[Rd] [R] Semantics of sequences in R

Stavros Macrakis macrakis at alum.mit.edu
Sun Feb 22 22:50:13 CET 2009

On Sun, Feb 22, 2009 at 4:12 PM, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
> 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
>> 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.

Yes, I wasn't questioning that.  This started out as an exploration of
Rolf's claim that "vectors can be considered to be lists", which I
think the table shows pretty clearly not to be true.  He did qualify
the claim with "At a certain level.", but I don't know what that level

> (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.)

>From my own experience, and the experience of colleagues who have
tried to learn R, I can tell you that these idiosyncracies make
learning the system more difficult.  A "pretty-looking matrix" is a
reflection of an orthogonal design, which is generally considered to
be a good thing. Many of the missing operations are perfectly
meaningful and useful.

> ...But it may make more sense to completely hide pairlists,

I agree that the pairlist cases are the least interesting.

> (BTW, your description of your last row doesn't match what you did, as far as I can see.)

Yes, sorry, older draft....

>> 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?

In many cases, the orthogonal design is pretty straightforward.  And
in the cases where the operation is currently an error (e.g.
sort(list(...))), I'd hope that wouldn't break existing code. There
are certainly cases which would be hard to change without breaking
existing code....

> Generally R core members are reluctant to take on work just because someone
> else thinks it would be nice if they did.

I understand this principle quite well, having been a contributor to
other similar projects.  I was simply starting the discussion.  After
all, if the core group disagrees that the functions should be made
more orthogonal, it is a waste of my time to submit code.

>  If you want to do this, that's one thing,

I have already suggested code changes in some (pretty trivial) cases
-- see r-help Feb 6, 2009 6:17 PM "Operations on difftime (abs, /, c)"
-- but perhaps r-help was the wrong place to send them.  I will
forward to r-devel.  And I will be happy to work on some of the
consistency issues I've mentioned here.


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