title: How to create a data.frame with a unknown number of columns?
body: <p>I would like to create, in a function, a loop to create a data.frame with a variable number of columns.</p>
  
  <p>With something like :</p>
  
  <pre><code>a = c("a","b")
  b = c(list(1,2,3), list(4,5,6))
  data.frame(a,b)
  </code></pre>
  
  <p>I would like to get a data-frame like :</p>
  
  <pre><code>a 1 2 3
  b 4 5 6
  </code></pre>
  
  <p>Instead of I obtain:</p>
  
  <pre><code>a  1  2  3  4  5  6
  b  1  2  3  4  5  6
  </code></pre>
  
  <p>Thank you!</p>
  
  <p>PS : I also tried with rbind, but it doesn't work...</p>