[R] column statistics

[Ivan Calandra]

Hi everybody,

I would like to compute the mean for 1 variable between the rows with 
the same levels.

For example, with the dataset below:
Factor1      Factor2      Value
A               X               1
A               X               2
A               Y               3
A               Y               4
B               X               5
B               X               6
B               Y               7
B               Y               8

I would like to get:
Factor1      Factor2      Value
A               X               1.5
A               Y               3.5
B               X               5.5
B               Y               7.5

Up to now, I worked in Statistica and Systat, and it was called "column 
statistics" in Statistica (and I had a script with the "BY" function in 
Systat).

Of course it is a simplified case. For my dataset I have 4 factors and 
15 variables, so a general method would be nice. However, my skills are 
not that great, so if you could please give some explanations (I mean 
other than what is in the ?function of course).

Thanks a lot in advance
Ivan