title: cross2() vs cross_df() to make a tibble with purrr
body: <p>I think I am misunderstanding how <code>purrr::cross_df()</code> is supposed to work. </p>
  
  <p>As an example, lets say that I have 20 stores and 104 weeks. I would like to make a tibble that has stores and weeks as the columns. The resulting tibble should be 2 columns by 20*104 rows to represent all store weeks.</p>
  
  <p>The way I approached this was</p>
  
  <pre><code>stores &lt;- 1:20
  weeks &lt;- 1:104
  cross2(stores,weeks) 
  </code></pre>
  
  <p>the problem is that I would like this to be a tibble not lists. I tried using replacing <code>cross</code> with <code>cross_df</code>, but I did not have any success. </p>