Substitute values in dataframe

Question

I have a dataframe with 3 columns(A,B,C). I wanted to update column C with column A when column B=22. I have written update statement like this, but it's updating NaN for non matching rows. Could you tell me how to update data in dataframe?

df = pd.DataFrame(data=[[10,20,30],[11,21,31],[12,22,32]], columns=['A','B','C'])
df.C = df[df.B==22].A

Answer 1

Let us try mask

df.C.mask(df.B==22, df.A,inplace=True)
df
    A   B   C
0  10  20  30
1  11  21  31
2  12  22  12