如何在按组计数后创建新列？

Question

我试图用group by和count创建一个新列。 但是，它将引发错误“插入的列的索引与框架索引不兼容”

import pandas as pd

#read csv
df1 = pd.read_csv('hi.txt',sep = '\t')#provide name and sheet of excel file
a=df1.groupby(['c','t']).count()
df1['difference']=a
print(df1)

输入：

coun    id  cat
A   12  90
U   13  91

Answer 1

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---

new_df=df.groupby(['category','country'],sort=False).country.count().to_frame('count').reset_index()
print(new_df)