[Rd] model.matrix.default chokes on backquote (PR#7202)

[Peter Dalgaard]

"Gabor Grothendieck" <ggrothendieck at myway.com> writes:

> Peter Dalgaard <p.dalgaard at biostat.ku.dk> writes:
> >
> > "Gabor Grothendieck" <ggrothendieck at myway.com> writes:
> > 
> > > > > as.character(attr(terms(`a(b)`~`c(d)`),"variables"))
> > > > [1] "list" "`a(b)`" "`c(d)`"
> > > >
> > > > whereas for instance
> > > >
> > > > > sapply(attr(terms(`a(b)`~`c(d)`),"variables")[-1],as.character)
> > > > [1] "a(b)" "c(d)"
> > >
> > > 1. That is quite subtle but a fix based on that would appear to
> > > solve it.
> > 
> > Hmm, not quite. I tried, and terms like offset(foo) gets me in
> > trouble. Probably, I was fixing the wrong end of the original problem:
> > In the comparisons, we can't have one side with backquotes and the
> > other without them. That doesn't have to mean that they should be
> > removed from both sides, and indeed it would get us in trouble if
> > someone was perverse enough to do things like
> > 
> > y ~ `offset(foo)` + offset(foo)
> > 
> > I.e. perhaps the real issue is that names(data) gets generated without
> > backquotes.
> > 
> > Anyways, this is a real can of worms and I'm not sure we're not too
> > close to 2.0.0 to start tampering with it...
> 
> How about a partial fix that does not address pathological 
> cases where the variable names themselves have embedded 
> backquotes but does address the common cases such as:
> 
> 	y <- ts(1:10); x1 <- y^2; x2 <- y^4
> 	lm(`lag(y)` ~ ., cbind(lag(y), x1, diff(x2)) )
> 
> without having to resort to:
> 
> 	lm(lag.y ~ ., cbind(lag.y = lag(y), x1, diff(x2)) )

I've committed a fix that at least handles Gabor's examples without
breaking the test suite. 

This will *not* cure the issues paraphrased as y ~ `offset(foo)` +
offset(foo), i.e. that a "funny name" and a function call or special
construct can lead to the same variable name in the model frame. I
strongly suspect that this cannot be done without splitting everything
apart and reassembling it into a new design. So, well, just don't do
that! 

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907