title: unsupported operand in pandas dataframe operation
body: <p>I have pandas DataFrame. I would like to get single value from a column based on a condition involving two another column. I am looking for the value from the column3 for which is the biggest distance in the column1 and 2.</p>
  
  <p>I build the simple example which works:</p>
  
  <pre><code>d = pd.DataFrame({'c1':[.1,3,11.3],'c2':[3,6,.6],'c3':[8,.8,10.9]})
  print'data d=\n%s\n' % d                                               
  x = float(d.c3[abs(d.c1-d.c2)==max(abs(d.c1-d.c2))].values)
  print 'the value of x= \n%s\n' % x
  </code></pre>
  
  <p>The output from this example is as I expect:</p>
  
  <pre><code>     c1   c2    c3
  0   0.1  3.0   8.0
  1   3.0  6.0   0.8
  2  11.3  0.6  10.9
  
  the value of x= 
  10.9
  </code></pre>
  
  <p>I try to apply exactly the same logic to my original problem with large dataframe inside a class. The code is:</p>
  
  <pre><code>yInit = float(self.DenFrame.Depth[abs(self.DenFrame.Hper-self.DenFrame.Vper)==max(abs(self.DenFrame.Hper-self.DenFrame.Vper))].values)
  </code></pre>
  
  <p>but this code produce an error:</p>
  
  <pre><code>...
    File "C:\Python27\lib\site-packages\pandas-0.9.0-py2.7-win32.egg\pandas\core\series.py", line 73, in wrapper
  return Series(na_op(self.values, other.values),
    File "C:\Python27\lib\site-packages\pandas-0.9.0-py2.7-win32.egg\pandas\core\series.py", line 59, in na_op
  result[mask] = op(x[mask], y[mask])
  TypeError: unsupported operand type(s) for -: 'str' and 'str'
  </code></pre>
  
  <p>I found in <a href="https://stackoverflow.com/questions/14408122/unsupported-operand-types-for-numpy-ndarray-and-numpy-float64">here</a> that there could be a problem with type of the columns but Depth is type <code>numpy.float64</code> Hper is type <code>float</code> Vper is type <code>float</code> so I understand how it can apply to my problem.</p>
  
  <p>I don't know from this point what to do as I understand the same code works in one case but not in another and I cannot spot the problem.</p>