title: Searching for a value in a Panda Data Frame returns inconsistent results
body: <p>I'm trying to search for a match in a Panda Data Frame.  I find that the results are not consistent or I'm using the wrong type of code.  I'm using the in command which is not always consistent.  If I compare values (see example) it works.  Is there an issues with the in command?</p>
  
  <p>Code example:</p>
  
  <pre><code>import pandas as pd
  report = pd.DataFrame(columns = (['col1','col2']))
  report
  i = 0
  while i &lt; 100:
      a = str(i)
      addthis = pd.Series({'col1':a,'col2':'AG100'})
      report = report.append(addthis,ignore_index=True)
      i = i + 1
  ###this will find a match but not 100 of the time%
  i = 0
  while i &lt; len(report):
      if str(i) in str(report[0:len(report)]):
          print('found match on ',i)
      else:
          print('No match found on ',i)
      i = i + 1
  ###this will find a match 100of the time%    
  i = 0
  while i &lt; len(report):
      if str(i) == report.ix[i,0]:
          print('found match on ',i)
      else:
          print('No match found on ',i)
      i = i + 1
  </code></pre>