[FIXED] Pandas df: Replace the value in a column for the 1st instance in another column

Issue

I have a panda df that may have duplicate id. For each id group, for the 1st occurrence, I want wt to be filled with wt_x. The below shows the initial df and the desired df. Thank you

Initial DF

id wt wt_x
1  12  33  
1  23  33
2  45  99
2  45  99
2  45  99
3  12  100

Desired DF

id wt wt_x
1  33  33  
1  23  33
2  99  99
2  45  99
2  45  99
3  100  100

Solution

Try like this

import pandas as pd

data = {
    'id': [1, 1, 2, 2, 2, 3],
    'wt': [12, 23, 45, 45, 45, 12],
    'wt_x': [33, 33, 99, 99, 99, 100]
}
df = pd.DataFrame(data)
first_occurrence_mask = ~df.duplicated(subset='id')
df.loc[first_occurrence_mask, 'wt'] = df.loc[first_occurrence_mask, 'wt_x']
print(df)

Output

   id   wt  wt_x
0   1   33    33
1   1   23    33
2   2   99    99
3   2   45    99
4   2   45    99
5   3  100   100

Answered By - Mahboob Nur