if df['a'] > 0:
    df['c'] = df['a']
if df['b'] > 0:
    df['c'] = min(df['a'],df['b'])

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-17-00945ffeddda> in <module>()
----> 1 if df['a'] > 0:
      2     df['c'] = df['a']
      3 if df['b'] > 0:
      4     df['c'] = min(df['a'],df['b'])

/opt/python/python35/lib/python3.5/site-packages/pandas/core/generic.py in __nonzero__(self)
   1574         raise ValueError("The truth value of a {0} is ambiguous. "
   1575                          "Use a.empty, a.bool(), a.item(), a.any() or a.all()."
-> 1576                          .format(self.__class__.__name__))
   1577
   1578     __bool__ = __nonzero__

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().