Rob Dixon wrote:
> Hacksaw wrote:
> >
> > > now, a stupid solution is:
> > >
> > > for (my $value = -1000; $value <= 1000; $value += 100) {
> > > print $value/1000, "\n";
> > > }
> > >
> > > hehe,
> > >
> >
> > Sadly, it's not as stupid as you think. Unless I misunderstand things, what
> > you are seeing here is a problem called IEEE 754 floating point.
> >
> > I'm sure there is some explanation that someone could offer as to why it's the
> > "best" thing, but IEEE754 doesn't represent simple decimals very well. It
> > converts them into binary using an odd method allowing it to represent the
> > number in one chunk, avoiding the mantissa and exponent form.
> >
> > However, this encoding can't represent any decimal not ending in 5 finitely,
> > much the same way it's not possible to represent 1/3 in decimal finitely.
> >
> > The upshot of this is that unless you *really* need floating point math, and
> > are willing to do what is necessary to compensate for the error that will
> > creep in, you should stay away from it.
> >
> > Smart folks will often represent monetary values in hundreths or thousanths of
> > cents, just to avoid floating point math.
>
> It's also worth pointing out here that rational values are almost always what is
> wanted in this sort of situation.
>
> The (very nice indeed) Math::Fraction module lets you do just this. The only
> changes to the code are to initialise $value as a Math::Fraction object with
>
> my $value = frac -1
>
> and to convert the value back to decimal for printing (otherwise
> the values would appear as ".. 1/10 1/5 3/10 .." etc.).
>
> HTH,
>
> Rob
Sounds good. I've always tried to do any multiplication steps [at least within the
range expressed as true integers, which in Perl is pretty large] before dividing.
Mostly I just ahve a hunch that rounding errors will be minimized if numbers are
kept in integfer frm as long as possible.
Joseph
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