title: Unique strings in a pandas dataframe
body: <p>I have following sample DataFrame <code>d</code> consisting of two columns 'col1' and 'col2'. I would like to find the list of unique names for the whole DataFrame d. </p>
  
  <pre><code>    d = {'col1':['Pat, Joseph', 
                   'Tony, Hoffman', 
                   'Miriam, Goodwin', 
                   'Roxanne, Padilla',
                   'Julie, Davis', 
                   'Muriel, Howell', 
                   'Salvador, Reese', 
                   'Kristopher, Mckenzie',
                   'Lucille, Thornton', 
                   'Brenda, Wilkerson'],
  
       'col2':['Kristopher, Mckenzie', 
               'Lucille, Thornton',
               'Pete, Fitzgerald; Cecelia, Bass; Julie, Davis', 
               'Muriel, Howell', 'Harriet, Phillips',
               'Belinda, Drake;David, Ford', 'Jared, Cummings;Joanna, Burns;Bob, Cunningham',
               'Keith, Hernandez;Pat, Joseph', 'Kristopher, Mckenzie', 'Lucille, Thornton']}
  
      df = pd.DataFrame(data=d)
  </code></pre>
  
  <p>For column col1 i can get it done by using function unique(). </p>
  
  <pre><code>df.col1.unique()
  array(['Pat, Joseph', 'Tony, Hoffman', 'Miriam, Goodwin',
         'Roxanne, Padilla', 'Julie, Davis', 'Muriel, Howell',
         'Salvador, Reese', 'Kristopher, Mckenzie', 'Lucille, Thornton',
         'Brenda, Wilkerson'], dtype=object)
  </code></pre>
  
  <blockquote>
  <pre><code>len(df.col1) 10 # total number of rows
  len(df.col1.unique())  9 # total number of unique rows
  </code></pre>
  </blockquote>
  
  <p>For col2 some of the rows have multiple names separated by a semicolon. e.g. <code>'Pete, Fitzgerald; Cecelia, Bass; Julie, Davis'</code>.</p>
  
  <p>How can I get the unique names from the col2 using vector operation? I am trying to avoid the for loop since the actual data set is large.</p>