title: cbind coerces a data frame to matrix
body: <p>I'm having trouble When using cbind. Prior to using cbind the object is a data.frame of two character vectors.  </p>
  
  <p>After I add a column using cbind, the data.frame object changes class to matrix.  I've tried as.vector, declaring h as an empty character vector, etc. but couldn't fix it.  Thank you for any suggestions and help.</p>
  
  <pre><code>output &lt;- data.frame(h = character(), st = character()) ## empty dataframe
  st &lt;- state.abb 
  h &lt;- (rep("a", 50))
  output &lt;- cbind(output$h, h) ## output changes to matrix class here
  output &lt;- cbind(output, st) ## adding a second column
  </code></pre>