python - 比较 pandas 中的两列 - 错误 : The truth value of a Series is ambiguous

Question

我有一个名为 m 的数据框，它包含三列 a、b、c。我想将 b 列与 a、c 进行比较，并将值放入 d 列中。

a   b   c
1   5   7
2   7   8
3   1   9
4   8   6

But when I try this in the following code, I got the error as

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

Code:

if m['b'] < m['a']:
    m['d'] = m['a']
elif m['d'] > m['c']:
    m['d'] = m['c'] 
else:
    m['d'] = m['b']
m

有办法做到这一点吗？

Answer 1

您可以使用双 numpy.where :

m['d'] = np.where(m['b'] < m['a'], m['a'], np.where(m['b'] > m['c'], m['c'], m['b']) )
print m    
   a  b  c  d
0  1  5  7  5
1  2  7  8  7
2  3  1  9  3
3  4  8  6  6