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swapbytes order problem with uint64?

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Tony Tse
Tony Tse on 10 Sep 2017
Commented: Walter Roberson on 10 Sep 2017
I find that swapbytes doesn't seem to be behaving correctly for my problem. I have a 64 bits hexidecimal string printed in big-endian: 'b0120c0a7799ba3e'
Manually swapping the bytes give me the expected answer:
typecast(uint64(hex2dec('3eba99770a0c12b0')), 'double')
ans =
1.5855e-06
If I follow the example code in swapbytes however, it seems behave incorrectly. Either:
typecast(swapbytes(uint64(hex2dec('b0120c0a7799ba3e'))), 'double')
ans =
3.5031e-305
or
typecast(uint64(swapbytes(hex2dec('b0120c0a7799ba3e'))), 'double')
ans =
0
The function seem to work correctly for a single precision hexadecimal:
typecast(swapbytes(uint32(hex2dec('0cc9d435'))),'single')
ans =
single
1.5854e-06
Am I missing something, I can certainly write some loops to parse the string but is there a better solution?

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Accepted Answer

Matt J
Matt J on 10 Sep 2017
Edited: Matt J on 10 Sep 2017
I suspect the problem is actually in hex2dec(). You are exceeding the flintmax limit described in the hex2dec documentation:
d = hex2dec('hex_value') converts hex_value to its floating-point integer representation. The argument hex_value is a hexadecimal integer stored as text. If the value of hex_value is greater than the hexadecimal equivalent of the value returned by flintmax, then hex2dec might not return an exact conversion.

  3 Comments

Matt J
Matt J on 10 Sep 2017
Tony Tse commented:
That actually makes a lot of sense so I definitely need to swap the order of the hex string before conversion wit my own routine before conversion then. Thanks.
Matt J
Matt J on 10 Sep 2017
Even if you swap the hex string yourself, I suspect you still have to make sure the swapped result obeys flintmax.
Tony Tse
Tony Tse on 10 Sep 2017
In my case, because the string comes from real measured data. I think it should be never by out of range? so ended just up doing something like this:
s = 'b0120c0a7799ba3e';
s = fliplr(s);
ii = 1;
while ii < length(s)
s([ii ii+1]) = s([ii+1 ii]);
ii = ii + 2;
end
typecast(uint64(hex2dec(s)), 'double');
Actually, hex2num should work without conversion to decimal now since I am not using swapbytes and don't need to typecast:
hex2num(s)

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More Answers (1)

Walter Roberson
Walter Roberson on 10 Sep 2017
Do not use hex2dec() for this purpose. Use sscanf()
sscanf('b0120c0a7799ba3e', '%lx')
ans =
uint64
12687216339351878206
%lx is an unsigned 64 bit format.

  2 Comments

Tony Tse
Tony Tse on 10 Sep 2017
Thanks! This worked perfectly.
typecast(swapbytes(sscanf('b0120c0a7799ba3e', '%lx')), 'double')
Walter Roberson
Walter Roberson on 10 Sep 2017
If you are working with something that is an IEEE double, then
swapbytes(hex2num('b0120c0a7799ba3e'))

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