Ordering of values returned by unique

AFAIK, it has always worked that way in S-plus and R.  Furthermore, 
the documentation in R for 'unique' says that it removes duplicated 
elements.  This does seem to leave the possibility that element other 
than the first of a set of duplicates is retained, which could mess up 
the order.  However, the documentation for 'duplicated' is clearer: it 
says that 'duplicated' identifies duplicates of earlier elements.  
Also in the examples for 'duplicated', it says that x[!duplicated(x)] 
== unique(x) (paraphrased).

I depend on this all the time, so I also checked some references.  In 
the Blue book the documentation for the functions unique and 
duplicated is combined and implies the above.  In MASS 4th Ed, the 
page referred to by the index entry for 'unique' (p48, #9 in my copy) 
states that 'unique' removes duplicates as identified by 'duplicated', 
which implies that the order of retained elements is not changed.  The 
Green book has no index entry for 'unique'.  In S-plus the 
implementation of unique.default(x) uses x[!duplicated(x)].

So, I think the evidence is pretty strong that unique(x) will always 
return elements in the same order as they first appear in x.  But it 
would be nice if the documentation for 'unique' explicitly stated that 
this is the behavior for all methods.  (It does state this for the 
array method for 'unique').

-- Tony Plate

At Wednesday 09:17 AM 9/29/2004, Witold Eryk Wolski wrote:

Hi,

Is the ordering of the values returned something on what I can rely 
on, a form of a standard,  that a function called unique in R (in 
futher versions) will return the uniq elements in order of they first 
occurcence.

x<-c(2,2,1,2)
unique(x)

[1] 2 1

Its seems not to be the standard. E.g. matlab

x=[2,2,1,2]

x =
    2     2     1     2

unique(x)

ans =
    1     2

I just noted it because, the way how it is working now is extremely 
usefull for some applications (e.g tree traversal), so i use it in a 
script. But I am a little woried if I can rely on this behaviour in 
further versions. And furthermore can I assume that someone reading 
the code will think that it works in that way?
Or is it better to define a additional function?
keeporderunique<-function(x)
{
   res<-rep(NA,length(unique(x))
   count<-0
   for(i in x)
   {
       if(!i%in%res)
           {
                   count<-count+1
                    res[count]<-i
           }
   }
   res
}

/E



-- 
Dipl. bio-chem. Witold Eryk Wolski
MPI-Moleculare Genetic
Ihnestrasse 63-73 14195 Berlin           _
tel: 0049-30-83875219                   'v'
http://www.molgen.mpg.de/~wolski       /   \
mail: witek96@users.sourceforge.net  ---W-W----
     wolski@molgen.mpg.de