If a dataframe contains a value in a column, how to perform a calculation in another column? – Pandas/ Python

Original Dataframe:

Output:

Logic needed: If column A contains 123, then for column C, take the value of B and multiply it with 0.0008, all the other values in column C should not be altered.

>Solution :

You could check if column "A" values are 123 or not and use mask on "C" to replace values there:

df['C'] = df['C'].mask(df['A']==123, df['B']*0.0008)

Output:

     A     B    C
0  123  1500  1.2