Pandas mathematical operation, conditional on column value

Question

I need to make a mathematical operation which is conditional on the value in a second column. Here is the setup.

Given a simple dataframe (df):

Answer 1

Use:(Not a safe way to achieve it see the comment below )

df['New']=df.col3[df.col1=='B']+10
df
Out[11]: 
  col1  col2  col3   New
0    A     2     0   NaN
1    A     1     1   NaN
2    B     9     9  19.0
3  NaN     8     4   NaN
4    D     7     2   NaN
5    C     4     3   NaN

Update

pd.concat([df,(df.col3[df.col1=='B']+10).to_frame('New')],1)
Out[51]: 
  col1  col2  col3   New
0    A     2     0   NaN
1    A     1     1   NaN
2    B     9     9  19.0
3  NaN     8     4   NaN
4    D     7     2   NaN
5    C     4     3   NaN