Pandas 中组内的交替值

Question

I have the following dataframe:

df1
    a   b                                  
4   0   1      
5   0   1      
6   0   2      
2   0   3          
3   1   2   
15  1   3   
12  1   3   
13  1   1     
15  3   1   
14  3   1   
8   3   1   
9   3   1   
10  3   2       

I need another column that groups by a,b and assigning the values 1,2 alternating, always starting with 1, within each group of a. It should look like this:

    a   b   c                               
4   0   1   1   
5   0   1   1   
6   0   2   2   
2   0   3   1       
3   1   2   1
15  1   3   2
12  1   3   2
13  1   1   1   
15  3   1   1
14  3   1   1
8   3   1   1
9   3   1   1
10  3   2   2

Answer 1

Use transform + factorize

df.groupby('a').b.transform(lambda x : (x.factorize()[0]+1)//2+1)
4     1
5     1
6     2
2     1
3     2
15    2
12    2
13    2
15    1
14    1
8     1
9     1
Name: b, dtype: int64

Answer 2

One approach could be to groupby , take the ngroup which basically enumerates each group, then take the 2 modulo and use the result to index from either 1 or 2 :

import numpy
df['c'] = np.array([1,2])[df.groupby(['a','b']).ngroup().values%2]

---

print(df)

    a  b  c
4   0  1  1
5   0  1  1
6   0  2  2
2   1  2  1
3   1  3  2
15  1  3  2
12  1  3  2
13  1  4  1
15  3  1  2
14  3  1  2
8   3  1  2
9   3  1  2

---

Update so that each now group in a starts with 1 (adapting @wen's answer):

df['c'] = df.groupby('a').b.transform(lambda x : x.factorize()[0]%2+1)

print(df)

    a  b  c
4   0  1  1
5   0  1  1
6   0  2  2
2   0  3  1
3   1  2  1
15  1  3  2
12  1  3  2
13  1  1  1
15  3  1  1
14  3  1  1
8   3  1  1
9   3  1  1
10  3  2  2