"already preserved" error when running preserve and restore in a loop

[Camilo Saldarriaga]

Hello community.

I am trying to run this loop with preserve and restore, however, when I run the code it says throws "already preserved" even when I am including the restore command at the end of the loop.

Any idea of what is the problem?
This is the code:

foreach country_name in "Antigua and Barbuda" "Argentina" "Aruba" "Bahamas" "Barbados" "Belize" "Bermuda" "Bolivia" "Brazil" "British Virgin Islands" "Cayman Islands" "Chile" "Colombia" "Costa Rica" "Cuba" "Curacao" "Dominica" "Dominican Republic" "Ecuador" "El Salvador" "Equatorial Guinea" "Grenada" "Guatemala" "Guyana" "Haiti" "Honduras" "Mexico" "Nicaragua" "Panama" "Paraguay" "Peru" "St. Kitts and Nevis" "St. Lucia" "St. Martin (French part)" "St. Vincent and the Grenadines" "Trinidad and Tobago" "Uruguay" "Venezuela, RB" {
preserve
drop if Country!= "`country_name'"
tsset year
dfuller ln_CO2_emissions_kt, lags(0)
dfuller ln_CO2_emissions_kt, lags(1)
dfuller ln_CO2_emissions_kt, lags(2)
dfuller ln_Energy_use_oil_kg_equ, lags(0)
dfuller ln_Energy_use_oil_kg_equ, lags(1)
dfuller ln_Energy_use_oil_kg_equ, lags(2)
dfuller ln_GDP_constant2015_USD, lags(0)
dfuller ln_GDP_constant2015_USD, lags(1)
dfuller ln_GDP_constant2015_USD, lags(2)
restore
}

[Maarten Buis]

I don't immediately see what is going wrong. But you can use frames instead, which seems to work.

frames reset

frame create data // store the whole data in frame data
frame change data
sysuse nlsw88

levelsof industry
local levs = r(levels)

frame create playground // this frame we can change as we wish
frame change playground

foreach lev of local levs{
    frame copy data playground, replace // get the whole data again
    
    keep if industry == `lev' // select the subsample
    sum wage                  // do stuff to that subsample
}

[Nick Cox]

dfuller seems to work fine if you specify a single panel, so the file heaving seems unnecessary.

In addition, see Method 1 in https://www.stata.com/support/faqs/d...-with-foreach/ for an easier way to loop over panels.