title: iGraph creating subgraph
body: <p>I have the following problem:<br>
  I have the following graph: <code>g6</code> which does not include <code>isolate vertices</code>. The vertices   are mainly grouped into two <code>ethnic groups</code>, given as attribute, namely: <code>mestizo and   saraguro</code>.<br>
  So I run the <code>subgraph</code>option for saraguros as follows:   </p>
  
  <blockquote>
    <p>g2saraguro &lt;- subgraph(g6, which(V(g6)$ethnic=="saraguro"))<br>
        summary(g2saraguro)  </p>
  </blockquote>
  
  <p>which did work:  </p>
  
  <blockquote>
    <p>Vertices: 166<br>
        Edges: 330<br>
        Directed: TRUE<br>
        No graph attributes.<br>
        Vertex attributes: ii, village, parroquia, canton, provinz, kindR, ethnic,   situation, researcha.<br>
        Edge attributes: question, activity.  </p>
  </blockquote>
  
  <p>However, if I try the same with <code>mestizo</code>, there is a proble, since it just identify:  </p>
  
  <blockquote>
    <p>s2mestizo &lt;- subgraph(g6,V(g6)$ethnic=="mestizo")<br>
        summary(s2mestizo)<br>
        Vertices: 2<br>
        Edges: 3<br>
        Directed: TRUE<br>
        No graph attributes.<br>
        Vertex attributes: ii, village, parroquia, canton, provinz, kindR, ethnic,   situation, researcha.<br>
        Edge attributes: question, activity.  </p>
  </blockquote>
  
  <p>but in the reality that is no the case, if I see: <code>V(g6)$ethnic</code>, <code>mestizo</code> are the  majority <code>!!</code><br>
  I really don´t know where the problem is, with <code>igraph 0.6</code> the procedure to form a  <code>subgraph</code> should be in that or a similar way, without <code>1-based indexing</code>.<br>
  thanks a lot for any help<br>
  regards<br>
  Vladimir  </p>