可能的错误：熊猫试图用浮点数填充int列

Question

In this example:

In [1]: df
Out[1]: 
   a    b
0  1  1.1
1  2  2.2
2  1  3.3
3  2  4.4

In [2]: df['c'] = 0

In [2]: df.c[df.a == 1] = df.b[df.a == 1]

In [3]: df
Out[3]: 
   a    b                    c
0  1  1.1  4607632778762754458
1  2  2.2                    0
2  1  3.3  4614613358185178726
3  2  4.4                    0

It seems like the resulting behavior is confusing. I know that the problem is that I have initialized 'c' as an int column. If, instead, I use df['c'] = 0. then everything works fine. So I get why my example doesn't work. It just seems like there could be a more useful result or at least an error message rather than a seemingly random large number. Definitely a nice to have, rather than need to have.

Answer 1

You are chain indexing, see here .

Use:

df.loc[df.a==1,'c'] = df['b']