increment variable for each group

Question

I have the following data:

ID    Time
1     01-01-01
1     02-01-01
1     02-01-01
2     01-01-01

I would like to start with 0 and increase one by one for each new time by group ID. SO I need to get something like that

ID    Time       Result
1     01-01-01   0
1     02-01-01   1
1     02-01-01   1
2     01-01-01   0

Is there a smart way to do it shortly? (Note that this is just for illustration, real data is large enough.)

Answer 1

You can use groupby with pd.factorize:

df['Result'] = df.groupby('ID')['Time'].transform(lambda x: pd.factorize(x)[0])
df

Output:

   ID      Time  Result
0   1  01-01-01       0
1   1  02-01-01       1
2   1  02-01-01       1
3   2  01-01-01       0