Creating day count using pandas

Question

I have following dataframe,

ID	d_of_stay
1	2021-03-01
1	2021-03-02
1	2021-03-03
2	2021-03-05
2	2021-03-06

I have to create a column like below,

ID	d_of_stay	day
1	2021-03-01	Day 0
1	2021-03-02	Day 1
1	2021-03-03	Day 2
2	2021-03-05	Day 0
2	2021-03-06	Day 1

How to do that using pandas/python?

Answer 1

This will give you the results you expect IIUC

df['day'] = df.groupby('ID')['d_of_stay'].cumcount()
df['day'] = 'Day ' + df['day'].astype(str)