OpenEuphoria

RE: BUG! Erratic for loop behavior

• Posted by Andy Serpa <renegade at earthling.net> Mar 23, 2002
• 485 views

Chris Bensler wrote:
> Do a search for "floating point precision" in the mailing list archives.
> 

Ok, I guess there's nothing to be done about it.  I just don't 
understand why the effect is allowed to become cumulative...


> Chris
> 
> Andy Serpa wrote:
> > 
> > Rolf Schröder wrote:
> > > Andy Serpa wrote:
> > > > ...
> > > > ------------------------------
> > > > for i = 31.1 to 34.2 by .1 do
> > > >   ? i
> > > > end for
> > > > ------------------------------
> > > > stops at 34.1 (!), NOT as expected.
> > > >
> > > Hi Andy,
> > >
> > > that's ok. Look at this:
> > >
> > >     for i = 31.1 to 34.1 by 0.1 do
> > >         printf(1,"%20.15f\n",i)
> > >     end for
> > >
> > > and you will notice why: the decimal given value 0.1 is exactly
> > > representable in the computer for it has to stores it internally as an
> > > binary value with a limited number of bits. Actually, 0.1 (dec.) is has
> > > an unlimited binary representation: 0.0001100110011..... . This is the
> > > correct way to do it (have in mind that 31.1 and 34.1 are also not
> > > exactly representable!):
> > >
> > >     for i = 311 to 341 by 1 do
> > >         printf(1,"%20.15f\n",i/10)
> > >     end for
> > >
> > 
> > Yeah, that's not really good enough for me -- .1 should be .1 in a loop.
> >  The Euphoria manual even has an example under the FOR loop section
> > using a .3 "by" value...  Is this also a "feature"?
> > 
> >