title: Is there a _merge indicator available after a merge?
body: <p>Is there a way to get the equivalent of a <code>_merge</code> indicator variable after a merge in <code>dplyr</code>? </p>
  
  <p>Something similar to <strong>Pandas'</strong> <code>indicator = True</code> option that essentially tells you how the merge went (how many matches from each dataset, etc).</p>
  
  <p>Here is an example in <code>Pandas</code></p>
  
  <pre><code>import pandas as pd
  
  df1 = pd.DataFrame({'key1' : ['a','b','c'], 'v1' : [1,2,3]})
  df2 = pd.DataFrame({'key1' : ['a','b','d'], 'v2' : [4,5,6]})
  
  match = df1.merge(df2, how = 'left', indicator = True)
  </code></pre>
  
  <p>Here, after a <code>left join</code> between <code>df1</code> and <code>df2</code>, you want to immediately know how many rows in <code>df1</code> found a match in <code>df2</code> and how many of them did not</p>
  
  <pre><code>match
  Out[53]: 
    key1  v1   v2     _merge
  0    a   1  4.0       both
  1    b   2  5.0       both
  2    c   3  NaN  left_only
  </code></pre>
  
  <p>and I can tabulate this <code>merge</code> variable: </p>
  
  <pre><code>match._merge.value_counts()
  Out[52]: 
  both          2
  left_only     1
  right_only    0
  Name: _merge, dtype: int64
  </code></pre>
  
  <p>I don't see any option available after a, say, left join in <code>dplyr</code></p>
  
  <pre><code>key1 = c('a','b','c')
  v1 = c(1,2,3)
  key2 = c('a','b','d')
  v2 = c(4,5,6)
  df1 = data.frame(key1,v1)
  df2 = data.frame(key2,v2)
  
  &gt; left_join(df1,df2, by = c('key1' = 'key2'))
    key1 v1 v2
  1    a  1  4
  2    b  2  5
  3    c  3 NA
  </code></pre>
  
  <p>Am I missing something here?
  Thanks!</p>