title: convert data.frame to ff
body: <p>I would like to convert a data.frame to a ff object, with as.ffdf as described <a href="https://stackoverflow.com/questions/15787221/how-can-i-apply-ffdf-to-non-atomic-data-frames">here</a></p>
  
  <pre><code>df.apr=as.data.frame(df.apr) # from data.table to data.frame
  cols=df.apr[1,] 
  cols=sapply(cols,class)
  df_apr=as.ffdf(df.apr,vmode=cols)
  </code></pre>
  
  <p>gives an error:</p>
  
  <pre><code>Error in ff(initdata = initdata, length = length, levels = levels, ordered = ordered,
  : vmode 'numeric' not implemented
  </code></pre>
  
  <p>without the 'vmode' argument, the following error is given:</p>
  
  <pre><code>Error in ff(initdata = initdata, length = length, levels = levels, ordered = ordered, 
  : vmode 'character' not implemented
  </code></pre>
  
  <p>writing away to a table and then reading directly into ff works however:</p>
  
  <pre><code>write.table(df.apr,file='df_apr.txt',sep='\t',row.names=F)
  df.apr.ff=read.table.ffdf(file='df_apr.txt',header=F,VERBOSE=T)
  </code></pre>
  
  <p>but this is time consuming [and clumsy]. 
  is there a better way?</p>