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From: John Nagle on 30 Mar 2010 14:14
aditya wrote:
> On Mar 30, 10:49 am, Raymond Hettinger <pyt...(a)rcn.com> wrote:
>> On Mar 30, 8:13 am, aditya <bluemangrou...(a)gmail.com> wrote:
>>
>>> To get the decimal representation of a binary number, I can just do
>>> this:
>>> int('11',2) # returns 3
>>> But decimal binary numbers throw a ValueError:
>>> int('1.1',2) # should return 1.5, throws error instead.
>>> Is this by design? It seems to me that this is not the correct
>>> behavior.
>> The int() constructor returns integers.
>> So, look to float() for non-integral values.
>> Binary representation isn't supported yet,
>> but we do have hex:
>>
>> >>> float.fromhex('1.8')
>> 1.5
>>
>> Raymond
>
> That looks very elegant, thanks!

Hex floats are useful because you can get a string representation
of the exact value of a binary floating point number. It should
always be the case that

float.fromhex(float.hex(x)) == x

That's not always true of decimal representations, due to rounding problems.

Long discussion of this here: "http://bugs.python.org/issue1580"


John Nagle



From: Mensanator on 30 Mar 2010 14:03
On Mar 30, 10:49 am, Raymond Hettinger <pyt...(a)rcn.com> wrote:
> On Mar 30, 8:13 am, aditya <bluemangrou...(a)gmail.com> wrote:
>
> > To get the decimal representation of a binary number, I can just do
> > this:
>
> > int('11',2) # returns 3
>
> > But decimal binary numbers throw a ValueError:
>
> > int('1.1',2) # should return 1.5, throws error instead.
>
> > Is this by design? It seems to me that this is not the correct
> > behavior.
>
> The int() constructor returns integers.
> So, look to float() for non-integral values.
> Binary representation isn't supported yet,

It is supported in the gmpy module.

>>> import gmpy
>>> help(gmpy.mpf)
Help on built-in function mpf in module gmpy:

mpf(...)
mpf(n): builds an mpf object with a numeric value n (n may be any
Python number, or an mpz, mpq, or mpf object) and a
default
precision (in bits) depending on the nature of n
mpf(n,bits=0): as above, but with the specified number of bits (0
means to use default precision, as above)
mpf(s,bits=0,base=10): builds an mpf object from a string s made
up of
digits in the given base, possibly with fraction-part
(with
period as a separator) and/or exponent-part (with exponent
marker 'e' for base<=10, else '@'). If base=256, s must be
a gmpy.mpf portable binary representation as built by the
function gmpy.fbinary (and the .binary method of mpf
objects).
The resulting mpf object is built with a default precision
(in
bits) if bits is 0 or absent, else with the specified
number
of bits.

>>> gmpy.mpf('1.1',0,2)
mpf('1.5e0')


> but we do have hex:
>
>     >>> float.fromhex('1.8')
>     1.5
>
> Raymond

From: Grant Edwards on 30 Mar 2010 14:08
On 2010-03-30, John Nagle <nagle(a)animats.com> wrote:

> Hex floats are useful because you can get a string representation of
> the exact value of a binary floating point number. It should always
> be the case that
>
> float.fromhex(float.hex(x)) == x

Until you try running your program on a machine that represents floats
using a radix other than 2,4, or 16.

;)

And it works for NaN and Inf too!

It would have been nice to have had that 5-6 years ago when I had to
write my own pickle/unpickle methods for floating point values so that
inf and nan were portable between Windows and Linux.

--
Grant Edwards grant.b.edwards Yow! But they went to MARS
at around 1953!!
gmail.com
From: MRAB on 30 Mar 2010 14:52
John Nagle wrote:
> aditya wrote:
>> On Mar 30, 10:49 am, Raymond Hettinger <pyt...(a)rcn.com> wrote:
>>> On Mar 30, 8:13 am, aditya <bluemangrou...(a)gmail.com> wrote:
>>>
>>>> To get the decimal representation of a binary number, I can just do
>>>> this:
>>>> int('11',2) # returns 3
>>>> But decimal binary numbers throw a ValueError:
>>>> int('1.1',2) # should return 1.5, throws error instead.
>>>> Is this by design? It seems to me that this is not the correct
>>>> behavior.
>>> The int() constructor returns integers.
>>> So, look to float() for non-integral values.
>>> Binary representation isn't supported yet,
>>> but we do have hex:
>>>
>>> >>> float.fromhex('1.8')
>>> 1.5
>>>
>>> Raymond
>>
>> That looks very elegant, thanks!
>
> Hex floats are useful because you can get a string representation
> of the exact value of a binary floating point number. It should
> always be the case that
>
> float.fromhex(float.hex(x)) == x
>
> That's not always true of decimal representations, due to rounding
> problems.
>
Floats have a limited length, unlike ints which are virtually unlimited.

> Long discussion of this here: "http://bugs.python.org/issue1580"
>
From: Mensanator on 30 Mar 2010 17:05
On Mar 30, 1:52 pm, MRAB <pyt...(a)mrabarnett.plus.com> wrote:
> John Nagle wrote:
> > aditya wrote:
> >> On Mar 30, 10:49 am, Raymond Hettinger <pyt...(a)rcn.com> wrote:
> >>> On Mar 30, 8:13 am, aditya <bluemangrou...(a)gmail.com> wrote:
>
> >>>> To get the decimal representation of a binary number, I can just do
> >>>> this:
> >>>> int('11',2) # returns 3
> >>>> But decimal binary numbers throw a ValueError:
> >>>> int('1.1',2) # should return 1.5, throws error instead.
> >>>> Is this by design? It seems to me that this is not the correct
> >>>> behavior.
> >>> The int() constructor returns integers.
> >>> So, look to float() for non-integral values.
> >>> Binary representation isn't supported yet,
> >>> but we do have hex:
>
> >>>     >>> float.fromhex('1.8')
> >>>     1.5
>
> >>> Raymond
>
> >> That looks very elegant, thanks!
>
> >    Hex floats are useful because you can get a string representation
> > of the exact value of a binary floating point number.  It should
> > always be the case that
>
> >      float.fromhex(float.hex(x)) == x
>
> > That's not always true of decimal representations, due to rounding
> > problems.
>
> Floats have a limited length, unlike ints which are virtually unlimited.

gmpy gives you arbitrary precision floats, also.

>
>
>
> > Long discussion of this here: "http://bugs.python.org/issue1580"- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -

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