title: Character vectors as ff objects in R
body: <p>I am trying to convert a standard (RAM) character vector to an ff object (vector). The code below returns an error:</p>
  
  <pre><code>&gt; as.ff(c('a', 'b'))
  Error in ff(initdata = initdata, length = length, levels = levels, ordered = ordered,: 
  vmode 'character' not implemented
  </code></pre>
  
  <p>This thread (<a href="https://stackoverflow.com/questions/17744525/r-difficulties-facing-with-read-csv-ffdf-physicalmode-and-virtualmode">https://stackoverflow.com/questions/17744525/r-difficulties-facing-with-read-csv-ffdf-physicalmode-and-virtualmode</a>) suggests that ff objects do not accept characters at all, only factors. Still, the below does not work:</p>
  
  <pre><code>&gt; as.ff(c('a', 'b'), vmode = 'factor')
  Error in ff(initdata = initdata, length = length, levels = levels, ordered = ordered,:
  vmode 'factor' not implemented
  </code></pre>
  
  <p>The list below does not include 'factors':</p>
  
  <pre><code>.vimplemented
  boolean   logical      quad    nibble      byte     ubyte     short    ushort 
   TRUE      TRUE      TRUE      TRUE      TRUE      TRUE      TRUE      TRUE 
  integer    single    double   complex      raw  character 
   TRUE      TRUE      TRUE     FALSE      TRUE     FALSE 
  </code></pre>
  
  <p>So is it possible at all to create an ff vector of characters?</p>