使用pandas标记每组的每第N行

Question

我有一个带有客户信息的Dataframe及其购买详情.我正在尝试添加一个新列,指示同一客户完成的每次第三次购买.

下面给出的是Dataframe

customer_name,bill_no,date
Mark,101,2018-10-01
Scott,102,2018-10-01
Pete,103,2018-10-02
Mark,104,2018-10-02
Mark,105,2018-10-04
Scott,106,2018-10-21
Julie,107,2018-10-03
Kevin,108,2018-10-07
Steve,109,2018-10-02
Mark,110,2018-10-06
Mark,111,2018-10-02
Mark,112,2018-10-05
Mark,113,2018-10-05

我写的是过滤同一客户完成的每次第三次购买.所以在这种情况下,我想为下面的bill_no添加一个标志

Mark,105,2018-10-04
Mark,112,2018-10-05

基本上每个3个账单的倍数为同一个客户生成.

Answer 1

使用groupby.cumcount:

n = 3
df['flag'] = df.groupby('customer_name').cumcount() + 1
df['flag'] = ((df['flag'] % n) == 0).astype(int)

print(df)
   customer_name  bill_no        date  flag
0           Mark      101  2018-10-01     0
1          Scott      102  2018-10-01     0
2           Pete      103  2018-10-02     0
3           Mark      104  2018-10-02     0
4           Mark      105  2018-10-04     1
5          Scott      106  2018-10-21     0
6          Julie      107  2018-10-03     0
7          Kevin      108  2018-10-07     0
8          Steve      109  2018-10-02     0
9           Mark      110  2018-10-06     0
10          Mark      111  2018-10-02     0
11          Mark      112  2018-10-05     1
12          Mark      113  2018-10-05     0