Insights sought into floating point division strangeness

EvdB has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Insights sought into floating point division strangeness by swampyankee (Parson) on Feb 07, 2006 at 17:44 UTC
The reason you're getting output like `103 / 103 ! = 1 (==1)` is that the default format used for printing is not showing enough precision; 1 +/- epsilon is getting rounded to 1. You can either reset `$#` to something like "%15.12f" or use a `printf` with an explicit format. emc " When in doubt, use brute force." — Ken Thompson	[reply] [d/l]
Re: Insights sought into floating point division strangeness by samtregar (Abbot) on Feb 07, 2006 at 18:58 UTC
That is very odd. Those aren't floats and neither should the result be. What does this output: `use Devel::Peek; Dump($int); Dump($number);` [download] That will show whether $int and $number are being stored as IVs (as they obviously should be) or NVs. -sam	[reply] [d/l]
Re: Insights sought into floating point division strangeness by Roy Johnson (Monsignor) on Feb 07, 2006 at 17:46 UTC
I get no output at all from that, and I wouldn't expect any. Floating point error doesn't creep in when you're dividing something by itself. Both the numerator and denominator will have exactly the same internal representation. Caution: Contents may have been coded under pressure.	[reply]
Re: Insights sought into floating point division strangeness by spiritway (Vicar) on Feb 08, 2006 at 01:37 UTC
I've got perl 5.8.7 on Windows (ActiveState) - and I get no output. I'm wondering whether there is some OS-specific issue where your numbers (or some of them) are being converted to floats silently. I'm also wondering whether it would help to try something like this: `use strict; use warnings; my $number_one=1; foreach my $int ( 1 .. 600 ) { my $number = $int / $int; next if $number == $number_one; warn "$int / $int != 1 (== $number)\n"; }` [download] I'm thinking that by comparing variables to another variable, instead of a literal constant, you may get everyone to be in the same form, and thus avoid this problem.	[reply] [d/l]
Re: Insights sought into floating point division strangeness by EvdB (Deacon) on Feb 08, 2006 at 09:17 UTC
I think that the reason I am getting this odd behaviour is that perl was compiled with fastmath support. It is now being taken out so I'll test again later. Cheers for the comments.	[reply]
Re^2: Insights sought into floating point division strangeness by BrowserUk (Patriarch) on Feb 08, 2006 at 09:49 UTC
perl was compiled with fastmath support `-ffast-math` was one of the options from your -V log that caused me ask how your Perl was configured/built. The other was the `-march=pentium2`, which is probably perfectly valid, but seems way back level unless it's a very old machine or the `-mcpu=athlon-xp` requires it? If your looking at your build configuration, it might be worth questioning these two also. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal? "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l] [select]
Re^3: Insights sought into floating point division strangeness by abs0 (Beadle) on Feb 08, 2006 at 13:17 UTC
As the perpetrator of the aforementioned perl, we have a standard set of perl binaries across each os/arch, and as the lowest spec machine we have in service is a PII everything still gets -march=pentium2. The -ffast-math was probably a little over enthusiastic. Amusingly enough checking an alpha box shows it gets compiled with -mieee which ensure it has (slower) IEE 754 compliant behaviour, and so does not show this up. Needless to say our perl binaries are now !-ffast-math ...	[reply]
Re^4: Insights sought into floating point division strangeness by BrowserUk (Patriarch) on Feb 08, 2006 at 14:17 UTC
Re: Insights sought into floating point division strangeness by socketdave (Curate) on Feb 07, 2006 at 17:44 UTC
I don't see this behavior on 5.8.4 on Linux.	[reply]
Re^2: Insights sought into floating point division strangeness by EvdB (Deacon) on Feb 07, 2006 at 17:59 UTC
This is my setup (5.8.7 on NetBSD): Read more... (5 kB)	[reply] [d/l]
Re^3: Insights sought into floating point division strangeness by ikegami (Patriarch) on Feb 07, 2006 at 18:40 UTC
Nothing is printed with Perl v5.8.6 built for i386-freebsd-64int	[reply]
Re: Insights sought into floating point division strangeness by BrowserUk (Patriarch) on Feb 07, 2006 at 17:55 UTC
What version/platform of perl are you running on? I don't see, nor would I expect to see any "hits" from this? Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal? "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l]
Re: Insights sought into floating point division strangeness by BrowserUk (Patriarch) on Feb 07, 2006 at 18:51 UTC
Does your output change if you add `use integer;` to your test? Also, is this a self-built (or self configured) perl, or a vendored install? Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal? "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l]


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