Pandas number rows within group in increasing order

Question

Given the following data frame:

import pandas as pd import numpy as np df=pd.DataFrame({'A':['A','A','A','B','B','B'],                 'B':['a','a','b','a','a','a'],                 }) df      A   B 0   A   a  1   A   a  2   A   b  3   B   a  4   B   a  5   B   a 

I'd like to create column 'C', which numbers the rows within each group in columns A and B like this:

    A   B   C 0   A   a   1 1   A   a   2 2   A   b   1 3   B   a   1 4   B   a   2 5   B   a   3 

I've tried this so far:

df['C']=df.groupby(['A','B'])['B'].transform('rank') 

...but it doesn't work!

Answer 1

Use groupby/cumcount:

In [25]: df['C'] = df.groupby(['A','B']).cumcount()+1; df Out[25]:     A  B  C 0  A  a  1 1  A  a  2 2  A  b  1 3  B  a  1 4  B  a  2 5  B  a  3