ggplot2/plyr interaction with latest R-devel?
On 02/09/2014 12:43, peter dalgaard wrote:
Impressive. Never ceases to amaze me what computers can do these days. ;-)
It's even more impressive given that we have
static double logbase(double x, double base)
{
#ifdef HAVE_LOG10
if(base == 10) return x > 0 ? log10(x) : x < 0 ? R_NaN : R_NegInf;
#endif
#ifdef HAVE_LOG2
if(base == 2) return x > 0 ? log2(x) : x < 0 ? R_NaN : R_NegInf;
#endif
return R_log(x) / R_log(base);
}
which, except possibly for base-10 and base-2, would seem to be quite hard to convince to return anything other than 1 if x == base....
I suspect PD knows (or has known) why, but for the sake of those who are not much bitten by ix86 compilers .... It's the curse of extended-precision arithmetic (and not enough registers). It does compute in an FPU register, then store z1 = R_log(x) compute z2 = R_log(base) in the FPU load z1 to an FPU register. compute z1/z2 (at least at -O2 with the version of gcc whose assembler output I looked at). So z1 is has a 64-bit mantissa and z2 a 53-bit one. x86_64 has more registers, and so avoids the store. This can be circumvented on i686 by compiling with -msse2, which for some reason Linux distros do not make the default. floor(log2(x)/3) is more reliable (and this is the underlying reason why we have log10 and log2).
-pd On 02 Sep 2014, at 03:27 , Ben Bolker <bbolker at gmail.com> wrote:
log(8, base=8L)-1 log(8, base=8)-1 logvals <- setNames(log(2:25,base=2:25)-1,2:25) logvals[logvals!=0] ## 5,8,14,18,19,25 all == .Machine$double.eps/2
Brian D. Ripley, ripley at stats.ox.ac.uk Emeritus Professor of Applied Statistics, University of Oxford 1 South Parks Road, Oxford OX1 3TG, UK