title: convert a numbers in a data.frame in significance stars in R
body: <p>I am looking for a function or a library to convert a number in the significance stars in a data.frame.</p>
  
  <p>Using the function <em>add.significance.stars()</em> of <a href="http://www.inside-r.org/packages/cran/pander/docs/add.significance.stars" rel="nofollow">pander package</a> i can convert numbers in significance stars. I wish to build a function in order to convert the number is a data.frame in   significance stars. Example</p>
  
  <pre><code>mydata &lt;-  data.frame("Su1"=c(1,1,0.0004),"Su2"=c(NA,1,0.03),"Su3"=c(NA,NA,0.007))
  rownames(mydata) &lt;- c("Su2","Su3","Su4")
  
  
  mydata
         Su1  Su2   Su3
  Su2 1.0000   NA    NA
  Su3 1.0000 1.0000   NA
  Su4 0.0004 0.03 0.007
  </code></pre>
  
  <p>with <em>add.significance.stars()</em>  i wish a table like:</p>
  
  <pre><code>mydata
             Su1  Su2   Su3
      Su2 "_1.0000_"   NA    NA
      Su3 "_1.0000_" "_1.0000_"    NA
      Su4 "_0.0004_ * * *" "_0.03_ *" "_0.007_ * *"
  </code></pre>