hu>> wrote:
>>
>>     Hi John,
>>
>>     You should avoid passing graphs (and other large data structures) as
>>     values between functions. Use references or pointers instead. For
>>     example:
>>
>>     FilterNodes<ListDigraph>& get_subgraph(void)
>>     {
>>         FilterNodes<ListDigraph> subgraph;
>>         ...
>>         return g;
>>     }
>>
>>     Note the '&' sign in the return type of the function. (If its
>>     meaning is not clear, read some C++ tutorials about references and
>>     pointers.)
>>
>>     Regards,
>>     Péter
>>
>>
>>
>>         Péter,
>>
>>         It turns out that the issue I am having is related to trying to
>>         return
>>         a FilterNodes<ListDigraph> issue from a function.  If
>>         the FilterNodes<ListDigraph> object is local I can iterate
>>         through the
>>         nodes as expected.  If the object is returned from a function i.e.
>>
>>         FilterNodes<ListDigraph> get_subgraph(void)
>>         {
>>         }
>>
>>         void another_function()
>>         {
>>              auto x = get_subgraph();
>>
>>              for (FilterNodes<ListDigraph>::NodeIt n(x); n != INVALID;
>> ++n)
>>              ...
>>         }
>>
>>         I get an exception when trying to iterate through the nodes.
>>
>>         John
>>
>>
>>
>>
>>         On Thu, Dec 29, 2016 at 5:28 PM, Kovács Péter
>>         <kpeter at inf.elte.hu <mailto:kpeter at inf.elte.hu>
>>         <mailto:kpeter at inf.elte.hu <mailto:kpeter at inf.elte.hu>>> wrote:
>>
>>             Hi John,
>>
>>             What kind of write method are you looking for? Could you
>>         send a code
>>             snippet that works for the graph, but does not apply to the
>>         subgraph?
>>
>>             Regards,
>>             Péter
>>
>>
>>                 I have a subgraph as follows:
>>
>>                 FilterNodes<ListDigraph> sub(*graph, filter);
>>
>>                 There doesn't appear to be a write method for the
>>         subgraph as
>>                 there is
>>                 for the graph.
>>
>>                 Suggestions?
>>
>>
>>                 --
>>                 John Lagerquist
>>                 Chief Engineer
>>                 RallyTronics LLC
>>                 801-866-5981
>>
>>
>>
>>
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>>
>>
>>
>>
>>
>>         --
>>         John Lagerquist
>>         Chief Engineer
>>         RallyTronics LLC
>>         801-866-5981
>>
>>
>>
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>>
>>
>>
>


-- 
John Lagerquist
Chief Engineer
RallyTronics LLC
801-866-5981
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