[R] Selecting elements from all items in a list

[David Winsemius]

On Feb 14, 2012, at 8:44 AM, geotheory wrote:

> Basic question (if you know the answer)...  I am dealing with a list  
> of
> commonly-formatted sub-lists, for example:
>
> l <- list("")
> l[[1]] <- c("A1","A2","A3")
> l[[2]] <- c("B1","B2","B3")
> l[[3]] <- c("C1","B2","B3")
>
> Lets say I need to extract every 2nd item (i.e. A2, B2, C2).  [[]]  
> cannot be
> used.   l[2] returns practically the same as l[[2]].  Is there a way  
> to
> achieve this without having to loop or convert to data.frame?

Any such extraction would of necessity have a loop-like basis, but  
perhaps you meant without an explicit for-loop?

 > sapply(l, "[[", 2)
[1] "A2" "B2" "B2"


-- 

David Winsemius, MD
West Hartford, CT