OpenEuphoria

RE: BUG! Erratic for loop behavior

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

Here's another example:

atom sum
sum = 60.0

for i = 1 to 45 do
  sum += .1
end for
? sum
? equal(sum,64.5)
? (sum = 64.5)
Output is:

64.5
0
0

Please don't tell me this is desirable behavior.  Explaining the cause 
of the problem doesn't remove the problem.  I am well aware of how 
computers represent numbers but since I'm only dealing with one decimal 
place I should get at least that much precision (actually, reverse that 
-- I am getting too much precision) when doing comparsions...


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"?
> 
>