title: list of character vectors of unequal length to data.frame
body: <p>I have a named list that looks like this:</p>
  
  <pre><code>&gt; head(pathways)
  $&lt;NA&gt;
  NULL
  $`2`
  [1] "hsa04610"
  $`9`
  [1] "hsa00232" "hsa00983" "hsa01100"
  $`10`
  [1] "hsa00232" "hsa00983" "hsa01100"
  $&lt;NA&gt;
  NULL
  $&lt;NA&gt;
  NULL
  </code></pre>
  
  <p>To describe it more formerly. The name of each list is an id number, and the entries of each element of the character vector that are elements of the list is another id number. I can filter out the <code>$&lt;NA&gt;</code> entries easily with <code>is.na()</code>, but then I want to change the rest so it would look like:</p>
  
  <pre><code>id   another_id
  2    hsa04610   
  9    hsa00232   
  9    hsa00983   
  9    hsa01100   
  10   hsa00232  
  10   hsa00983
  10   hsa01100
  
  
  &gt; dput(test)
  structure(list(`NA` = NULL, `2` = "hsa04610", `9` = c("hsa00232", 
  "hsa00983", "hsa01100"), `10` = c("hsa00232", "hsa00983", "hsa01100"
  ), `NA` = NULL, `NA` = NULL), .Names = c(NA, "2", "9", "10", 
  NA, NA))
  </code></pre>
  
  <p>Any ideas?</p>