普通にソートしたら意図した順にならない時は
> sort(c("first", "3rd", "_second"))
[1] "_second" "3rd" "first"
ordered = TRUE
オプションを指定して順序つきの因子にするとできる
> vec <- factor(c("first", "3rd", "_second"), levels = c("first", "_second", "3rd"), ordered = TRUE)
> vec
[1] first 3rd _second
Levels: first < _second < 3rd
> sort(vec)
[1] first _second 3rd
Levels: first < _second < 3rd
Enjoy!