title: R igraph: label vertex by condition
body: <p>I have a graph g with a set of vertices and a list with the names of some "special" vertices.
  I want the graph to display a label with the vertex name ONLY for these special vertices.</p>
  
  <p>I tried something like this:</p>
  
  <pre><code>plot(g, vertex.size = 4, vertex.label = ifelse(V(g) %in% usernames, V(g)$label, ""), asp = F)
  </code></pre>
  
  <p>But apparently I'm missing something, because the predicate never enter TRUE.
  Also, it seems like V(g)$label results in the following error:</p>
  
  <blockquote>
    <p>Error in ans[test &amp; ok] &lt;- rep(yes, length.out = length(ans))[test &amp; ok] : replacement has length zero</p>
  </blockquote>
  
  <p>Your help will be appreciated! :)</p>