graphs: how to apply functions for GRAPH to DIGRAPH

Thanks, that works! In the meantime I inspected the DIMACS format and 
changed the keyword "arcs" to "edges". Then the graph is read as undirected.

Rolf

Andrej Vodopivec wrote:
> On Sat, May 8, 2010 at 7:59 AM, Rolf Fankhauser
> <rolf.fankhauser at gepdata.ch>  wrote:
>    
>> I defined a directed graph, exported and imported it in format DIMACS. That
>> works fine.
>> But if I want to check if the graph is planar with function is_planar, I get
>> the error "Argument 1 to 'IS_PLANAR' is not a graph: Structure [DIGRAPH].
>> Why is function is_planar not applicable to DIGRAPHs ? How can I convert my
>> directed graph to an undirected graph?
>>      
> The function underlying_graph converts directed graphs to undirected.
>
> HTH, Andrej
>