title: Forcing NAs to be shown in regression output from xtable
body: <p><code>xtable</code> defaults to removing <code>NA</code> values from regression output. I would like to force those coefficients to show up in the output, with their <code>NA</code> values. There is an argument <code>NA.string</code> to <code>print.xtable</code> that I would have thought would provide this functionality, but it does not do anything with <code>lm</code> objects at least.</p>
  
  <p>For example,</p>
  
  <pre><code>df &lt;- data.frame(A=1:3, B=2:4, C=2:4)
  reg &lt;- lm(A ~ B + C, df)
  print(xtable(reg), NA.string = "Foo") 
  </code></pre>
  
  <p>creates the following output</p>
  
  <pre><code>% latex table generated in R 3.0.2 by xtable 1.7-1 package
  % Wed Feb  5 11:04:09 2014
  \begin{table}[ht]
  \centering
  \begin{tabular}{rrrrr}
     \hline
   &amp; Estimate &amp; Std. Error &amp; t value &amp; Pr($&gt;$$|$t$|$) \\ 
    \hline
  (Intercept) &amp; -1.0000 &amp; 0.0000 &amp; -Inf &amp; 0.0000 \\ 
     B &amp; 1.0000 &amp; 0.0000 &amp;  Inf &amp; 0.0000 \\ 
     \hline
  \end{tabular}
  \end{table}
  </code></pre>
  
  <p>while what I would like to have output is </p>
  
  <pre><code>% latex table generated in R 3.0.2 by xtable 1.7-1 package
  % Wed Feb  5 11:04:09 2014
  \begin{table}[ht]
  \centering
  \begin{tabular}{rrrrr}
     \hline
   &amp; Estimate &amp; Std. Error &amp; t value &amp; Pr($&gt;$$|$t$|$) \\ 
    \hline
  (Intercept) &amp; -1.0000 &amp; 0.0000 &amp; -Inf &amp; 0.0000 \\ 
     B &amp; 1.0000 &amp; 0.0000 &amp;  Inf &amp; 0.0000 \\ 
     C &amp; NA &amp; NA &amp; NA &amp; NA \\ %%% This is the new line
     \hline
  \end{tabular}
  \end{table}
  </code></pre>