[R] problem about set operation and computation after split

[R. Michael Weylandt]

On Wed, Jun 6, 2012 at 2:43 AM, bestbird <bestbird7788 at gmail.com> wrote:
> 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 ?

Take a look at Reduce()

>
> 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?

lapply ?

possibly in conjunction with sweep.


>
> 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

lapply

probably you'll need order for this one.

>
> 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

lapply

which()

and logical indexing.

Michael

>
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