Python中的双向查找

Question

So basically I have 2 DataFrames like this:

Table_1

Apple	Banana	Orange	Date
1	2	4	2020
3	5	2	2021
7	8	9	2022

Table_2

fruit	year
Apple	2020
Apple	2021
Apple	2022
Banana	2020
Banana	2021
Banana	2022
Orange	2020
Orange	2021
Orange	2022

So I want to lookup the values for the fruits for Table_2 from the Table_1 based on the fruit name and the respective year.

The final outcome should look like this:

fruit	year	number
Apple	2020	1
Apple	2021	3
Apple	2022	7
Banana	2020	2
Banana	2021	5
Banana	2022	8
Orange	2020	4
Orange	2021	2
Orange	2022	9

In the Excel for an example one can do something like this:

=INDEX(Table1[[Apple]:[Orange]],MATCH([@year],Table1[Date],0),MATCH([@fruit],Table1[[#Headers],[Apple]:[Orange]],0))

But what is the way to do it in Python?

Answer 1

Assuming pandas , you can melt and merge :

out = (df2
   .merge(df1.rename(columns={'Date': 'year'})
              .melt('year', var_name='fruit', value_name='number'),
           how='left'
          )
)

output:

    fruit  year  number
0   Apple  2020       1
1   Apple  2021       3
2   Apple  2022       7
3  Banana  2020       2
4  Banana  2021       5
5  Banana  2022       8
6  Orange  2020       4
7  Orange  2021       2
8  Orange  2022       9