creating a factor from dates by subject?

Thu, Dec 20, 2007 7:33 AM

Dear R-help,

I have a data set consisting of measurements made on multiple
subjects.  Measurement sessions are repeated for each subject on
multiple dates.  Not all subjects have the same number of
sessions.  To create a factor that represents the session, I do
the following:

data <- read.csv('test-data.csv') # data appended below
data$date <- as.Date(data$date, format='%m/%d/%Y')
data$session <- rep(NA,nrow(data))
for (i in unique(data$ID)) {
  data$session[data$ID==i] <- as.numeric(factor(data$date[data$ID==i]))
}
data$session <- factor(data$session)

This results in a session column in the data frame that runs from
1 to the number of sessions for each subject ID. 

What do you R gurus think of this?  Is there a better more R-ish
way to do this with without creating the session variable in the
data frame and then looping?  I find myself doing this sort of
thing all the time and it feels crufty to me.  

Thanks, Mike

Michael A. Miller                               mmiller3 at iupui.edu
  Imaging Sciences, Department of Radiology, IU School of Medicine


"ID","date","session"
1,05/24/2006,1
1,02/01/2007,2
1,05/23/2007,3
1,07/06/2007,4
2,07/28/2006,1
2,09/24/2006,2
2,01/18/2007,3
3,07/24/2006,1
3,01/17/2007,2
3,03/22/2007,3
4,05/08/2006,1
4,07/24/2006,2
4,09/26/2006,3
4,03/16/2007,4
5,07/19/2006,1
5,01/11/2007,2
5,05/04/2007,3
6,06/27/2006,1
6,08/15/2006,2
6,10/31/2006,3
6,02/27/2007,4
7,08/01/2006,1
7,10/06/2006,2
7,03/16/2007,3
8,06/06/2006,1
8,11/16/2006,2
8,04/24/2007,3
9,03/13/2007,1
9,04/27/2007,2
9,05/13/2007,3
10,08/03/2006,1
10,01/03/2007,2
10,04/25/2007,3
10,06/12/2007,4
11,05/24/2005,1
11,08/31/2006,2
11,04/10/2007,3
12,01/25/2007,1
12,04/30/2007,2
12,06/11/2007,3
1,05/24/2006,1
1,02/01/2007,2
1,05/23/2007,3
1,07/06/2007,4
2,07/28/2006,1
2,09/24/2006,2
2,01/18/2007,3
3,07/24/2006,1
3,01/17/2007,2
3,03/22/2007,3
4,05/08/2006,1
4,07/24/2006,2
4,09/26/2006,3
4,03/16/2007,4
5,07/19/2006,1
5,01/11/2007,2
5,05/04/2007,3
6,06/27/2006,1
6,08/15/2006,2
6,10/31/2006,3
6,02/27/2007,4
7,08/01/2006,1
7,10/06/2006,2
7,03/16/2007,3
8,06/06/2006,1
8,11/16/2006,2
8,04/24/2007,3
9,03/13/2007,1
9,04/27/2007,2
9,05/13/2007,3
10,08/03/2006,1
10,01/03/2007,2
10,04/25/2007,3
10,06/12/2007,4
11,05/24/2005,1
11,08/31/2006,2
11,04/10/2007,3
12,01/25/2007,1
12,04/30/2007,2
12,06/11/2007,3
1,05/24/2006,1
1,02/01/2007,2
1,05/23/2007,3
1,07/06/2007,4
2,07/28/2006,1
2,09/24/2006,2
2,01/18/2007,3
3,07/24/2006,1
3,01/17/2007,2
3,03/22/2007,3
4,05/08/2006,1
4,07/24/2006,2
4,09/26/2006,3
4,03/16/2007,4
5,07/19/2006,1
5,01/11/2007,2
5,05/04/2007,3
6,06/27/2006,1
6,08/15/2006,2
6,10/31/2006,3
6,02/27/2007,4
7,08/01/2006,1
7,10/06/2006,2
7,03/16/2007,3
8,06/06/2006,1
8,11/16/2006,2
8,04/24/2007,3
9,03/13/2007,1
9,04/27/2007,2
9,05/13/2007,3
10,08/03/2006,1
10,01/03/2007,2
10,04/25/2007,3
10,06/12/2007,4
11,05/24/2005,1
11,08/31/2006,2
11,04/10/2007,3
12,01/25/2007,1
12,04/30/2007,2
12,06/11/2007,3
1,05/24/2006,1
1,02/01/2007,2
1,05/23/2007,3
1,07/06/2007,4
2,07/28/2006,1
2,09/24/2006,2
2,01/18/2007,3
3,07/24/2006,1
3,01/17/2007,2
3,03/22/2007,3
4,05/08/2006,1
4,07/24/2006,2
4,09/26/2006,3
4,03/16/2007,4
5,07/19/2006,1
5,01/11/2007,2
5,05/04/2007,3
6,06/27/2006,1
6,08/15/2006,2
6,10/31/2006,3
6,02/27/2007,4
7,08/01/2006,1
7,10/06/2006,2
7,03/16/2007,3
8,06/06/2006,1
8,11/16/2006,2
8,04/24/2007,3
9,03/13/2007,1
9,04/27/2007,2
9,05/13/2007,3
10,08/03/2006,1
10,01/03/2007,2
10,04/25/2007,3
10,06/12/2007,4
11,05/24/2005,1
11,08/31/2006,2
11,04/10/2007,3
12,01/25/2007,1
12,04/30/2007,2
12,06/11/2007,3

creating a factor from dates by subject?

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