Pandas/Python: Set value of one column based on value in another column

Question

I need to set the value of one column based on the value of another in a Pandas dataframe. This is the logic:

if df[\'c1\'] == \'Value\':
    df[\'c2\'] = 10         


        

Answer 1

Note the tilda that reverses the selection. It uses pandas methods (i.e. is faster than if/else).

df.loc[(df['c1'] == 'Value'), 'c2'] = 10
df.loc[~(df['c1'] == 'Value'), 'c2'] = df['c3']

Answer 2

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