[R] Semantics of sequences in R

Raubertas, Richard richard_raubertas at merck.com
Tue Feb 24 03:22:45 CET 2009


> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Duncan Murdoch
> Sent: Sunday, February 22, 2009 4:13 PM
> I think this was posted to the wrong list, so my followup is going to 
> R-devel.
> On 22/02/2009 3:42 PM, Stavros Macrakis wrote:
> > Inspired by the exchange between Rolf Turner and Wacek 
> Kusnierczyk, I
> > thought I'd clear up for myself the exact relationship among the
> > various sequence concepts in R, including not only generic vectors
> > (lists) and atomic vectors, but also pairlists, factor sequences,
> > date/time sequences, and difftime sequences.
> > 
> > I tabulated type of sequence vs. property to see if I could 
> make sense
> > of all this.  The properties I looked at were the predicates
> > is.{vector,list,pairlist}; whether various sequence operations (c,
> > rev, unique, sort, rle) can be used on objects of the various types,
> > and if relevant, whether they preserve the type of the 
> input; and what
> > the length of class( as.XXX (1:2) ) is.
> > 
> > Here are the results (code to reproduce at end of email):
> > 
> >              numer list  plist fact  POSIXct difft
> > c_keep?      TRUE  TRUE  FALSE FALSE TRUE    FALSE
> > rev_keep?    TRUE  TRUE  FALSE TRUE  TRUE    TRUE
> > unique_keep? TRUE  TRUE  "Err" TRUE  TRUE    FALSE
> > sort_keep?   TRUE  "Err" "Err" TRUE  TRUE    TRUE
> > rle_len      2     "Err" "Err" "Err" "Err"   "Err"
> > 
> > Alas, this tabulation, rather than clarifying things for me, just
> > confused me more -- the diverse treatment of sequences by various
> > operations is all rather bewildering.
> But you are asking lots of different questions, so of course 
> you should 
> get different answers.  For example, the first three rows are 
> behaving 
> exactly as documented.  (Perhaps the functions should have 
> been designed 
> differently, but a pretty-looking matrix isn't an argument for that. 
> Give some examples of how the documented behaviour is causing 
> problems.)
> I think some of the operations in the later rows are undocumented 
> (generally pairlists tend not to be documented, even if in some cases 
> they are supported), and it might make sense to make them more 
> consistent in the undocumented cases.  But it may make more sense to 
> completely hide pairlists, for instance, and then several more of the 
> examples are behaving as documented.  (BTW, your description of your 
> last row doesn't match what you did, as far as I can see.)
> > Wouldn't it be easier to teach, learn, and use R if there were more
> > consistency in the treatment of sequences?  
> Which ones in particular should change?  What should they change to? 
> What will break when you do that?

Okay, here is one that should change:  'c()' should do something useful 
with factors, for example return a factor whose levels are the union of
levels of the arguments.  Note that precedent for this already exists 
in base R:

> f1 <- factor(letters[1:3])
> f2 <- factor(letters[3:5])
> c(f1, f2)
[1] 1 2 3 1 2 3
> str(rbind(data.frame(f=f1), data.frame(f=f2)))
'data.frame':   6 obs. of  1 variable:
 $ f: Factor w/ 5 levels "a","b","c","d",..: 1 2 3 3 4 5

So the code and documentation already exist in 'rbind.data.frame'.  As 
for what would break, well, it is hard to imagine any possible use for 
the current behavior, or who could have made use of it.  But you never 
know I guess ...

Rich Raubertas
Merck & Co.

>  > I understand that in
> > long-running projects like S/R, there is an accumulation of
> > contributions by a variety of authors, but perhaps the time has come
> > for some cleanup at least for the base library?
> Generally R core members are reluctant to take on work just because 
> someone else thinks it would be nice if they did.  If you want to do 
> this, that's one thing, but if you are just saying that it 
> would be nice 
> if someone else did it, then it's much less likely to get 
> done.  To get 
> someone else to do it you need to convince them that it's a 
> valuable use 
> of their time, and I don't see that yet.
> Duncan Murdoch
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