[R] Help with ddply to eliminate a for..loop

[Bos, Roger]

I created a small example to show something that I do a lot of.  "scale"
data by month and return a data.frame with the output.  "id" represents
repeated observations over "time" and I want to scale the "slope"
variable.  The "out" variable shows the output I want.  My for..loop
does the job but is probably very slow versus other methods.  ddply
seems ideal, but despite playing with the baseball examples quite a bit
I can't figure out how to get it to work with my sample dataset.  

TIA for any help, Roger

Here is the sample code:

dat <- data.frame(id=rep(letters[1:5],3),
time=c(rep(1,5),rep(2,5),rep(3,5)), slope=1:15)
dat

for (i in 1:3) {
    mat <- dat[dat$time==i, ]
    outi <- data.frame(mat$time, mat$id, slope=scale(mat$slope))
    if (i==1) {
        out <- outi
    } else {
        out <- rbind(out, outi)
    }
}
out

Here is the sample output:

> dat <- data.frame(id=rep(letters[1:5],3),
time=c(rep(1,5),rep(2,5),rep(3,5)), slope=1:15)

> dat
   id time slope
1   a    1     1
2   b    1     2
3   c    1     3
4   d    1     4
5   e    1     5
6   a    2     6
7   b    2     7
8   c    2     8
9   d    2     9
10  e    2    10
11  a    3    11
12  b    3    12
13  c    3    13
14  d    3    14
15  e    3    15

> for (i in 1:3) {
+     mat <- dat[dat$time==i, ]
+     outi <- data.frame(mat$time, mat$id, slope=scale(mat$slope))
+     if (i==1) {
+         out  .... [TRUNCATED] 

> out
   mat.time mat.id      slope
1         1      a -1.2649111
2         1      b -0.6324555
3         1      c  0.0000000
4         1      d  0.6324555
5         1      e  1.2649111
6         2      a -1.2649111
7         2      b -0.6324555
8         2      c  0.0000000
9         2      d  0.6324555
10        2      e  1.2649111
11        3      a -1.2649111
12        3      b -0.6324555
13        3      c  0.0000000
14        3      d  0.6324555
15        3      e  1.2649111
> 
***************************************************************

This message is for the named person's use only. It may\...{{dropped:20}}