Re: [Rd] documenation for arrows() is backwards (PR#7839)

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Thu 05 May 2005 - 11:07:26 GMT

hoffman.mm+R-project.org@ebi.ac.uk writes:

> Full_Name: Michael Hoffman
> Version: 2.1.0
> OS: Linux (Fedora Core 3, kernel 2.6.11-1.14_FC3)
> Submission from: (NULL) (193.62.199.8)
>
>
> help(arrows) says:
>
> """
> arrows(x0, y0, x1, y1, length = 0.25, angle = 30, code = 2,
> col = par("fg"), lty = NULL, lwd = par("lwd"), xpd = NULL)
>
> ...
>
> If 'code=2' an arrowhead is drawn at '(x0[i],y0[i])' and if
> 'code=1' an arrowhead is drawn at '(x1[i],y1[i])'. If 'code=3' a
> head is drawn at both ends of the arrow. Unless 'length = 0',
> when no head is drawn.
> """
>
> If you do:
>
> plot(c(-1, 1), c(-1, 1), col=0)
> arrows(x0=0, y0=0, x1=0, y1=1, code=2)
>
> it is pretty clear that the arrowhead is drawn at x1, y1 with code=2. If you
> switch to code=1, the arrowhead is drawn at x0, y0. Either the documentation or
> the function is incorrect.

One way to decide is to compare with Splus. Now that doesn't have a 'code' argument and the arguments are named x1,y1,x2,y2 (I wonder why R wanted to be different here?), but they do put the arrowheads at the *to* end, which does seem to be the sensible thing to do.

Arguably, using 'code=2' as the default is a bit weird, but changing it could be quite painful. I.e., we should fix the docs.

BTW, the docs also might say that code=0 makes arrows behave like segments(). It is also a bit strange that the code argument isn't vectorized, which might have been useful.

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

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Received on Thu May 05 21:15:22 2005

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