functions within a list

Hi,

I am trying to perform various functions on a list with a number of  
elements. For example. I would like to take the mean of different  
variable values in the entire list. As an example,
say I have a list with 1000 elements and variables called Id and  
Tick. What I would like to do is take the mean of a variable called  
X for each Tick in the data element. So, there can be say 1 to 600  
tick values per element in a list, that would mean I would like to  
find the 600 mean values for each of the ticks values in the 1000  
elements.


I tried a simple attempt below, but I am sure it is way off as it  
didn't produce what I expected:

//res=res[[1..1000]]
weightX<-sapply(res, function(.df) {mean(.df$X[.df$Id>0 & .df 
$Tick<601])})

Basically, I was trying to get the mean for all tick values less  
than 600 that have an Id variable greater than 0. So, since there  
are 600 ticks I would like to return a result with 600 mean values,  
for each of the 600 ticks, that factors all the 1000 occurrences of  
each tick, starting from 1.

I hope this is clear. Thanks in advance.

Mark