> ID <- c("a","b")
> Num <- c(33,44)
> library(data.table)
> dt <- data.table(ID=ID, Num=Num)
> setkey(dt,ID)
> dt["a"]
   ID Num
1:  a  33
> dt["b"]
   ID Num
1:  b  44

index <- c("b","a","b")
> dt[index]
   ID Num
1:  b  44
2:  a  33
3:  b  44