mhx@[EMAIL PROTECTED]
(Marcel Hendrix) writes:
>anton@[EMAIL PROTECTED]
(Anton Ertl) writes Re: [SPOILER] Re:
Euler problem #187
>
>> mhx@[EMAIL PROTECTED]
(Marcel Hendrix) writes:
>> >anton@[EMAIL PROTECTED]
(Anton Ertl) writes Re: [SPOILER] Re:
Euler problem #187
>[..]
>>>Your code, pasted into Gforth-fast, generates a number that is rejected
by
>>>the Euler solution checker?
>
>> If I run it on a 64-bit system, the result is correct. If I run it on
>> a 32-bit system, or if I run it on iForth, it is too high by 1. It's
>> not obvious where the bug is.
I found and fixed the bug:
: count-p2 { n1 -- n2 }
0 limit 1- n1 / 3 - 2/ 1+ n1 3 - 2/ 0 max +do
flags i + c@[EMAIL PROTECTED]
+ loop ;
Without the "0 MAX", for n1=2, this accessed element -1 of FLAGS; this
element corresponds to the number 1, but is obviously not initialized.
Apparently it happened that 32-bit Gforth and iForth had a 1 there and
64-bit Gforth had a 0 there. Strange, there could have been 254 other
values there.
>You have strong evidence that the 64-bit system (and the algorithm)
>are perfect?
Well, having it do the right thing for limit=30 and then getting the
result accepted by the Euler website was made me pretty confident, but
obviously that was overconfidence.
> I can not read the code with these funny loops.
Read up on them in
http://www.complang.tuwien.ac.at/forth/gforth/Docs-html/Counted-Loops.html
They are just improved versions of the regular counted loops. I guess
I would have made another error or two with the regular counted loops.
However, the reverse loop in SOLUTION can be replaced with a forward
loop here (which could be implemented with DO...LOOP), but if we want
to do the optimization I mentioned at the end of the program, we need
the reverse loop.
BTW, I now think that we can get a significant speedup by making
PRIMES stop when it reaches sqrt(plimit).
- anton
--
M. Anton Ertl http://www.complang.tuwien.ac.at/anton/home.html
comp.lang.forth FAQs: http://www.complang.tuwien.ac.at/forth/faq/toc.html
New standard: http://www.forth200x.org/forth200x.html
EuroForth 2008:
http://www.complang.tuwien.ac.at/anton/euroforth/ef08.html
|
