Indeed. The previous post was done using 5.10.1; this is 5.22:

C:\Program Files>\Perl22\bin\perl.exe \perl22\bin\p1.pl
printf "% 25.17g\n", -8.2727285363069939e-293;;
                     -8.2727285363069883e-293        ## manually reali
+gned to highlight the difference.
[0]{} Perl>

[download]

Of more importance is the source of the numbers that apparently cannot be represented by 64-bit floating point. At least in perl.

They are produced & output by the C++ code I'm optimising. I originally thought that the C++ math was producing denormals and C++ was outputting them unnormalised, but that does not seem to be the case:

print join ' ', unpack 'a1 a11 a52', scalar reverse unpack 'b64', pack
+ 'd', $_ for -8.2727285363069939e-293, -8.2727285363069883e-293;;
1 00000110100 1010011010101110110010001111100101000001001101001100
1 00000110100 1010011010101110110010001111100101000001001101000111

[download]

Even the latest perl seems just to be getting the conversion wrong!? Both numbers are representable with 64-bit Ieee754 numbers, so why Perl should silently convert one to the other is something of a mystery. Basically, Perl's (or the underlying CRT) input routine just seems to be broken.

Update: The 5.22 perl I'm using is built with the same compiler and libraries as the C++ code; so this seems to be a PerlIO issue rather than a CRT issue.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)
In the absence of evidence, opinion is indistinguishable from prejudice. Not understood.

In reply to Re^2: Determining the minimum representable increment/decrement possible? (Perl's scanf broken?) by BrowserUk
in thread Determining the minimum representable increment/decrement possible? by BrowserUk

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