[R] Best way to compute the difference between two levels of a factor ?

[wphantomfr]

Dear R-help Members,


I am wondering if anyone think of the optimal way of computing for 
several numeric variable the difference between 2 levels of a factor.


To be clear let's generate a simple data frame with 2 numeric variables  
collected for different subjects (ID) and 2 levels of a TIME factor 
(time of evaluation)

data=data.frame(ID=c("AA","AA","BB","BB","CC","CC"),TIME=c("T1","T2","T1","T2","T1","T2"),X=rnorm(6,10,2.3),Y=rnorm(6,12,1.9))

   ID TIME         X         Y
1 AA   T1  9.959540 11.140529
2 AA   T2 12.949522  9.896559
3 BB   T1  9.039486 13.469104
4 BB   T2 10.056392 14.632169
5 CC   T1  8.706590 14.939197
6 CC   T2 10.799296 10.747609

I want to compute for each subject and each variable (X, Y, ...) the 
difference between T2 and T1.

Until today I do it by reshaping my dataframe to the wide format (the 
columns are then ID, X.T1, X.T2, Y.T1,Y.T2) and then  compute the 
difference between successive  columns one by one :
data$Xdiff=data$X.T2-data$X.T1
data$Ydiff=data$Y.T2-data$Y.T1
...

but this way is probably not optimal if the difference has to be 
computed for a large number of variables.

How will you handle it ?


Thanks in advance

Sylvain Clément