Question

If I have some R list mylist, you can append an item obj to it like so:

mylist[[length(mylist)+1]] <- obj

But surely there is some more compact way. When I was new at R, I tried writing lappend() like so:

lappend <- function(lst, obj) {
    lst[[length(lst)+1]] <- obj
    return(lst)
}

but of course that doesn’t work due to R’s call-by-name semantics (lst is effectively copied upon call, so changes to lst are not visible outside the scope of lappend(). I know you can do environment hacking in an R function to reach outside the scope of your function and mutate the calling environment, but that seems like a large hammer to write a simple append function.

Can anyone suggest a more beautiful way of doing this? Bonus points if it works for both vectors and lists.

Answer 1

If it’s a list of string, just use the c() function :

R> LL <- list(a="tom", b="dick")
R> c(LL, c="harry")
$a
[1] "tom"

$b
[1] "dick"

$c
[1] "harry"

R> class(LL)
[1] "list"
R> 

That works on vectors too, so do I get the bonus points?

Edit (2015-Feb-01): This post is coming up on its fifth birthday. Some kind readers keep repeating any shortcomings with it, so by all means also see some of the comments below. One suggestion for list types:

newlist <- list(oldlist, list(someobj))

In general, R types can make it hard to have one and just one idiom for all types and uses.