[Rd] Ordering of values returned by unique
Witold Eryk Wolski
wolski at molgen.mpg.de
Wed Sep 29 18:24:28 CEST 2004
Thanks for this explanation and pointing me to value section of the
documention and the function duplicate. Indeed the documentation entry
in the value section states:
An object of the same type of 'x'. but if an element is equal to one
with a smaller index, it is removed. Could have found it by myself.
Tony Plate wrote:
> 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:
>> 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
>> > x<-c(2,2,1,2)
>> > unique(x)
>>  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?
>> for(i in x)
>> 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 at users.sourceforge.net ---W-W----
>> wolski at molgen.mpg.de
>> R-devel at stat.math.ethz.ch mailing list
Dipl. bio-chem. Witold Eryk Wolski
Ihnestrasse 63-73 14195 Berlin _
tel: 0049-30-83875219 'v'
http://www.molgen.mpg.de/~wolski / \
mail: witek96 at users.sourceforge.net ---W-W----
wolski at molgen.mpg.de
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