From reid_huntsinger at merck.com  Mon Jan 31 18:42:05 2005
From: reid_huntsinger at merck.com (Huntsinger, Reid)
Date: Mon Jan 31 18:42:29 2005
Subject: [Rd] type of list elements in .Call
Message-ID: <D9A95B4B7B20354992E165EEADA31999056A929B@uswpmx00.merck.com>

This just means that R doesn't currently do anything complicated when you
ask for a pointer to the data in an R vector object. But that doesn't
matter; the behavior of REAL etc doesn't depend on that. 

Note that INTEGER, REAL, etc just give you pointers of type int *, double *,
etc. It's up to you to make sure that a particular use makes sense or you'll
get garbage at best. As discussed in the manual, you can either do this on
the R side, or in C: first check the type of the object, then coerce if that
makes sense and is necessary, and handle errors as you see fit.

Coercion between types requires a copy, in general, so you need to
explicitly ask for that. 

Reid Huntsinger


-----Original Message-----
From: r-devel-bounces@stat.math.ethz.ch
[mailto:r-devel-bounces@stat.math.ethz.ch] On Behalf Of Faheem Mitha
Sent: Monday, January 31, 2005 10:52 AM
To: Prof Brian Ripley
Cc: r-devel@stat.math.ethz.ch
Subject: Re: [Rd] type of list elements in .Call




On Mon, 31 Jan 2005, Prof Brian Ripley wrote:

> This is covered, copiously, in the examples in `Writing R Extensions'. 
> Hint: search for coerceVector.

I see. I thought that INTEGER and its relatives did coercion too, but now 
I see that is not stated anywhere.

Eg. REAL is first used in "Writing R Extensions" in 4.7.1 as in

REAL(ab)[0] = 123.45
...

and I cannot find a discussion about what it does previous to that, or 
indeed afterwards.

In Rinternals header file, it says

/* Under the generational allocator the data for vector nodes comes
    immediately after the node structure, so the data address is a
    known ofset from the node SEXP. */

This does not mean much to me. Perhaps a short comment in the 
documentation would be helpful, if not already present.

                                                                   Faheem.

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[Rd] type of list elements in .Call
Huntsinger, Reid