How to subtract multiple columns from different multiple columns?
The dataset looks something like this (note I have monthly values, on the picture you see yearly):
What I want to achieve is that I substract from column DI (Pre-Discount) 2018 the column DI (Post-Discount) 2018 - and so fort for each corresponding year and month - creating new columns called Deferred Discount 2018
Example of consequitive iterations:
DI (Pre-Discount) 2018 - DI (Post-Discount) 2018 = Deferred Discount 2018
DI (Pre-Discount) 2018 - DI (Post-Discount) 2019 = Deferred Discount 2019
DI (Pre-Discount) 2018 - DI (Post-Discount) 2020 = Deferred Discount 2020
DI (Pre-Discount) 2018 - DI (Post-Discount) 2021 = Deferred Discount 2021
How should I best approach the problem?