题解 | #统计运动会项目报名人数#

pd.DataFrame.join: Join columns with other DataFrame either on index or on a key column

join(self, other, on=None, how='left', lsuffix='', rsuffix='', sort=False) -> 'DataFrame'

If we want to join using the key columns, we need to set key to be the index in both df and other. The joined DataFrame will have key as its index.

思路：

1. 先根据两个表的 item_id 字段内连接两个表 join(on='item_id', how='inner')（注意到题目要求只输出报名人数不为0的项目及其对应的报名人数，因此使用内连接）
2. 再进行分组聚合统计各个项目的人数 groupby('item_name')['employee_id'].count()

import pandas as pd
signup = pd.read_csv('signup.csv')
items = pd.read_csv('items.csv')
df = items.set_index('item_id').join(signup.set_index('item_id'), 
                                     on='item_id', how='inner')
print(df.groupby('item_name')['employee_id'].count())