[R] funny arrows

[Prof Brian D Ripley]

On 6 Sep 2000, Peter Dalgaard BSA wrote:

> "Ritter, Christian C SRTCL-CTGAS" <Christian.C.Ritter at OPC.shell.com> writes:
> 
> > The result of 
> > > plot(c(-1,1),c(-1,1),type="n")
> > > arrows(0,0,1,1)
> > > arrows(0,0,0,0)
> > is not what I expected. The first arrow is drawn correctly. The second
> > does not collapse to a point but goes from (0,0) off to
> > (-infinity,infinity).

I thought screens of infinite size were infinitely expensize and/or
infinitely slow.

> > I guess something is divided by zero in the internal function.
> 
> R-1.1.1 on Linux behaves OK (if a lonely arrowhead at (0,0) pointing
> left is OK)

Clearly it is not, as there is no information on the arrowhead direction.

As others have reported, what happens depends on the precise run-time
system in use (so `Linux' is too vague here).  Internally R computes
atan2(0,0), and most systems will give 0.0 but that is hardly a definitive
answer.  For what it is worth, the prototype of the next version of
R for Windows gives a left-pointing arrow too.

S-PLUS gives nothing, and I think that is the right answer.  Indeed,
arrows(0,0,1e-8, 0) gives nothing too.

-- 
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 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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