get element of set in R with package "sets"

Question

I have generate following set. How can i get the elements for further usage?
Plus, how can i delete {}?

> library(sets)
> A<-set("item1","item2","item3")
> A
{"item1", "item2", "item3"}
> A[1]
{}        #i would like to have {}
> A[2]
{}        #i would like to have "item1"
> A[3]    
{}        #i would like to have "item2"

Additional question is that:

> A1<-set(paste("item",1:3,sep=""))
> A1
{<<character(3)>>}    # how can i generate the same result as A?

Thanks for your gentle reply!

Answer 1

For sets as mathematical objects, element order (as well as possible repetition) makes no difference. It "doesn't make sense" to talk about a set's "first" or "second" or "third", so the sets package's authors have not supplied a subsetting method that would allow you to retrieve indexed elements of "set"-class objects.

To get a sense of why that's a good design decision that directly follows from the objects' faithful representation of sets' important mathematical structure, examine the following:

library(sets)
a <- set("item3", "item3", "item2", "item1")
b <- set("item1", "item2", "item3")
identical(a,b)
# [1] TRUE

If you insist on extracting set elements with a numeric index, you can always do something like this:

set(as.character(a)[1])
# {"item1"}

As for your second question, the outcome you're after can be got by using the enormously useful function do.call():

do.call(set, as.list(paste("item",1:3,sep="")))
# {"item1", "item2", "item3"}