Re: [Rd] (PR#7326)(inappropriate) manipulation of expression objects

From: <ripley_at_stats.ox.ac.uk>
Date: Tue 02 Nov 2004 - 05:50:25 EST

On 29 Oct 2004, Peter Dalgaard wrote:

> deepayan@stat.wisc.edu writes:
>
> > > foo <- expression(alpha, beta, gamma)
> > > foo[2]
> > expression(beta)
> > > foo[2] <- NA
> > > foo ## or str(foo)
> > Segmentation fault
> >
> > (Same behaviour in R 1.9.1)
> >
> > 'foo[[2]] <- NA' works fine, though.
>
> Right. The NA is not important; foo[2] <- 1 crashes R just as
> effectively. We should likely either give an error or demote [ to [[.
> The latter is what happens with lists, and expressions are basically
> just list of call objects (or names or constants). To wit:
>
> > x <- list(1,2,3)
> > x[2] <- 1
> > x
> [[1]]
> [1] 1
>
> [[2]]
> [1] 1
>
> [[3]]
> [1] 3

That's not the issue: VectorAssign has identical code for the vector list and expression cases. The issue is in SubassignTypeFix which says

    case 2010:	/* expression <- logical    */
	/* Note : No coercion is needed here. */
	/* We just insert the RHS into the LHS. */
	/* FIXME: is this true or should it be just like the "vector" case? */

and at level=1 the FIXME is correct.

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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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Received on Tue Nov 02 06:11:10 2004

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