title: Pandas/Python: Set value of one column based on value in another column
body: <p>I need to set the value of one column based on the value of another in a Pandas dataframe. This is the logic:</p>
  
  <pre><code>if df['c1'] == 'Value':
      df['c2'] = 10
  else:
      df['c2'] = df['c3']
  </code></pre>
  
  <p>I am unable to get this to do what I want, which is to simply create a column with new values (or change the value of an existing column: either one works for me). </p>
  
  <p>If I try to run the code above or if I write it as a function and use the apply method, I get the following:</p>
  
  <pre><code>ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().
  </code></pre>