[R] how to reverse a list

[Gabor Grothendieck]

Here is a minor shortening of that solution:

   tapply(t0, unlist(t0), names)


On 4/11/07, rolf at math.unb.ca <rolf at math.unb.ca> wrote:
> Weiwei Shi wrote:
>
> > I am wondering if there is a quick way to "reverse" a list like this:
> >
> > t0 <- list(a=1, b=1, c=2, d=1)
> >
> > reverst t0 to t1
> >
> > > t1
> > $`1`
> > [1] "a" "b" "d"
> >
> > $`2`
> > [1] "c"
>
> t1 <- tapply(unlist(t0),unlist(t0),function(x){names(x)})
>
> works for your example.  Not clear how ``general'' an answer
> you want.
>
> If the entries of your list are not scalars, then the problem is much
> harder I think.  (And may not even be well posed.)  But then you
> should be using a *vector*, not a list.
>
> Don't use a chainsaw when you need a fretsaw.
>
>                                cheers,
>
>                                        Rolf Turner
>                                        rolf at math.unb.ca
>
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