R devel - Aug 1999 - ts and defaults

Brian

The example below may be helpful to further illustrate the concern I have with
the effects of your ts library on defaults. According to the "Blue
Book", in
Splus, in R up to 0.64.2, and in today's snapshot of 0.65 without the
"ts"
library attached
> end(ts(rnorm(10), start=c(1991,1), frequency=1))
gives
[1] 2000    1

However, with the "ts" library attached the result is

[1] 2000

This change in the default behavior will break a lot of user code, effectively
meaning that many people who do a lot of time series work will not use the
library. Hopeful that is not what you intend. Even casual users, I believe, will
be confused by the change in behavior depending on whether  ts is attached or
not.

Possibly a much better way to define a new improved class is to give it a new
name, rather than a name that introduces conflicts with the old defaults.

I hope that you will have time to consider changing this before 0.65 is
released. I will be away for the next two week, but will be happy to continue
testing when I return.

Paul Gilbert



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On Fri, 6 Aug 1999, Paul Gilbert wrote:
> Brian
> 
> The example below may be helpful to further illustrate the concern I have
with
> the effects of your ts library on defaults. According to the "Blue
Book", in
> Splus, in R up to 0.64.2, and in today's snapshot of 0.65 without the
"ts"
> library attached
> 
> > end(ts(rnorm(10), start=c(1991,1), frequency=1))
> 
> gives
> [1] 2000    1
How is anyone supposed to know that means 2000 not 2001 without reading
the help page?
> However, with the "ts" library attached the result is
> 
> [1] 2000
Isn't that much more readable?
> This change in the default behavior will break a lot of user code [ ... ]
Statistical evidence, please.

Note that all the S (and R) code is written to accept start=1991, so this
user code must depart from that convention. Maybe it would be a good
opportunity to re-write that code to conform to the standard conventions?
As in

start:    starting date for the series, e.g., in years, February,
       1970  would  be  1970+(1/12)  or  1970.083.  If start is a
       vector with  at  least  two  data  values,  the  first  is
       interpreted  as  the  time  unit,  e.g., the year, and the
       second as  the  number  of  positions  into  the  sampling
       period;  e.g., February, 1970 could be c(1970,2).

Note the two possibilities, and that the one you advocate is the
alternate. I at least find c(1970, 2) = 1970+(1/12) a _very_ confusing
notation, fortunately one that can be avoided.

The problem here is that ts in R is fundamentally incompatible with S, as R
has a ts class and S does not. I believe start.default should not be 
implemented in terms of class ts (and any user can change the
behaviour of a class), and I trust the `lots of user code' does not do
things like that. The plan is to separate the the default methods from the
ts class methods.  We do not consider that S compatibility applies 
to class ts, which S does not have.

Brian

-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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