interpret 4 byte as 32-bit float (IEEE-754)

Terry Reedy tjreedy at
Sat Jan 15 14:36:13 EST 2005

"Scott David Daniels" <Scott.Daniels at Acm.Org> wrote in message 
news:41e952b9$1 at
> franzkowiak wrote:
>> I've read some bytes from a file and just now I can't interpret 4 bytes 
>> in this dates like a real value.  An extract from my program:
>> def l32(c):
>>     return ord(c[0]) + (ord(c[1])<<8) + (ord(c[2])<<16) + 
>> (ord(c[3])<<24)
>> ...
>> value = l32(      <---  3F 8C CC CD  should be 1.11
> OK, here's the skinny (I used blocks & views to get the answer):
> import struct
> bytes = ''.join(chr(int(txt, 16)) for txt in '3F 8C CC CD'.split())
> struct.unpack('>f', bytes)
> I was suspicious of that first byte, thought it might be an exponent,
> since it seemed to have too many on bits in a row to be part of 1.11.

I believe exponents are typically stored as a positive offset from the 
largest negative exponent.  3F8 is about half of 7FF, so that seems about 
right for an actual exponent of 0.

Terry J. Reedy

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