[Rd] segments() with zero-length arguments (PR#11192)

[Prof Brian Ripley]

[Taken off R-bugs, at least for now.]

On Thu, 17 Apr 2008, Peter Dalgaard wrote:

> ripley at stats.ox.ac.uk wrote:
>> I think we should allow only all-zero arguments (at present any
>> zero-length argument is an error), as per the R-level proposed code.
>>
>> arrows() and rect() share the code so it is much cleaner to do this
>> internally.
>>
>>
> There are precedents for not requiring all-or-none zero-length args. In
> arithmetic, "recycling" treats the case of one zero length item as if
> all had length zero
>
>> 2+numeric(0)
> numeric(0)

There are also rules for array arithmetic.

> cbind() and rbind() are a bit anomalous in that they just throw away the
> offending item:
>
>> cbind(a=1:2,b=numeric(0),c=3:4)
>     a c
> [1,] 1 3
> [2,] 2 4
>
> Only the former variant would make sense for segments(), but segments()
> does recycle, unlike lines() which complains if vectors are of different
> length.

And neither is documented. Also S has changed its recycling rules in 
various ways over its lifetime.

> I think there are cases where you might want it to just do nothing
> rather than warn and do nothing or cause an error. Consider rug()-like code:
> segments(x, ytop, x, ybot) with an empty x. (I realize that the real
> rug() uses Axis(), but the point remains.)

I agree it is moot, which is why I commented on this.  Rich's proposal was 
the one I ended up implementing, but it was not my first idea. There are 
also cases when you pass zero-length arguments in error, and I thought 
those more likely.

>
>> Done for R-devel.
>>
>> On Thu, 17 Apr 2008, rmh at temple.edu wrote:
>>
>>
>>> Uwe Ligges suggested I post this on R-bugs as a wishlist item with a
>>> proposed patch.  R considers zero-length arguments to segments() to be
>>> an error.  I would like R to allow this and to return without an
>>> error.  It occurs naturally in settings like
>>>
>>> valid <- c(FALSE, FALSE, FALSE)
>>> segments(x0[valid], y0[valid], x1[valid], y1[valid])
>>>
>>> For what it may be worth, S-Plus does not consider zero-length
>>> arguments to segments() be an error.
>>>
>>>
>>> plot(1:10)
>>> segments(1,1,10,10,col='green')
>>> segments(numeric(0), numeric(0), numeric(0), numeric(0), col='green')
>>> Error in segments(x0, y0, x1, y1, col = col, lty = lty, lwd = lwd, ...) :
>>>        invalid first argument
>>>
>>>
>>> segments.proposal <-
>>>  function (x0, y0, x1, y1, col = par("fg"), lty = par("lty"),
>>>            lwd = par("lwd"), ...) {
>>>    if (length(x0)==0 && length(y0)==0 && length(x1)==0 && length(y1)==0)
>>>      return(invisible(NULL))
>>>    .Internal(segments(x0, y0, x1, y1, col = col, lty = lty, lwd = lwd, ...))
>>> }
>>>
>>> segments.proposal(numeric(0), numeric(0), numeric(0), numeric(0), col='green')
>>>
>>> ______________________________________________
>>> R-devel at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>>>
>>
>>
>
>
> --
>   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
>  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
> (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
> ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)              FAX: (+45) 35327907
>
>
>

-- 
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 272866 (PA)
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