Python：删除每个组中具有最大值的行

Question

I have a pandas data frame df like this.

In [1]: df
Out[1]:
      country     count
0       Japan        78
1       Japan        80
2         USA        45
3      France        34
4      France        90
5          UK        45
6          UK        34
7       China        32
8       China        87
9      Russia        20
10      Russia        67

I want to remove rows with the maximum value in each group. So the result should look like:

      country     count
0       Japan        78
3      France        34
6          UK        34
7       China        32
9      Russia        20

My first attempt:

idx = df.groupby(['country'], sort=False).max()['count'].index
df_new = df.drop(list(idx))

My second attempt:

idx = df.groupby(['country'])['count'].transform(max).index
df_new = df.drop(list(idx))

But it didn't work. Any ideas?

Answer 1

groupby / transform('max')

You can first calculate a series of maximums by group. Then filter out instances where count is equal to that series. Note this will also remove duplicates maximums.

g = df.groupby(['country'])['count'].transform('max')
df = df[~(df['count'] == g)]

The series g represents maximums for each row by group. Where this equals df['count'] (by index), you have a row where you have the maximum for your group. You then use ~ for the negative condition.

print(df.groupby(['country'])['count'].transform('max'))

0    80
1    80
2    45
3    90
4    90
5    45
6    45
7    87
8    87
9    20
Name: count, dtype: int64

sort + drop

Alternatively, you can sort and drop the final occurrence:

res = df.sort_values('count')
res = res.drop(res.groupby('country').tail(1).index)

print(res)

  country  count
9  Russia     20
7   China     32
3  France     34
6      UK     34
0   Japan     78