On Wednesday 26 April 2006 22:12, Christoph Bartoschek wrote:
> Am Mittwoch, 26. April 2006 22:27 schrieb Dmitry Suzdalev:
> > On Thursday 27 April 2006 00:12, Will Entriken wrote:
> > > std::numeric_limits<double>::min() return the minimum positive value.
> >
> > Hmm... yes? :)
> > Bahhh, I should read docs more carefully, thanks! :)
> >
> > > you could try maxd = -1 if d will be non-negative. that will be pretty
> > > obvious what is going on.
> >
> > Nope, d might be negative - that's why I didn't use -1 for maxd.
> > So, then another question:
> > Maybe I'm wrong when I say that
> > "-std::numeric_limits<double>::max()" is messy-looking?
>
> If you need a value that is smaller than all numbers, you could use:
>
> maxd = - std::numeric_limits<double>::infinity();

That's all well and good, but it won't work using gcc (3.2, 3.3 and 3.4 at 
least) on Linux. The key point is that

  std::numeric_limits<double>::has_infinity

is false on these systems (or at least it is for me).

You could store a flag to indicate whether this is the first time through the 
loop, and if so then set maxd. Or you can use this:

  double minDbl = -std::numeric_limits<double>::max();

This will work for all IEEE-754 compliant systems (x86, x86_64, Solaris, etc), 
and it's what I use at work (where those are the only platforms we support), 
but it is a little messy. Alternatively, you can use something like this:

double makeInfinity()
{
  double x = std::numeric_limits<double>::max();
  while (x * x > x) x = x * x;
  return x;
}
double infinity()
{
  static double inf = makeInfinity();
  return inf;
}

This will work on all systems which actually have an infinity (whether the C++ 
standard library knows about it or not). On systems which don't (if you can 
find any), you'll probably get a floating point exception of some kind when 
it overflows.

One final thing... on Intel-based machines, computations involving infinity or 
NaN floating-point values are VERY VERY slow (hundreds of times slower than 
normal floating-point operations), so you probably don't want to use infinity 
after all. Best to just use -std::numeric_limits<double>::max() or a flag.

Richard
 
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