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signed to unsigned

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  signed to unsigned Brad Tilley <kj4eit@gmail.com> - 2012-02-17 10:51 -0800
    Re: signed to unsigned Chris Rebert <clp2@rebertia.com> - 2012-02-17 11:05 -0800
      Re: signed to unsigned Brad Tilley <kj4eit@gmail.com> - 2012-02-17 11:10 -0800
    Re: signed to unsigned Peter Otten <__peter__@web.de> - 2012-02-17 20:14 +0100
      Re: signed to unsigned Brad Tilley <kj4eit@gmail.com> - 2012-02-17 11:22 -0800
        Re: signed to unsigned Dave Angel <d@davea.name> - 2012-02-17 14:37 -0500

#20562 — signed to unsigned

In C or C++, I can do this for integer conversion:

unsigned int j = -327681234; // Notice this is signed.

j will equal 3967286062. I thought with Python that I could use struct
to pack the signed int as an unsigned int, but that fails:

>>> x = struct.pack("<I", -327681234)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: integer out of range for 'I' format code

Is there an easy way in Python to do the same conversion that C or C++
code does? Thanks for any advice.

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#20564

On Fri, Feb 17, 2012 at 10:51 AM, Brad Tilley <kj4eit@gmail.com> wrote:
> In C or C++, I can do this for integer conversion:
>
> unsigned int j = -327681234; // Notice this is signed.
>
> j will equal 3967286062. I thought with Python that I could use struct
> to pack the signed int as an unsigned int, but that fails:
>
>>>> x = struct.pack("<I", -327681234)
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
> struct.error: integer out of range for 'I' format code
>
> Is there an easy way in Python to do the same conversion that C or C++
> code does? Thanks for any advice.

Pack it as the actual type, then unpack it as the desired type:

Python 2.7.1 (r271:86832, Jul 31 2011, 19:30:53)
Type "help", "copyright", "credits" or "license" for more information.
>>> from struct import pack, unpack
>>> unpack('=I', pack('=i',-327681234))
(3967286062,)

I would think there's some more efficient way to do this though.

Cheers,
Chris

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#20565

> Pack it as the actual type, then unpack it as the desired type:
>
> Python 2.7.1 (r271:86832, Jul 31 2011, 19:30:53)
> Type "help", "copyright", "credits" or "license" for more information.>>> from struct import pack, unpack
> >>> unpack('=I', pack('=i',-327681234))
>
> (3967286062,)
>
> I would think there's some more efficient way to do this though.
>
> Cheers,
> Chris


Thanks Chris! I was doing it backwards. I only have a few of these
right now, so performance isn't a concern. I appreciate the advice.

Brad

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#20566

Brad Tilley wrote:

> In C or C++, I can do this for integer conversion:
> 
> unsigned int j = -327681234; // Notice this is signed.
> 
> j will equal 3967286062. I thought with Python that I could use struct
> to pack the signed int as an unsigned int, but that fails:
> 
>>>> x = struct.pack("<I", -327681234)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> struct.error: integer out of range for 'I' format code
> 
> Is there an easy way in Python to do the same conversion that C or C++
> code does? Thanks for any advice.

>>> 0xffffffff & -327681234
3967286062

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#20567

> >>> 0xffffffff & -327681234
>
> 3967286062

Very nice! Thanks for that example. Unsigned long longs:

0xffffffffffffffff & -9151314442815602945
9295429630893948671L

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#20568

On 02/17/2012 02:22 PM, Brad Tilley wrote:
>>>>> 0xffffffff&  -327681234
>> 3967286062
> Very nice! Thanks for that example. Unsigned long longs:
>
> 0xffffffffffffffff&  -9151314442815602945
> 9295429630893948671L
Or more generally, use modulo

-13452324 % 2^64

-- 

DaveA

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