[Lemon-user] graphCopy() does not overwrite target

[Kovács Péter]

Dear Allan,

> Hello,
> 
> I've been using graphCopy() to backup and restore a graph's state and 
> noticed that rather than overwriting the contents of the target graph it 
> seems to append the source graph's contents to it somehow.

Yes, it seems to be true.

> This does not 
> seem to be the intended behavior and is not mentioned in the 
> documentation.

I think, graph copying must clear the target graph before adding nodes 
and edges to it. However, it is not performed currently.

I made a bug report ticket about it:
http://lemon.cs.elte.hu/trac/lemon/ticket/371

> There also does not seem to be a built-in way to clear 
> the contents of a graph.

All the graph structures that can be used as a target graph provide a 
clear() function to remove all nodes and edes.

> As a work around I had to use something like 
> the following code segment:
> 
> while(countNodes(graph) != 0) {
>   ListGraph::NodeIt n(graph);
>   graph.erase(n);
> }
> graphCopy(backupGraph, graph).nodemap(backupNodemap, nodemap).run()

Consider to use:

graph.clear();
graphCopy(backupGraph, graph).nodemap(backupNodemap, nodemap).run();

(It seems to be a bug that clear() is necessary, but you can use this 
work around until the bug is fixed.)

> I am also aware of the Snapshot class for saving the state of a graph 
> but because I contract nodes in my graph it would invalidate the snapshot.

The Snapshot feature cannot handle this case, use graphCopy().

Regards,
Peter