title: add one column (subtraction) to a dataframe with python
body: <p>I try to add one column in a dataframe df2  which contains the value 0 <code>if(df2['P_ACT_KW'] - df2['P_SOUSCR']) &lt; 0 else  df2['P_ACT_KW']- df2['P_SOUSCR']</code>.</p>
  
  <pre><code>if (df2['P_ACT_KW'] - df2['P_SOUSCR']) &lt;0:
      df2['depassement']=0
  else:
      df2['depassement']= (df2['P_ACT_KW'] - df2['P_SOUSCR']) 
  </code></pre>
  
  <p>I got this error message : </p>
  
  <blockquote>
    <p>ValueError                                Traceback (most recent call
    last)  in ()
    ----> 1 if (df2['P_ACT_KW'] - df2['P_SOUSCR']) &lt;0:
          2     df2['depassement']=0
          3 else:
          4     df2['depassement']= (df2['P_ACT_KW'] - df2['P_SOUSCR'])</p>
    
    <p>C:\Users\Demonstrator\Anaconda3\lib\site-packages\pandas\core\generic.py
    in <strong>nonzero</strong>(self)
        890         raise ValueError("The truth value of a {0} is ambiguous. "
        891                          "Use a.empty, a.bool(), a.item(), a.any() or a.all()."
    --> 892                          .format(self.<strong>class</strong>.<strong>name</strong>))
        893 
        894     <strong>bool</strong> = <strong>nonzero</strong></p>
    
    <p>ValueError: The truth value of a Series is ambiguous. Use a.empty,
    a.bool(), a.item(), a.any() or a.all().</p>
  </blockquote>
  
  <p>Any idea please?</p>
  
  <p>Thank you</p>