[R] how to reverse a list

[rolf at math.unb.ca]

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