Here are two ways to sort the data frame. The dplyr package used in the second method has many very handy functions for manipulating data frame.
DAT <- data.frame(Country = c("R", "A", "E", "R", "E", "A", "E", "R", "A"),
Year = c(2010, 2010, 2010, 2015, 2015, 2015, 2004, 2004, 2004))
DAT
#> Country Year
#> 1 R 2010
#> 2 A 2010
#> 3 E 2010
#> 4 R 2015
#> 5 E 2015
#> 6 A 2015
#> 7 E 2004
#> 8 R 2004
#> 9 A 2004
DAT[order(DAT$Country, DAT$Year),]
#> Country Year
#> 9 A 2004
#> 2 A 2010
#> 6 A 2015
#> 7 E 2004
#> 3 E 2010
#> 5 E 2015
#> 8 R 2004
#> 1 R 2010
#> 4 R 2015
library(dplyr)
arrange(DAT, Country, Year)
#> Country Year
#> 1 A 2004
#> 2 A 2010
#> 3 A 2015
#> 4 E 2004
#> 5 E 2010
#> 6 E 2015
#> 7 R 2004
#> 8 R 2010
#> 9 R 2015
Created on 2020-04-19 by the reprex package (v0.3.0)