如果if语句groupby pandas数据框，则应用。 系列的真值是模棱两可的错误

Question

I can not apply an if statement on the column difference_bearing. If the values of the column difference_bearing(float) > 20. The expected result would be -1. In all other cases the expected results would be 0.

df_FCD_big.groupby(['tripID'])['difference_bearing'].apply(lambda x: -1 if x >20 else 0)

The following warning pops up.

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

Thanks in advance

Answer 1

As mentioned by @coldspeed.
If you need to know metrics within groups, you can do so by applying groupby after that, which would be lot more efficient.

df['result'] = np.where(df['difference_bearing'] > 20, -1, 0)
df_FCD_big.groupby(['tripID'])['result'].apply(sum)

Answer 2

这是克服错误的方法：

df_FCD_big.groupby(['tripID'])['difference_bearing'].apply(lambda x: x.apply(lambda y: -1 if y > 20 else 0))