**Subject: **RE: Is a ts of length one a ts? (PR#245)

**From: **Martyn Plummer (*plummer@iarc.fr*)

**Date: **Thu 12 Aug 1999 - 03:08:40 EST

**Next message:**Martin Maechler: "weighted.residuals() ?"**Previous message:**Kurt Hornik: "Re: INSTALL script does not handle spaces in pathnames correctly (PR#242)"**In reply to:**Prof Brian Ripley: "RE: Is a ts of length one a ts? (PR#245)"**Reply:**Martyn Plummer: "RE: Is a ts of length one a ts? (PR#245)"

Message-ID: <XFMail.990811190840.plummer@iarc.fr>

On 11-Aug-99 Prof Brian Ripley wrote:

*>> Date: Wed, 11 Aug 1999 13:52:51 +0200 (CEST)
*

*>> From: Martyn Plummer <plummer@iarc.fr>
*

*>> To: Prof Brian D Ripley <ripley@stats.ox.ac.uk>
*

*>> Subject: RE: Is a ts of length one a ts? (PR#245)
*

*>> Cc: r-devel@stat.math.ethz.ch, R-bugs@biostat.ku.dk
*

*>>
*

*>> On 11-Aug-99 Prof Brian D Ripley wrote:
*

*>> > On Wed, 11 Aug 1999, Martyn Plummer wrote:
*

*>> >
*

*>> >> On 10-Aug-99 ripley@stats.ox.ac.uk wrote:
*

*>> >> > Is there any good reason why a time series of length one is not
*

*>> >> > allowed by [.ts (you can certainly create one with ts)?
*

*>> >>
*

*>> >> It certainly has a start and an end value, but what's the
*

*>> >> frequency? If you want the tsp attribute of a time series
*

*>> >> to be well defined, then maybe you should disallow series
*

*>> >> of length 1.
*

*>> >
*

*>> > I think the frequency is perfectly well defined, as that of the series you
*

*>> > subsetted.
*

*>>
*

*>> Is it?
*

*>>
*

*>> R 0.64.2:
*

*>>
*

*>> R> x <- ts(1:10)
*

*>> R> frequency(x)
*

*>> [1] 1
*

*>> R> y <- x[c(2,4,6,8,10)]
*

*>> R> frequency(y)
*

*>> [1] 0.5
*

*>>
*

*>
*

*> I was only discussing the case of a subset of one, but you omitted
*

*> the next sentence. What I actually said was
*

*>
*

*> `I think the frequency is perfectly well defined, as that of the series you
*

*> subsetted. How can extracting one observation change the frequency of
*

*> observation?'
*

*>
*

*> Obviously, subsetting a regularly spaced series can change the frequency.
*

Yes, I am sorry for clipping too much of your e-mail in my reply.

I didn't develop my argument properly and it looks facetious.

Let me try again.

x <- ts(1:10)

y <- x[c(1,3,5,7,9)]

x1 <- x[1]

y1 <- y[1]

The question we disagree on, I think, is "Are x1 and y1 the same?".

I would say yes. I don't think that the attributes of a subset should

depend on the sequence of commands used to derive it, and I can't think of

another example in R or S where this is the case. Although R 0.64.2 and

S-PLUS behave differently here, they both conform to this notion.

Underlying my argument is the notion that two time series with a single

observation and the same start value are identical. The fact that ts()

can construct two such time series with different frequencies blows a

large hole in my argument so I'll stop.

Martyn

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**Next message:**Martin Maechler: "weighted.residuals() ?"**Previous message:**Kurt Hornik: "Re: INSTALL script does not handle spaces in pathnames correctly (PR#242)"**In reply to:**Prof Brian Ripley: "RE: Is a ts of length one a ts? (PR#245)"**Reply:**Martyn Plummer: "RE: Is a ts of length one a ts? (PR#245)"

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