[R] Accessing elements of a list

[David Winsemius]

On May 25, 2011, at 3:25 PM, Seth W Bigelow wrote:

> I have a list that is made of lists of varying length. I wish to  
> create a
> new vector that contains the last element of each list. So far I  
> have used
> sapply to determine the length of each list, but I'm stymied at the  
> part
> where I index the list to make a new vector containing only the last  
> item
> of each list
>
> mylist = list(c(1,2,3),c("cat","dog"),c("x","y","z","zz"))      #  
> Create
> list
>
> last <- sapply(mylist,length) # Make vector with list lengths
>
> last_only <- mylist[[1:length(mylist)]][last]   # Crash and burn  
> trying to
> make new vector with last items!

If you wanted to apply the successive values of last using "[" to  
successive values of mylist there is a list-ish method via mapply:

 > mapply("[", mylist, last)
[1] "3"   "dog" "zz"

`mapply` is also the function underlying `Vectorise`
-- 

David Winsemius, MD
West Hartford, CT