RE: [Rd] 1/tan(-0) != 1/tan(0)

2005-06-01 Thread Ted Harding
On 01-Jun-05 Martin Maechler wrote:
 Testing the code that Morten Welinder suggested for improving
 extreme tail behavior of  qcauchy(),
 I found what you can read in the subject.
 namely that the tan() + floating-point implementation on all
 four different versions of Redhat linux, I have access to on
 i686 and amd64 architectures,
 
  1/tan(c(-0,0))
 gives
 -Inf  Inf
 
 and of course, that can well be considered a feature, since
 after all, the tan() function does jump from -Inf to +Inf at 0. 
 I was still surprised that this even happens on the R level,
 and I wonder if this distinction of -0 and 0 shouldn't be
 mentioned in some place(s) of the R documentation.

Indeed I would myself consider this a very useful feature!

However, a query: Clearly from the above (ahich I can reproduce
too), tan() can distinguish between -0 and +0, and return
different results (otherwise 1/tan() would not return different
results).

But how can the user tell the difference between +0 amnd -0?
I've tried the following:

   sign(c(-0,0))
  [1] 0 0
   sign(tan(c(-0,0)))
  [1] 0 0
   sign(1/tan(c(-0,0)))
  [1] -1  1

so sign() is not going to tell us. Is there a function which can?

Short of wrting one's own:

  sign0 -
function(x){
  if(abs(x)0) stop(For this test x must be +0 or -0)
  return(sign(1/tan(x)))
}

;)

Best wishes,
Ted.



E-Mail: (Ted Harding) [EMAIL PROTECTED]
Fax-to-email: +44 (0)870 094 0861
Date: 01-Jun-05   Time: 10:50:06
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Re: [Rd] 1/tan(-0) != 1/tan(0)

2005-06-01 Thread Simon Urbanek

On Jun 1, 2005, at 5:50 AM, (Ted Harding) wrote:

However, a query: Clearly from the above (ahich I can reproduce  
too), tan() can distinguish between -0 and +0, and return different  
results (otherwise 1/tan() would not return different results).


But how can the user tell the difference between +0 amnd -0?


That's indeed a good question - by definition (-0)==(+0) is true,  
-00 is false and signum of both -0 and 0 is 0.


I don't see an obvious way of distinguishing them at R level. Besides  
computational ways (like the 1/tan trick) the only (very ugly) way  
coming to my mind is something like:

a==0  substr(sprintf(%f,a),1,1)==-
Note that print doesn't display the sign, only printf does.

At C level it's better - you can use the signbit() function/macro  
there. Any other ideas?


Cheers,
Simon

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Re: [Rd] 1/tan(-0) != 1/tan(0)

2005-06-01 Thread Gabor Grothendieck
On 6/1/05, Simon Urbanek [EMAIL PROTECTED] wrote:
 On Jun 1, 2005, at 5:50 AM, (Ted Harding) wrote:
 
  However, a query: Clearly from the above (ahich I can reproduce
  too), tan() can distinguish between -0 and +0, and return different
  results (otherwise 1/tan() would not return different results).
 
  But how can the user tell the difference between +0 amnd -0?
 
 That's indeed a good question - by definition (-0)==(+0) is true,
 -00 is false and signum of both -0 and 0 is 0.
 
 I don't see an obvious way of distinguishing them at R level. Besides
 computational ways (like the 1/tan trick) the only (very ugly) way
 coming to my mind is something like:
 a==0  substr(sprintf(%f,a),1,1)==-
 Note that print doesn't display the sign, only printf does.

On my XP machine running R 2.1.0 patched 2005-05-14

 sprintf(%f,-0)
[1] 0.00

does not print the sign.

however, the tan trick can be done without tan using just division:

R sign0 - function(x) if (x != 0) stop(x not zero) else sign(1/x)
R sign0(0)
[1] 1
R sign0(-0)
[1] -1

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