swapbytes order problem with uint64?
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Tony Tse
el 10 de Sept. de 2017
Comentada: Walter Roberson
el 10 de Sept. de 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
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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Matt J
el 10 de Sept. de 2017
Editada: Matt J
el 10 de Sept. de 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 comentarios
Matt J
el 10 de Sept. de 2017
Even if you swap the hex string yourself, I suspect you still have to make sure the swapped result obeys flintmax.
Más respuestas (1)
Walter Roberson
el 10 de Sept. de 2017
Do not use hex2dec() for this purpose. Use sscanf()
sscanf('b0120c0a7799ba3e', '%lx')
ans =
uint64
12687216339351878206
%lx is an unsigned 64 bit format.
2 comentarios
Walter Roberson
el 10 de Sept. de 2017
If you are working with something that is an IEEE double, then
swapbytes(hex2num('b0120c0a7799ba3e'))
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