Seekers of Perl Wisdom provide us with a steady diet of questions about why arithmetic in Perl sometimes doesn't "add up" as expected, and PM's wisdom providers get regular exercise explaining how floating point arithmetic is inherently limited in precision.

But as I was playing with some code in Snippets (file/language entropy calculator), I was surprised to find that the precision limits seem to vary from one run to the next -- or rather, the behavior of floating-point arithmetic beyond the precision limit was not constant, as I would have expected.

Both the OP's code in that Snippet thread and my trimmed-down version of same seem to produce different values on successive runs over the same input text file. Here are a few sample output values from repeating the same computation on the same input (this is running on a G4 powerbook with macosx 10.3.6, Darwin kernel version 7.6.0, perl v5.8.1-RC3 built for darwin-thread-multi-2level): 

5.35423847163199795318
5.35423847163199884136
5.35423847163199972954
5.35423847163200150590
5.35423847163200239407

[download]
and if I run it more times, I get more distinct values.

Now, it's obvious that no one should be paying attention to more than 12 or 13 digits in terms of floating-point accuracy (so wufnik should adjust his ideas about "printf" formatting), but I'm wondering whether this variability is observed on other systems.

I did take the time to check the same code and data on solaris/sparc (perl 5.5.3) and freebsd (i386 and amd64, perl 5.8.5) -- none of them showed variable behavior like I saw on the mac (solaris/sparc always came up with one answer, which happened to match one of the answers on the mac; the i386 and amd64 freebsd's both came with a consistent single answer every time, which differed from the sparc answer and matched another of the mac's results).

So it smells like a mac/G4 issue, rather than a perl issue. But it struck me as noteworthy, and I'd be interested in others' reactions.

In reply to Floating point (in)accuracy: more "interesting" than expected by graff

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