.... but swapping entire bytes ...

In addition to the endianness issue, there's also the "precision" issue that can trip one up when print()ing NV values.
On a perl whose nvtype is double: 
D:\>perl -le "print unpack 'd', pack 'd', sqrt 2;"
1.4142135623731

[download]
On a perl whose nvtype is the 80-bit extended precision long double: 
D:\>perl -le "print unpack 'd', pack 'd', sqrt 2;"
1.41421356237309515

[download]
On a perl whose nvtype is either the IEEE 754 long double or the __float128: 
D:\>perl -le "print unpack 'd', pack 'd', sqrt 2;"
1.41421356237309514547462185873883

[download]
You might think that the differences in those 3 one liners occur because pack 'd', sqrt 2 has returned different bytes in each of those 3 one liners, but that's not so.
In each of those 3 one-liners pack 'd', sqrt 2 returned exactly the same byte.
So, it must be that the unpacking returned different values ? Wrong again.
It's the way that perl's print() function conceals the actual value returned by unpack 'd', pack 'd', sqrt 2 that accounts for the anomalies.

Perl is bullshitting in all 3 cases (and knows it).
On a perl whose nvtype is double: 
D:\>perl -le "print 'WTF' if (unpack 'd', pack 'd', sqrt 2) != 1.41421
+35623731;"
WTF

[download]
On a perl whose nvtype is the 80-bit extended precision long double: 
D:\>perl -le "print 'WTF' if (unpack 'd', pack 'd', sqrt 2) != 1.41421
+356237309515;"
WTF

[download]
On a perl whose nvtype is either the IEEE 754 long double or the __float128: 
D:\>perl -le "print 'WTF' if (unpack 'd', pack 'd', sqrt 2) != 1.41421
+356237309514547462185873883;"
WTF

[download]
3 different renditions of the same value - none of them accurate.
I recall a quote (probably from Larry) from a while ago, along the lines of "perl makes the hard things easy".
When it comes to certain aspects of the way perl deals with NVs, it's more a case of "perl makes the easy things hard".
I think it's ridiculous and, although I'm scathing of it, I'm also used to it and sort of like the way that it keeps me on my toes ;-)

So ... for a given $byte, what one expects to be returned by unpack "d", $byte can depend not only upon machine endianness, but also on NV type.

Cheers,
Rob

In reply to Re^6: Converting bytes to floats by syphilis
in thread Converting bytes to floats by Anonymous Monk

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