[Rd] ranges and contiguity checking

[William Dunlap]

> -----Original Message-----
> From: r-devel-bounces at r-project.org 
> [mailto:r-devel-bounces at r-project.org] On Behalf Of Duncan Murdoch
> Sent: Wednesday, May 12, 2010 11:35 AM
> To: bullard at stat.berkeley.edu
> Cc: r-devel at stat.math.ethz.ch
> Subject: Re: [Rd] ranges and contiguity checking
> 
> On 12/05/2010 2:18 PM, James Bullard wrote:
> > Hi All,
> >
> > I am interfacing to some C libraries (hdf5) and I have 
> methods defined for
> > '[', these methods do hyperslab selection, however, currently I am
> > limiting slab selection to contiguous blocks, i.e., things 
> defined like:
> > i:(i+k). I don't do any contiguity checking at this point, 
> I just grab the
> > max and min of the range and them potentially do an 
> in-memory subselection
> > which is what I am definitely trying to avoid. Besides 
> using deparse, I
> > can't see anyway to figure out that these things (i:(i+k) 
> and c(i, i+1,
> > ..., i+k)) are different.
> >
> > I have always liked how 1:10 was a valid expression in R 
> (as opposed to
> > python where it is not by itself.), however I'd somehow 
> like to know that
> > the thing was contiguous range without examining the un-evaluated
> > expression or worse, all(diff(i:(i+k)) == 1)

You could define a sequence class, say 'hfcSeq'
and insist that the indices given to [.hfc are
hfcSeq objects.  E.g., instead of
    hcf[i:(i+k)]
the user would use
    hcf[hfcSeq(i,i+k)]
or
    index <- hcfSeq(i,i+k)
    hcf[index]
max, min, and range methods for hcfSeq
would just inspect one or both of its
elements.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com  

> 
> You can implement all(diff(x) == 1) more efficiently in C, 
> but I don't 
> see how you could hope to do any better than that without 
> putting very 
> un-R-like restrictions on your code.  Do you really want to say that
> 
> A[i:(i+k)]
> 
> is legal, but
> 
> x <- i:(i+k)
> A[x]
> 
> is not?  That will be very confusing for your users.  The problem is 
> that objects don't remember where they came from, only arguments to 
> functions do, and functions that make use of this fact mainly 
> do it for 
> decorating the output (nice labels in plots) or making error messages 
> more intelligible. 
> 
> Duncan Murdoch
> 
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