如何根据给定列为行中出现的唯一值分配 pandas dataframe 中的数字

Question

Data Frame looks like

Unique Id     Date    
   H1         2/03/2022
   H1         2/03/2022
   H1         2/03/2022
   H1         3/03/2022
   H1         4/03/2022
   H2         9/03/2022
   H2         9/03/2022
   H2         10/03/2022

Expected Data Frame

    Unique Id     Date       Count
   H1         2/03/2022       1
   H1         2/03/2022       1
   H1         2/03/2022       1
   H1         3/03/2022       2
   H1         4/03/2022       3
   H2         9/03/2022       1
   H2         9/03/2022       1
   H2         10/03/2022      2

Repetitive dates should be assigned with number 1, else other should be assigned some other number

tried multiple approaches, please assist

Answer 1

There are a bunch of ways to do this, the primary issue is going to be that you need to treat the date as a date object so that October doesn't get moved ahead of September in your second group.

import pandas as pd
df = pd.DataFrame({'Unique_Id': ['H1', 'H1', 'H1', 'H1', 'H1', 'H2', 'H2', 'H2'],
 'Date': ['2/03/2022',
  '2/03/2022',
  '2/03/2022',
  '3/03/2022',
  '4/03/2022',
  '9/03/2022',
  '9/03/2022',
  '10/03/2022']})

Dense Rank

df.groupby('Unique_Id')['Date'].apply(lambda x: pd.to_datetime(x).rank(method='dense'))

Cat Codes

df.groupby('Unique_Id')['Date'].apply(lambda x: pd.to_datetime(x).astype('category').cat.codes+1)

Factorize

df.groupby('Unique_Id')['Date'].transform(lambda x: x.factorize()[0] + 1)

Answer 2

here is one way to do it making use of groupby and transform

" Repetitive dates should be assigned with number 1 , else other should be assigned some other number " is what the question stated, so I choose 2 where the values are unique

df['count'] = df.groupby('Date').transform(lambda x: 1 if (x.size > 1) else 2  )
df

    Unique_Id   Date    count
0   H1       2/03/2022    1
1   H1       2/03/2022    1 
2   H1       2/03/2022    1
3   H1       3/03/2022    2
4   H1       4/03/2022    2
5   H2       9/03/2022    1
6   H2       9/03/2022    1
7   H2       10/03/2022   2