[R] problem about set operation and computation after split

[bestbird]

hi,
    I met some problems in R, plz help me.
1. How to do a intersect operation among several groups in one list, without
a loop statement? (I think It may be a list)
   create data:
   myData <- data.frame(product = c(1,2,3,1,2,3,1,2,2),
year=c(2009,2009,2009,2010,2010,2010,2011,2011,2011),value=c(1104,608,606,1504,508,1312,900,1100,800))
   mySplit<- split(myData,myData$year)  
   mySplit
$`2009`
  product year value
1       1 2009  1104
2       2 2009   608
3       3 2009   606

$`2010`
  product year value
4       1 2010  1504
5       2 2010   508
6       3 2010  1312

$`2011`
  product year value
7       1 2011   900
8       2 2011  1100
9       2 2011   800
    I want to get intersection of product between every year. I know the
basic is:
    intersect(intersect(mySplit[[1]]$product,
mySplit[[2]]$product),mySplit[[3]]$product)   
    this will give the correct answer:
    [1] 1 2
    above code lacks reusability, so It should use a for loop:
    myIntersect<-mySplit[[1]]$product
    for (i in 1:length(mySplit)-1){ 
        myIntersect<-intersect(myIntersect,mySplit[[i+1]]$product)
    }
    It's correct too, but stll too complex, so my question is:
    Can I do the same thing just use another similar intersect function
(without for/repeat/while).
    What's this simple function's name ?

2.how to do a relative computation after split (notice: not befor split)?
   create data:
   myData1 <- data.frame(product = c(1,2,3,1,2,3),
year=c(2009,2009,2009,2010,2010,2010),value=c(1104,608,606,1504,508,1312),relative=0)
   mySplit1<- split(myData1,myData1$year)  
   mySplit1
$`2009`
  product year value relative
1       1 2009  1104        0
2       2 2009   608        0
3       3 2009   606        0

$`2010`
  product year value relative
4       1 2010  1504        0
5       2 2010   508        0
6       3 2010  1312        0
   I want compute relative value in the every group, what I mean is , I want
get the result is just like below:
   $`2009`
  product year value relative
1       1 2009  1104        0
2       2 2009   608        -496
3       3 2009   606        -2

$`2010`
  product year value relative
4       1 2010  1504        0
5       2 2010   508        -996
6       3 2010  1312        804
I think to use a loop maybe work, but Is there no direct method on list?

3.how to do a sorting after split, It's just like above question, what I
want is sorting by value:
   $`2009`
  product year value relative
3       3 2009   606        0
2       2 2009   608        0
1       1 2009  1104        0
$`2010`
  product year value relative
5       2 2010   508        0
6       3 2010  1312        0
4       1 2010  1504        0

4. how to do a filtering after split, Yes, It's just like above quetion,
what I want is filtering out data which value is more than 1000:
$`2009`
  product year value relative
1       1 2009  1104        0
$`2010`
  product year value relative
4       1 2010  1504        0
6       3 2010  1312        0

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