python - 获取每个元素出现的最快方法

Question

我有一个像这样的大数据框

                                             name     Country   ... 
1                                            Paul     Germany
2                                            Paul     Germany
3                                          George     Italy
3                                          George     Italy   
3                                          George     Italy
                       ...
N                                            John     USA

我正在寻找名称列中每个元素的出现情况，例如

                                             name     Country   Count 
1                                            Paul     Germany    2000
2                                          George     Italy       500
                          ...
N                                            John     USA       40000

知道什么是最好的方法吗？

因为这很长

df['count'] = df.groupby(['name'])['name'].transform(pd.Series.value_counts)

Answer 1

你可以这样做:

df.groupby(['name', 'Country']).size()

示例:

import pandas as pd

df = pd.DataFrame.from_dict({'name' : ['paul', 'paul', 'George', 'George', 'George'],
    'Country': ['Germany', 'Italy','Germany','Italy','Italy']})

df

输出:

    Country name
0   Germany paul
1   Italy   paul
2   Germany George
3   Italy   George
4   Italy   George

分组并获取计数:

df.groupby(['name', 'Country']).size()

输出:

name    Country
George  Germany    1
        Italy      2
paul    Germany    1
        Italy      1