title: get the distinct value of a column which each cell has multiple value R
body: <p>my data is look like this :</p>
  
  <pre><code>id  book_id                     numberofbook_id
   1     ["19167120","237494310","195166798"]    3      
   2     ["19167120","237494310"]                2       
   3     []                                      0
  </code></pre>
  
  <p>what I am trying to do is first to have a diffrent datafrane which has diffrent book_id as single</p>
  
  <pre><code> book_id
   "19167120"
   "237494310"
   "195166798"
  </code></pre>
  
  <p>and then based on this , group the id </p>
  
  <pre><code> book_id       id      numberofid
   "19167120"    [1,2]       2
   "237494310"   [1,2]       2
   "195166798"   [1]         1
  </code></pre>
  
  <p>Ps :I was thinking of first combine all the cell in to one by <code>unlist</code> and then use <code>unique</code> function to get the unique one and then put them into a column.but <code>unlist</code> wasn't the answer in here.</p>