Re: subgraph error (errata)

*To*: mathgroup at smc.vnet.net*Subject*: [mg125916] Re: subgraph error (errata)*From*: David Bailey <dave at removedbailey.co.uk>*Date*: Sat, 7 Apr 2012 05:54:35 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*References*: <jlh0l6$t5s$1@smc.vnet.net>

On 04/04/2012 09:27, dan jack wrote: > I write an errata for the previous message: > > I obtain the error in this case: > > gg = Graph[{11 -> 2, 11 -> 3, 4 -> 11, 5-> 11, 11 -> 11}] > Subgraph[gg, 1 ->_ ] > > Join::headsd: Expression {11,2} at position 1 is expected to have head > List for all subexpressions through level 11. > DeleteDuplicates::list: List expected at position 1 in > DeleteDuplicates[Join[{11,2},{11,3},11]]. > > but not in this case: > gg = Graph[{1 -> 2, 1 -> 3, 4 -> 1, 5-> 1, 1-> 1}] > > so it seems that the only difference is the vertex number: 1 works but > 11 do not works ...very strange. > > Obviously the arrows are the correct Directed Edges (ESC de ESC)... > > any help or solidarity comment is appreciated :-) > I think when you specify a subgraph using a pattern, you have to specify edges using the DirectedEdge construct. This actually prints rather like -> but there is a little blob at the left end (confusing eh?) Thus the correct syntax is: gr = Graph[{1 -> 2, 1 -> 3, 4 -> 1, 5 -> 1, 1 -> 1}] Subgraph[gr, DirectedEdge[1, _]] David Bailey http://www.dbaileyconsultancy.co.uk