[R] Create a new var reflecting the order of subjects in existing var

[Nguyen Dinh Nguyen]

Dear R helpers
I have a data set sth like this:
set.seed(123);dat <- data.frame(ID= c(rep(1,2),rep(2,3), rep(3,3), rep(4,4),
rep(5,5)),
                  var1 =rnorm(17, 35,2),
                  var2=runif(17,0,1))
dat
   ID     var1       var2
1   1 33.87905 0.02461368
2   1 34.53965 0.47779597
3   2 38.11742 0.75845954
4   2 35.14102 0.21640794
5   2 35.25858 0.31818101
6   3 38.43013 0.23162579
7   3 35.92183 0.14280002
8   3 32.46988 0.41454634
9   4 33.62629 0.41372433
10  4 34.10868 0.36884545
11  4 37.44816 0.15244475
12  4 35.71963 0.13880606
13  5 35.80154 0.23303410
14  5 35.22137 0.46596245
15  5 33.88832 0.26597264
16  5 38.57383 0.85782772
17  5 35.99570 0.04583117
I would like to create a new var in dat which reflects the order of each
subject (ID), like this
   ID     var1       var2	 IDorder
1   1 33.87905 0.02461368       1
2   1 34.53965 0.47779597       2
3   2 38.11742 0.75845954       1
4   2 35.14102 0.21640794       2
5   2 35.25858 0.31818101       3
6   3 38.43013 0.23162579       1
7   3 35.92183 0.14280002       2
8   3 32.46988 0.41454634       3
9   4 33.62629 0.41372433       1
10  4 34.10868 0.36884545       2
11  4 37.44816 0.15244475       3
12  4 35.71963 0.13880606       4
13  5 35.80154 0.23303410       1
14  5 35.22137 0.46596245       2
15  5 33.88832 0.26597264       3
16  5 38.57383 0.85782772       4
17  5 35.99570 0.04583117       5

Thank you very much for your help
Regards
Nguyen