R help - Feb 2008 - "Inverting" a list

Is there a built in function to invert a list?  i.e. to go from

list(
  a = c("1", "2", "3"),
  b = c("1"),
  d = c("2", "4")
)

to

list(
 "1" = c("a", "b"),
 "2" = c("a", "d"),
 "3" = "a",
 "4" = "2"
)

Hadley

-- 
http://had.co.nz/

This isn't a single command but its pretty short:

unstack(stack(L)[2:1])

On Feb 6, 2008 5:51 PM, hadley wickham <h.wickham at gmail.com>
wrote:
> Is there a built in function to invert a list?  i.e. to go from
>
> list(
>  a = c("1", "2", "3"),
>  b = c("1"),
>  d = c("2", "4")
> )
>
> to
>
> list(
>  "1" = c("a", "b"),
>  "2" = c("a", "d"),
>  "3" = "a",
>  "4" = "2"
> )
>
> Hadley
>
> --
> http://had.co.nz/
>
> ______________________________________________
> R-help at r-project.org mailing list
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

A more primitive method is about 5 times faster than Gabor's.

L <- list(
            a = c("1", "2", "3"),
            b = c("1"),
            d = c("2", "4")
)

system.time(
for (i in 1:100)
{t1 <- unlist(L)
names(t1) <- rep(names(L), lapply(L, length))
tapply(names(t1), t1, c)
}
)

system.time(
for (i in 1:100)
unstack(stack(L)[2:1])
)