How to identify the minimum year of membership to the agreement between the pairs that are symmetric

[Ridwan Sheikh]

Dear All,
I have data on trade agreements and membership to the WTO between country (i) and country (j). My data looks something like:

* Example generated by -dataex-. To install: ssc install dataex
clear
input str3(country_i country_j) int year byte(gatt pta)
"AUS" "BRA" 1986 0 0
"AUS" "BRA" 1987 0 0
"AUS" "BRA" 1988 0 1
"AUS" "BRA" 1989 0 1
"AUS" "BRA" 1990 0 1
"AUS" "BRA" 1991 0 1
"AUS" "BRA" 1992 1 1
"AUS" "BRA" 1993 1 1
"AUS" "BRA" 1994 1 1
"BRA" "AUS" 1986 0 0
"BRA" "AUS" 1987 0 0
"BRA" "AUS" 1988 1 0
"BRA" "AUS" 1989 1 0
"BRA" "AUS" 1990 1 1
"BRA" "AUS" 1991 1 1
"BRA" "AUS" 1992 1 1
"BRA" "AUS" 1993 1 1
"BRA" "AUS" 1994 1 1
end

The gatt membership year between AUS-BRA pair is 1992 whereas between BRA-AUS pair , it is year 1988. I want to pick the minimum year (earliest year between the symmetric pairs). How can i do that in STATA ?
similarly the pta between between AUS-BRA pair is 1988 whereas between BRA-AUS pair, the agreement year is1990. Similarily, i want to pick the minimum year of agreement signing between the country pairs which are symmetric .

Thank you

[Nick Cox]

. search dm0043, entry

Search of official help files, FAQs, Examples, and Stata Journals

SJ-8-4  dm0043  . Tip 71: The problem of split identity, or how to group dyads
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
        Q4/08   SJ 8(4):588--591                                 (no commands)
        tip on how to handle dyadic identifiers

dm0043 is an otherwise unpredictable search term for many discussions here on Statalist of precisely this issue.

gen first = cond(country_i < country_j, country_i, country_j) 
gen second = cond(country_i < country_j, country_j, country_i) 

egen wanted = min(pta) , by(first second)

See also https://www.statalist.org/forums/help#spelling

[Ridwan Sheikh]

Thanks Nick Cox