On Mon, 6 Dec 2010, William Dunlap wrote:
> I was writing some assertion tests for modelling-related
> code I had written and was surprised to see one test
> fail because the "specials" attribute of the output of
> terms() is a "pairlist" instead of a "list". In 2.12.0
> I get:
>
> > dput(attr(terms(y~Spec(x1)+x2, specials=c("Spec")),
"specials"))
> list(Spec = 2L)
> > all.equal(attr(terms(y~Spec(x1)+x2, specials=c("Spec")),
> "specials"), list(Spec=2L))
> [1] "Modes: pairlist, list"
> > all.equal(attr(terms(y~Spec(x1)+x2, specials=c("Spec")),
> "specials"), pairlist(Spec=2L))
> [1] TRUE
> > identical(attr(terms(y~Spec(x1)+x2, specials=c("Spec")),
> "specials"), pairlist(Spec=2L))
> [1] TRUE
>
> I was wondering if there was a reason for using pairlist
> instead of list here or it it was just an historical
> artifact. In general, when should one use pairlists?
Probably never directly. But indirectly, a lot as for example
argument lists are pairlists.
Looking at the code, I think this one is simply history. For
completeness I will correct ?terms.object.
--
Brian D. Ripley, ripley at 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)
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