从零开始计数(Start counting at zero by group)

 考虑以下数据框： 
 
>>> import pandas as pd
>>> df = pd.DataFrame({'group': list('aaabbabc')})
>>> df
  group
0     a
1     a
2     a
3     b
4     b
5     a
6     b
7     c
 
 我想统计每个组发生的累计次数。 我想要的输出如下： 
 
>>> df
  group  n
0     a  0
1     a  1
2     a  2
3     b  0
4     b  1
5     a  3
6     b  2
7     c  0
 
 我最初的做法是做这样的事情： 
 
df['n'] = df.groupby('group').apply(lambda x: list(range(x.shape[0])))
 
 基本上为每个组分配一个长度为n的数组，索引为零。 但这已证明难以转位和加入。 

Consider the following dataframe:
 
>>> import pandas as pd
>>> df = pd.DataFrame({'group': list('aaabbabc')})
>>> df
  group
0     a
1     a
2     a
3     b
4     b
5     a
6     b
7     c
 
I want to count the cumulative number of times each group has occurred. My desired output looks like this:
 
>>> df
  group  n
0     a  0
1     a  1
2     a  2
3     b  0
4     b  1
5     a  3
6     b  2
7     c  0
 
My initial approach was to do something like this:
 
df['n'] = df.groupby('group').apply(lambda x: list(range(x.shape[0])))
 
Basically assigning a length n array, zero-indexed, to each group. But that has proven difficult to transpose and join.

原文：https://stackoverflow.com/questions/50175299