msbin to ieee

[John Machin]

On May 7, 11:37 pm, revuesbio <revues... at gmail.com> wrote:
> On 7 mai, 14:56, John Machin <sjmac... at lexicon.net> wrote:
>
>
>
> > On May 7, 10:00 pm, revuesbio <revues... at gmail.com> wrote:
>
> > > On 7 mai, 13:21, John Machin <sjmac... at lexicon.net> wrote:
>
> > > > On May 7, 6:18 pm, revuesbio <revues... at gmail.com> wrote:
>
> > > > > On 7 mai, 03:52, John Machin <sjmac... at lexicon.net> wrote:
>
> > > > > > On May 7, 7:44 am, revuesbio <revues... at gmail.com> wrote:
>
> > > > > > > Hi
> > > > > > > Does anyone have the python version of the conversion from msbin to
> > > > > > > ieee?
> > > > > > > Thank u
>
> > > > > > Yes, Google has it. Google is your friend. Ask Google. It will lead
> > > > > > you to such as:
>
> > > > > >http://mail.python.org/pipermail/python-list/2005-August/337817.html
>
> > > > > > HTH,
> > > > > > John
>
> > > > > Thank you,
>
> > > > > I've already read it but the problem is always present. this script is
> > > > > for double precision MBF format ( 8 bytes).
>
> > > > It would have been somewhat more helpful had you said what you had
> > > > done so far,  even posted your code ...
>
> > > > > I try to adapt this script for single precision MBF format ( 4 bytes)
> > > > > but i don't find the right float value.
>
> > > > > for example : 'P\xad\x02\x95' will return '0.00024924660101532936'
>
> > > > If you know what the *correct* value is, you might like to consider
> > > > shifting left by log2(correct_value/erroneous_value) :-)
>
> > > > Do you have any known correct pairs of (mbf4 string, decimal_float
> > > > value)? My attempt is below -- this is based on a couple of
> > > > descriptive sources that my friend Google found, with no test data. I
> > > > believe the correct answer for the above input is 1070506.0 i.e. you
> > > > are out by a factor of 2 ** 32
>
> > > > def mbf4_as_float(s):
> > > >     m0, m1, m2, m3 = [ord(c) for c in s]
> > > >     exponent = m3
> > > >     if not exponent:
> > > >         return 0.0
> > > >     sign = m2 & 0x80
> > > >     m2 |= 0x80
> > > >     mant = (((m2 << 8) | m1) << 8) | m0
> > > >     adj = 24 + 128
> > > >     num = mant * 2.0 ** (exponent - adj)
> > > >     if sign:
> > > >         return -num
> > > >     return num
>
> > > > HTH,
> > > > John
>
> > > well done ! it's exactly what i'm waiting for !!
>
> > > my code was:>>> from struct import *
> > > >>> x = list(unpack('BBBB','P\xad\x02\x95'))
> > > >>> x
> > > [80, 173, 2, 149]
> > > >>> def conversion1(bytes):
>
> > > b=bytes[:]
> > > sign = bytes[-2] & 0x80
> > > b[-2] |= 0x80
> > > exp = bytes[-1] - 0x80 - 56
> > > acc = 0L
> > > for i,byte in enumerate(b[:-1]):
> > > acc |= (long(byte)<<(i*8))
> > > return (float(acc)*2.0**exp)*((1.,-1.)[sign!=0])
>
> > Apart from the 2**32 problem, the above doesn't handle *any* of the
> > 2**24 different representations of zero. Try feeding \0\0\0\0' to it
> > and see what you get.
>
> > > >>> conversion1(x)
>
> > > 0.00024924660101532936
>
> > > this script come from google groups but i don't understand bit-string
> > > manipulation (I'm a  newbie). informations about bit-string
> > > manipulation with python is too poor on the net.
>
> > The basic operations (and, or, exclusive-or, shift) are not specific
> > to any language. Several  languages share the same notation (& | ^ <<
>
> > >>), having inherited it from C.
>
> > > thank you very much for your script.
>
> > Don't thank me, publish some known correct pairs of values so that we
> > can verify that it's not just accidentally correct for 1 pair of
> > values.
>
> pairs of values :
> (bytes string, mbf4_as_float(s) result)                        right
> float value
> ('P\xad\x02\x95', 1070506.0)
> 1070506.0
> ('\x00\x00\x00\x02', 5.8774717541114375e-039)         0.0

There is no way that \x00\x00\x00\x02' could represent exactly zero.
What makes you think it does? Rounding?

> ('\x00\x00\x00\x81', 1.0)
> 1.0
> ('\x00\x00\x00\x82', 2.0)
> 2.0
> ('\x00\x00@\x82', 3.0)
> 3.0
> ('\x00\x00\x00\x83', 4.0)
> 4.0
> ('\x00\x00 \x83', 5.0)
> 5.0
> ('\xcd\xcc\x0c\x81', 1.1000000238418579)                 1.1
> ('\xcd\xcc\x0c\x82', 2.2000000476837158)                  2.2
> ('33S\x82', 3.2999999523162842)                              3.3
> ('\xcd\xcc\x0c\x83', 4.4000000953674316)                  4.4

It is not apparent whether you regard the output from the function as
correct or not.

4.4 "converted" to mbf4 format is '\xcd\xcc\x0c\x83' which is
4.4000000953674316 which is the closest possible mbf4 representation
of 4.4 (difference is 9.5e-008).

The next lower mbf4 value '\xcc\xcc\x0c\x83' is 4.3999996185302734
(difference is   -3.8e-007).

Note that floating-point representation of many decimal fractions is
inherently inexact. print repr(4.4) produces 4.4000000000000004

Have you read this:
    http://docs.python.org/tut/node16.html
?

If you need decimal-fraction output that matches what somebody typed
into the original software, or saw on the screen, you will need to
know/guess the precision that was involved, and round the numbers
accordingly -- just like the author of the original software would
have needed to do.

>>> ['%.*f' % (decplaces, 4.4000000953674316) for decplaces in range(10)]
['4', '4.4', '4.40', '4.400', '4.4000', '4.40000', '4.400000',
'4.4000001', '4.40000010', '4.400000095']

HTH,
John