R help - Apr 2006 - Indexing a vector by a list of vectors

Hello R-help -

I have

vec <- c("string1", "string2", "string3")
ind <- list(c(1,2),c(1,2,3))

I want "vec" indexed by each vector in the list "ind".
The first element of the list I want would be vec[c(1,2)],
the second element would be vec[c(1,2,3)], like the following.

[[1]]
[1] "string1" "string2"

[[2]]
[1] "string1" "string2" "string3"

Using for loops, this is simple.  For fun, I tried to implement it 
without a for loop using some combination of *apply() functions and
"[".

I succeeded with

myfunc <- function(x) {
   do.call("[",list(vec,x))
}
lapply(ind,myfunc)

I was not, however, able to get my desired result without defining my 
own dummy function.  Can anyone think of a way?  As I said, I already 
have a way that works, I'm just curious if there is a more 'elegant'
solution that does not rely on my having to define another function. 
Seems like it should be possible.

Thanks, Erik Iverson

The following is quite simple although it does use a function:

   lapply(ind, function(x) vec[x])

and if you really don't want to use a function then try:

   mapply("[", rep(list(vec), length(ind)), ind)


On 4/4/06, Erik Iverson <iverson at biostat.wisc.edu>
wrote:
> Hello R-help -
>
> I have
>
> vec <- c("string1", "string2", "string3")
> ind <- list(c(1,2),c(1,2,3))
>
> I want "vec" indexed by each vector in the list "ind".
> The first element of the list I want would be vec[c(1,2)],
> the second element would be vec[c(1,2,3)], like the following.
>
> [[1]]
> [1] "string1" "string2"
>
> [[2]]
> [1] "string1" "string2" "string3"
>
> Using for loops, this is simple.  For fun, I tried to implement it
> without a for loop using some combination of *apply() functions and
"[".
>
> I succeeded with
>
> myfunc <- function(x) {
>   do.call("[",list(vec,x))
> }
> lapply(ind,myfunc)
>
> I was not, however, able to get my desired result without defining my
> own dummy function.  Can anyone think of a way?  As I said, I already
> have a way that works, I'm just curious if there is a more
'elegant'
> solution that does not rely on my having to define another function.
> Seems like it should be possible.
>
> Thanks, Erik Iverson
>
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