title: Frequency of each unique combination in data frame
body: <p>In a dataset (N=6000) I would like to analyse how often combinations of (15 dummy)variables occur. </p>
  
  <pre><code>ID       Var1        Var2       Var3    Var15
  
  1          1          0          0        1
  
  2          0          1          1        1
  
  3          1          0          0        0
  
  6000       1          0          0        0
  </code></pre>
  
  <p>For this example what I would like to see is that the combination 1000 occurs twice, 1001 occurs once, and 0111 occurs also once. </p>
  
  <p>The only way I can think up is compute a variable for each possible combination...</p>
  
  <p>Is there an elegant and efficient way to do this?</p>
  
  <p>I have read through 
  <a href="https://stackoverflow.com/questions/46837639/how-to-summarize-all-possible-combinations-of-variables">How to summarize all possible combinations of variables?</a> But that is a slightly different question and <a href="https://stackoverflow.com/questions/5143575/aggregating-tally-counters">Aggregating Tally counters</a> transcends my knowledge (but if that is the answer to my question, I will go through it).</p>