title: Pick element in sublist and make it a column in a data frame
body: <p>I have a list of 2000 sublists each with 7 elements each of the same form. The fifth of these elements is a numeric vector with 40 elements. My aim is to get a data frame with 2000 rows and 40 columns, where each column represents one of the 40 elements in the fifth element of the 2000 sublists. Here is a downsized example:</p>
  
  <pre><code>sub_list_1 &lt;- list(a = c(1:5), b = "b")
  sub_list_2 &lt;- list(a = c(6:10), b = "b")
  sub_list_3 &lt;- list(a = c(11:15), b = "b")
  top_list &lt;- list(sub_list_1, sub_list_2, sub_list_3)
  </code></pre>
  
  <p>Now, suppose I want a data frame with 5 columns and three rows, where each columns represents one of the five elements in <code>a</code> and each row represents one sublist. Thus, the output should be</p>
  
  <pre><code> output
       [,1] [,2] [,3] [,4] [,5]
  [1,]    1    2    3    4    5
  [2,]    6    7    8    9   10
  [3,]   11   12   13   14   15
  </code></pre>