Dear Users,
Greetings of the day!

I am working on a gravity model and need help in data preparation. I have data of export and import for a set of reporters and partners countries for 20 years. Now I need to take out the export and import observation for the reverse pair of reporters and partners for each year. For example in the given below data for the year let's say 2005.
reporter partner export_b_usd import_b_usd
China India 1000 200
India Cina 180 1150
In this example data, I should have either China-India data or India-China data. I need to take out the reverse pair say India-China. I searched in stata but did not get a way out to solve this. Please suggest.

Thanks