比较数据框中的两列，如果值不同，则从另一列给出特定值

Question

I want to compare column_3 to column_1. If the value is the same give the value from column_2 back, if not search for the same value in column_1 and give the value from column_2. The output should be like column_4.

Thank you for your help!

Answer 1

You could apply a Series map function to column_3 using a dictionary with keys from column_1 and corresponding values in column_2 . This will then act as a lookup for values in column_3 and create a new column, column_4 :

import pandas as pd

df = pd.DataFrame({'column_1':[5,6,7,8,9], 
                   'column_2':[435,345,765,876,987], 
                   'column_3':[5,6,7,5,6]})

df['column_4'] = df['column_3'].map({k:v for k,v in zip(df['column_1'],df['column_2'])})


#    column_1   column_2    column_3    column_4
# 0    5          435          5          435
# 1    6          345          6          345
# 2    7          765          7          765
# 3    8          876          5          435
# 4    9          987          6          345