title: concatenate two lists of string and attach index in r
body: <p>Here is my sample:</p>
  
  <pre><code>stringa = c("a","b","c")
  stringb = c("high","low","average")
  index = c(1,2,3)
  
  data &lt;- data.frame(stringa,stringb,index)
  </code></pre>
  
  <p>I would like to concatenate stringa and stringb, and attach the corresponding index in another column. For example, the first row of the result should be "a high" with index "1".</p>
  
  <p>Now I have used this function to concatenate the two strings:</p>
  
  <pre><code>c(outer(a, b, paste))
  </code></pre>