From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>

Date: Tue 29 Mar 2005 - 08:09:57 GMT

Date: Tue 29 Mar 2005 - 08:09:57 GMT

On Tue, 29 Mar 2005, Berwin A Turlach wrote:

> Dear Brian,

*>
**>>>>>> "BDR" == Prof Brian Ripley <ripley@stats.ox.ac.uk> writes:
**>
**> BDR> On Tue, 29 Mar 2005, Berwin A Turlach wrote:
**> >> Dear all,
**> >>
**> >> I would like to suggest changing the pairs.formula command such
**> >> that a command like
**> >> [...]
**>
**> BDR> Perhaps you could explain what precisely 'the job' is
**> Sorry, for not making it clear enough, must be that English is my
**> second language. What I meant is, that with the above definition of
**> pairs.formula a command like
**> pairs(GNP ~ . - Year - GNP.deflator, longley)
**> would only produce a pairwise scatterplot of GNP and all other
**> variables in the dataframe except for Year and GNP.deflator. And,
**> incidentally, so would
**> pairs( ~ . - Year - GNP.deflator, longley)
*

Still not a description, just two examples. Since later on you took a description to be an example, I see the confusion.

> BDR> and why you chose such an unusual piece of code to do it?

> Mmh, I don't think of it as being so unusual, most of it was gleaned

*> from other R function. Well, I realise that R programing paradigms
**> change over the years, so I must have gotten them from quite old
**> routine.
*

I guess you got it from an S not R function.

> BDR> (E.g. what is the prescription for the ordering of terms, and

*> BDR> why do you think the rownames of the factors and the
**> BDR> variables might be in different orders? They are set the
**> BDR> same in the C code.)
**> In case that a user foolishly specifies a more complicated formula
**> having not read the help pages. It seemed to me that this was the
**> only construct to figure out which variables are actually appearing in
**> terms of the formula.
*

Really? Please check what I wrote: `variables' and `rownames of the factors' are always the same, apart from the response. Please show an example where you got something different.

> BDR> BTW, the help page specifically warns against a formula of

*> BDR> the type you specified.
**> Does it? I searched the help page (R 2.0.1) for `.' and couldn't find
**> anything about not using it in a formula. But then, again, perhaps I
**> missed something with English being my second language......
**>
**> BDR> Why do you want to allow a response?
*

> Why not, the current version allows it too. At least the help page in

*> my version of R states:
**>
**> (A response will be interpreted as another variable, but not
**> treated specially, so it is confusing to use one.)
**>
**> Actually, I noticed that the response variable will always be in the
**> top row, so it is treated specially. Try it out and compare:
**>
**>> pairs(longley)
**>> pairs(GNP ~ ., longley)
*

Yes, but those are different pairs() methods. It is the same as pairs(~ GNP + ., longley), so it is not treated specially.

> BDR> Currently only '+' is documented to work.

*> The help page for pairs in my version of R states:
**>
**> formula: a formula, such as '~ x + y + z'. Each term will give a
**> separate variable in the pairs plot, so terms should be
**> numeric vectors. (A response will be interpreted as another
**> variable, but not treated specially, so it is confusing to
**> use one.)
**>
**> I don't interpret this as "only '+' is documented to work". For me
**> this is just an example of an allowed formula, but an exhaustive
**> listing of those formulas that are allowed. But then, despite the
**> danger of repeating myself, English is only my second language....
*

There are several places where only the allowed form of formulae is specified in this way. You are not allowed interactions, for example, and it refers to `each term'. `.' is not documented to work (and used not to).

-- 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 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ R-devel@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-develReceived on Tue Mar 29 18:13:38 2005

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