How to apply formula on a column in Dataframe based on value from other column

I have a DataFrame and I want to apply formula on Column A based on Column B.

If Column B has some values then apply formula on column A (100 - column value). Below is the data I have.

DataFrame Input

      A         B 
0     35       TYLER
1     20       MACK
2     30       MACK
3     40       MITCH

If column B has ‘MACK’ and ‘MITCH’ then apply formula, else not. How can I do that?

DataFrame Output

       A         B 
0     35       TYLER
1     80       MACK
2     70       MACK
3     60       MITCH

>Solution :

You can limit your formula to just the matching rows with boolean indexing:

apply_rows = df["B"].isin(["MACK", "MITCH"])
df.loc[apply_rows, "A"] = 100 - df.loc[apply_rows, "A"]