if (!requireNamespace("remotes")) install.packages("remotes")
remotes::install_github("lolow/gdxtools")

library(gdxtools)
mygdx <- gdx("results.gdx")
print(mygdx)                    # <results.gdx, 8 symbols>
print(mygdx$parameters$name)    # parameters available
all_items(mygdx)                # lists of names by symbol type
# Parameter — returns a data.frame with domain columns + value
travel_cost <- mygdx["travel_cost"]
# Variable / equation fields: "l" (level, default), "m" (marginal),
# "lo" (lower bound), "up" (upper bound)
lo_travel_time     <- mygdx["travel_time",     field = "lo"]
m_time_constraint  <- mygdx["time_constraint", field = "m"]
# Extract the same item from several GDX files
allparam <- batch_extract("myparam", files = c("test1.gdx", "test2.gdx"))

# Parameters
param1 <- data.frame(x = c("1","2","4","8"), value = 1:4)
attributes(param1) <- c(attributes(param1), gams = "definition of parameter 1")
param2 <- data.frame(a = c("london","paris","tahiti"), value = c(50, 0.2, 1e-2))
write.gdx("test.gdx", params = list(param1 = param1, param2 = param2))
# Sets — pass each one as a data.frame; column names become its dimensions
myset1 <- data.frame(a = c("london","paris","tahiti"))
myset2 <- data.frame(a = c("london","paris","tahiti"),
                     b = c("tahiti","tahiti","paris"))
write.gdx("test1.gdx", sets = list(city = myset1, road = myset2))
# Variables — give level / lower / upper as separate named lists; missing
# entries fall back to free-var defaults (level 0, lower -Inf, upper +Inf)
var_l  <- data.frame(a = c("london","paris","tahiti"), value = 0.2)
var_lo <- data.frame(a = c("london","paris","tahiti"), value = 1e-2)
var_up <- data.frame(a = c("london","paris","tahiti"), value = 50)
write.gdx("test2.gdx",
          vars_l  = list(travel_time = var_l),
          vars_lo = list(travel_time = var_lo),
          vars_up = list(travel_time = var_up))
# Compressed output
write.gdx("test.gdx", params = list(param1 = param1), compress = TRUE)
# write2.gdx — alias of write.gdx for params + sets only (kept for
# backward compatibility; both use the same fast path)
write2.gdx("test.gdx", params = list(param1 = param1, param2 = param2))

# NA / NaN values in parameter `value` columns
write.gdx("out.gdx", params = list(p = p), na = "drop")   # default: silently drop
write.gdx("out.gdx", params = list(p = p), na = "keep")   # preserve as GAMS NA
write.gdx("out.gdx", params = list(p = p), na = "error")  # stop on any NA
# Duplicate index keys
write.gdx("out.gdx", params = list(p = p), dup = "first")  # default: keep first
write.gdx("out.gdx", params = list(p = p), dup = "last")   # keep last (GAMS-process semantic)
write.gdx("out.gdx", params = list(p = p), dup = "error")  # stop on duplicates