如何利用现有栏目在大熊猫中创建新栏目？

Question

我对Python非常陌生，我正在尝试使用预exisitng列(下面的代码)在我的熊猫df中创建一个列。

if ((df['a'] != -1) & (df['b'] == -1)):
    df['c'] = df['a']
elif ((df['a'] == -1 ) & (df['b'] != -1)):
    df['c'] = df['b']    
else: 
    df['c'] = 0

但是，我一直收到以下错误：

ValueError:系列的真值不明确。使用a.empty、a.bool()、a.item()、a.any()或a.all()。

当我添加'.any()‘时，它仍然没有按需要执行。我真的不干了吗？

Answer 1

df = pd.DataFrame( {'a': [-1, 1, 1, -1], 'b': [1, 1, -1, -1]})

df['c'] = 0
df.loc[(df['a'] != -1) & 
       (df['b'] == -1), 'c'] = df[(df['a'] != -1) & (df['b'] == -1)]['a'] 

df.loc[(df['a'] == -1) & 
       (df['b'] != -1), 'c'] = df[(df['a'] == -1) & (df['b'] != -1)]['b'] 

print (df)

输出：

   a  b  c
0 -1  1  1
1  1  1  0
2  1 -1  1
3 -1 -1  0

或

df['c'] = df.apply(lambda r: 
                   r['a'] if r['a'] != -1 and r['b'] == -1 else (
                       r['b'] if r['a'] == -1 and r['b'] != -1 else 0), 
                   axis=1)