You can use np.where()
to set values based on a specified condition:
#df
c1 c2 c3
0 4 2 1
1 8 7 9
2 1 5 8
3 3 3 5
4 3 6 8
Now change values (or set) in column ['c2']
based on your condition.
df['c2'] = np.where(df.c1 == 8,'X', df.c3)
c1 c3 c4
0 4 1 1
1 8 9 X
2 1 8 8
3 3 5 5
4 3 8 8