[R] conditional mean between two data frames with different levels

[albechan]

Hi guys, I have two data frames: one referred to 2008 and one to 2009. Their
structure is identical except for the different data in them. 
I need to create a vector alfa of the same length of the dataframe 2009 and
fill each element with the mean of 2008$var1 conditional to the subgroup
indicated by a factor variable in 2009$var2.
In this case it would be easy to use the function
alfa[i]<-ave(2008$var1,2008$var2==2009$var2[i],FUN=mean).
The problem is that 2008$var2 and 2009$var2 contain both 20 levels each but
only 18 of them are shared. So for those 18 I need to find the result that
I`d get applying the above formula (which in any case doesnt work if the
levels are not identical in the two data frames colmns) and for those two
different levels in 2009$var2 to use the average of the whole column
2008$var2.
Anybody has some ideas? Please help me...
Hope it`s clear enough what I need.
Thanks!
alberto 
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