Converting binary to double precision

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

I'm having difficulties converting binary to double precision floating point. Using an on-line calculator, here is what the number should be: 5.71713084433028663131071134273E-5 Here's the number in hex:

unpack("H16",$buffer);
#Returns 3f0d f967 3344 c570
[download]

Here is the number in "d":

unpack("d",$buffer);
#Returns 1.69044404965372e+235
[download]

Something is clearly wrong. Reading the unpack documentation, there is no option for "D". Is there another way I can convert this number?

Comment on Converting binary to double precision Select or Download Code

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Re: Converting binary to double precision (big-endian) by BrowserUk (Patriarch) on Feb 24, 2014 at 19:06 UTC
Your value comes from a big-endian system, so you'll need `'d>'` to decode it: `$n = pack 'H16', '3f0df9673344c570';; print unpack 'd>', $n;; 5.71713084433029e-005` [download] You could also do the byte swapping yourself: `$n = pack 'H16', '3f0df9673344c570';; print unpack 'd', scalar reverse $n;; 5.71713084433029e-005` [download] 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". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l] [select]
Re^2: Converting binary to double precision (big-endian) by ahoriuch (Acolyte) on Feb 24, 2014 at 21:34 UTC
In my case, this was the correct answer. I needed to switch from big to little-endian. It's amazing how little there is on the internet on this subject. Hopefully this will help future users. `my $n = pack 'H16', '3f0df9673344c570';; printf "Big\n"; print unpack ("d", scalar reverse $n); #Big #5.71713084433029e-05 printf "\nLittle\n"; print unpack ("d",$n); printf "\n"; #Little #1.69044404965372e+235` [download]	[reply] [d/l]
Re: Converting binary to double precision by toolic (Bishop) on Feb 24, 2014 at 18:25 UTC
Reading the unpack documentation, there is no option for "D" pack `D A float of long-double precision in native format. (Long doubles are available only if your system supports long double values _and_ if Perl has been compiled to support those. Raises an exception otherwise.)` [download]	[reply] [d/l]
Re^2: Converting binary to double precision by ahoriuch (Acolyte) on Feb 24, 2014 at 18:35 UTC
Is there a specific version/ install that I need? Invalid type 'D' in unpack at ... line 112, <FILE> chunk 1.	[reply]
Re^3: Converting binary to double precision by toolic (Bishop) on Feb 24, 2014 at 18:37 UTC
I'm running on 5.12.2, and I see it. What version of perl do you use? UPDATE: I don't see it in 5.8.8. Also, I can't find any mention of a change to pack in either perl5100delta or perl5120delta.	[reply]
Re^4: Converting binary to double precision by ahoriuch (Acolyte) on Feb 24, 2014 at 18:55 UTC
Re: Converting binary to double precision by Anonymous Monk on Feb 24, 2014 at 18:42 UTC
If you know that you are dealing with an x86 platform, this approach might work, but Perl is platform-independent. The binary representation of a floating point number among CPU-types is not the same. For that matter, there is more than one floating-point type available on a single CPU. The binary representation will vary considerably.	[reply]


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