[R] recode Variable in dependence of values of two other variables

[Julia Moeller]

Hi,

as an R-beginner, I have a recoding problem and hope you can help me:

I am working on a SPSS dataset, which I loaded into R (load("C:/...)

I have  2 existing Variables: "ID" and "X" ,
and one variable to be computed: meanX.dependID (=mean of X for all rows 
in which ID has the same value)

ID = subject ID.  Since it is a longitudinal dataset, there are repeated 
measurement points for each subject, each of which appears in a new row. 
So, each ID value appears in many rows. (e.g. ID ==1 in row 1:5; ID ==2 
in rows 6:8 etc).


Now: For all rows, in which ID has a certain value, meanX.dependID shall 
be the mean of X in for these rows. How can I automatisize that, without 
having to specify the number of the rows each time?

e.g.


ID    X    meanX.dependID
1    2    2.25
1    3    2.25
1    1    2.25
1    3    2.25
2    5    3.3
2    2    3.3
2    3    3.3
3    4    3
3    1    3
3    2    3
3    3    3
3    4    3
3    5    3


Thanks a lot! Hope this is the right place to post, if not, please tell me!
best,
Julia