[Rd] (no subject)

[Suharto Anggono Suharto Anggono]

------------------
>>>>> Henric Winell <[hidden email]>
>>>>>     on Wed, 21 Oct 2015 13:43:02 +0200 writes:

    > Den 2015-10-21 kl. 07:24, skrev Suharto Anggono Suharto Anggono via R-devel:
    >> Marius Hofert-4------------------------------
    >>> Den 2015-10-09 kl. 12:14, skrev Martin Maechler:
    >>> I think so: the code above doesn't seem to do the right thing.  Consider
    >>> the following example:
    >>>
    >>> > x <- c(1, 1, 2, 3)
    >>> > rank2(x, ties.method = "last")
    >>> [1] 1 2 4 3
    >>>
    >>> That doesn't look right to me -- I had expected
    >>>
    >>> > rev(sort.list(x, decreasing = TRUE))
    >>> [1] 2 1 3 4
    >>>
    >>
    >> Indeed, well spotted, that seems to be correct.
    >>
    >>>
    >>> Henric Winell
    >>>
    >> ------------------------------
    >>
    >> In the particular example (of length 4), what is really wanted is the following.
    >> ind <- integer(4)
    >> ind[sort.list(x, decreasing=TRUE)] <- 4:1
    >> ind

    > You don't provide the output here, but 'ind' is, of course,

    >> ind
    > [1] 2 1 3 4

    >> The following gives the desired result:
    >> sort.list(rev(sort.list(x, decreasing=TRUE)))

    > And, again, no output, but

    >> sort.list(rev(sort.list(x, decreasing=TRUE)))
    > [1] 2 1 3 4

    > Why is it necessary to use 'sort.list' on the result from
    > 'rev(sort.list(...'?

You can try all kind of code on this *too* simple example and do
experiments.  But let's approach this a bit more scientifically
and hence systematically:

Look at  rank  {the R function definition} to see that
for the case of no NA's,

 rank(x, ties.method = "first')   ===    sort.list(sort.list(x))

If you assume that to be correct and want to define "last" to be
correct as well (in the sense of being  "first"-consistent),
it is clear that

  rank(x, ties.method = "last)   ===  rev(sort.list(sort.list(rev(x))))

must also be correct.  I don't think that *any* of the proposals
so far had a correct version [but the too simplistic examples
did not show the problems].

In  R-devel (the R development) version of today, i.e., svn
revision >= 69549, the implementation of  ties.method = "last'
uses
        ## == rev(sort.list(sort.list(rev(x)))) :
        if(length(x) == 0) integer(0)
        else { i <- length(x):1L
               sort.list(sort.list(x[i]))[i] },

which is equivalent to using rev() but a bit more efficient.

Martin Maechler, ETH Zurich 
------------------

I'll defend that my code is correct in general.

All comes from the fact that, if p is a permutation of 1:n,
{ ind <- integer(n); ind[p] <- 1:n; ind }
gives the same result to
sort.list(p)
You can make sense of it like this. In ind[p] <- 1:n, ind[1] is the position where p == 1. So, ind[1] is the position of the smallest element of p. So, it is the first element of sort.list(p). Next elements follow.

That's why 'sort.list' is used for ties.method="first" and ties.method="random" in function 'rank' in R. When p gives the desired order,
{ ind <- integer(n); ind[p] <- 1:n; ind }
gives ranks of the original elements based on the order. The original element in position p[1] has rank 1, the original element in position p[2] has rank 2, and so on.

Now, I say that rev(sort.list(x, decreasing=TRUE)) gives the desired order for ties.method="last". With the order, the elements are from smallest to largest; for equal elements, elements are ordered by their positions backwards.