On Fri, Aug 31, 2007 at 08:39:02PM -0400, Duncan Murdoch wrote: [snip] > The other day I came across one in complex numbers, and it took me a > while to figure out that negative zero was what was happening: > > > x <- complex(real = -1) > > x > [1] -1+0i > > 1/x > [1] -1+0i > > x^(1/3) > [1] 0.5+0.8660254i > > (1/x)^(1/3) > [1] 0.5-0.8660254i > > (The imaginary part of 1/x is negative zero.) > > As a Friday question: are there other ways to create and detect > negative zero in R? > > And another somewhat more serious question: is the behaviour of > negative zero consistent across platforms? (The calculations above were > done in Windows in R-devel.)
Let me contribute to the statistics over platforms, which already contains powerpc-apple-darwin8.9.1 (Steven McKinney) and two different results on AMD 64 (deepayan.sarkar, Jeffrey Horner). I tried 1/Im(1/complex(real = -1)) on three different platforms, all of which are Linux, but with different CPU and different gcc version. In all cases, R-devel_2007-08-31 is used. The results differ. 1. [...]$ grep name /proc/cpuinfo model name : Pentium III (Coppermine) gcc version 4.0.2 20050901 (prerelease) (SUSE Linux) 1/Im(1/complex(real = -1)) # Inf 2. [...]$ grep name /proc/cpuinfo model name : Celeron (Coppermine) gcc version 4.1.0 (SUSE Linux) 1/Im(1/complex(real = -1)) # -Inf 3. [...]$ grep name /proc/cpuinfo # two processor machine model name : Intel(R) Xeon(TM) CPU 2.80GHz model name : Intel(R) Xeon(TM) CPU 2.80GHz gcc version 3.3.3 20040412 (Red Hat Linux 3.3.3-7) 1/Im(1/complex(real = -1)) # -Inf The result does not depend on option -ffloat-store. On Xeon CPU, it does not depend on, whether SSE (IEEE 754) arithmetic is used (-march=pentium4 -mfpmath=sse) or not. Let us try something more. z <- complex(real = -1) 1/Im(z) # [1] Inf 1/Im(Conj(z)) # [1] -Inf z <- complex(real = -1, imaginary = -0) 1/Im(z) # [1] -Inf 1/Im(Conj(z)) # [1] Inf These results are consistent across platforms. Let us look at 1/Im(1/z). Platform 1 (Pentium III): z <- complex(real = -1) 1/Im(1/z) # [1] Inf z <- complex(real = -1, imaginary = -0) 1/Im(1/z) # [1] Inf Platform 2 (Celeron): z <- complex(real = -1) 1/Im(1/z) # [1] -Inf z <- complex(real = -1, imaginary = -0) 1/Im(1/z) # [1] -Inf (*) Platform 3 (Xeon): z <- complex(real = -1) 1/Im(1/z) # [1] -Inf z <- complex(real = -1, imaginary = -0) 1/Im(1/z) # [1] Inf (*) So, besides the first difference between 1 and 2+3, there is also a difference between 2 and 3 (marked (*)). Petr Savicky. === ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
