RE: [Rd] 1/tan(-0) != 1/tan(0)
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 -- XFMail -- __ R-devel@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
Re: [Rd] 1/tan(-0) != 1/tan(0)
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 __ R-devel@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
Re: [Rd] 1/tan(-0) != 1/tan(0)
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 __ R-devel@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-devel