hex to signed integer

**Steven Taschuk** · Jul 18 '05, 12:37 AM

Re: hex to signed integer

Quoth Tom Goulet:[color=blue]
> My question basically is: What is the opposite of the following?
> | "%08X" % -1[/color]

Here's one way, very like what you already have:

def hex2signed(s):
return struct.unpack(' !i', binascii.unhexl ify(s))[0]

(This will not be the inverse of '%08x' % n in Python 2.4, when
'%x' % -1 will produce '-1', but I think it does what you want.)

Another approach:

def hex2signed(s):
value = long(s, 16)
if value > sys.maxint:
value = value - 2L*sys.maxint - 2
assert -sys.maxint-1 <= value <= sys.maxint
return int(value)

--
Steven Taschuk staschuk@telusp lanet.net
"Please don't damage the horticulturalis t."
-- _Little Shop of Horrors_ (1960)

**Tim Roberts** · Jul 18 '05, 12:37 AM

Re: hex to signed integer

Tom Goulet <tomg@em.ca> wrote:
[color=blue]
>Hello,
>
>My question basically is: What is the opposite of the following?
>| "%08X" % -1
>
>I want to convert a string of hexadecimal characters to the signed
>integer they would have been before the <print> statement converted
>them. How do I do this in such a way that is compatible with Python
>versions 1.5.2 through 2.4, and not machine-dependent?
>
>This is my current best:
>| struct.unpack(" !l", \
>| chr(string.atoi (hexbit[0:2], 16)) + \
>| chr(string.atoi (hexbit[2:4], 16)) + \
>| chr(string.atoi (hexbit[4:6], 16)) + \
>| chr(string.atoi (hexbit[6:8], 16)))[/color]

(Unrelated note: the blackslashes are unnecessary in this example, since it
is inside a set of parens.)

How slimy is this?

try:
temp = int(hexbit,16)
except:
temp = int(long(hexbit ,16)-2**32)

Equivalently:
if hexbit[0] < '8':
temp = int(hexbit,16)
else:
temp = int(long(hexbit ,16)-2**32)
--
- Tim Roberts, timr@probo.com
Providenza & Boekelheide, Inc.

**Tom Goulet** · Jul 18 '05, 12:37 AM

Re: hex to signed integer

Steven Taschuk wrote:
[color=blue]
> return struct.unpack(' !i', binascii.unhexl ify(s))[0][/color]

Hmm, <unhexlify> is not in <binascii> in Python 1.5.2.
[color=blue]
> (This will not be the inverse of '%08x' % n in Python 2.4, when
> '%x' % -1 will produce '-1', but I think it does what you want.)[/color]

| >>> print "%08X" % -1294044542
| __main__:1: FutureWarning: %u/%o/%x/%X of negative int will return a \
| signed string in Python 2.4 and up

Argh. Thanks for warning me.

I only want to store thirty-two bits as an integer and input and output
that value of hexadecimal in Python versions 1.5.2 through 2.4, and it's
causing me a lot of grief. It looks like I'm going to have to resort to
<struct> for both input and output.
[color=blue]
> value = long(s, 16)[/color]

A second argument to <long> is not in Python 1.5.2, either. Using
<atol> from <string> instead works. Thanks!

--
Tom Goulet, tomg@em.ca, D8BAD3BC, http://web.em.ca/~tomg/contact.html

**Miki Tebeka** · Jul 18 '05, 12:37 AM

Re: hex to signed integer

Hello Tom,
[color=blue]
> I want to convert a string of hexadecimal characters to the signed
> integer they would have been before the <print> statement converted
> them. How do I do this in such a way that is compatible with Python
> versions 1.5.2 through 2.4, and not machine-dependent?[/color]
eval?
e.g.:[color=blue][color=green][color=darkred]
>>> eval("0x%s" % "FF")[/color][/color][/color]
255

HTH.
Miki

**Tom Goulet** · Jul 18 '05, 12:38 AM

Re: hex to signed integer

Tim Roberts wrote:[color=blue]
> Tom Goulet <tomg@em.ca> wrote:[/color]
[color=blue][color=green]
>>I want to convert a string of hexadecimal characters to the signed
>>integer they would have been before the <print> statement converted
>>them. How do I do this in such a way that is compatible with Python
>>versions 1.5.2 through 2.4, and not machine-dependent?[/color][/color]
[color=blue]
> if hexbit[0] < '8':
> temp = int(hexbit,16)
> else:
> temp = int(long(hexbit ,16)-2**32)[/color]

The <int> function takes only one argument in Python 1.5.2.

--
Tom Goulet, tomg@em.ca, D8BAD3BC, http://web.em.ca/~tomg/contact.html

**Tom Goulet** · Jul 18 '05, 12:38 AM

Re: hex to signed integer

Miki Tebeka wrote:
[color=blue]
> eval?
> e.g.:[color=green][color=darkred]
>>>> eval("0x%s" % "FF")[/color][/color]
> 255[/color]

| >>> eval("0x"+"B2DE 7282", {}, {})
| <string>:0: FutureWarning: hex/oct constants > sys.maxint will return \
| positive values in Python 2.4 and up
| -1294044542

There you have it. Using the <eval> function doesn't work on 2.4 and
using the <int> function doesn't work on 1.5.2. Besides, I want to
avoid use of the <eval> function.

--
Tom Goulet, tomg@em.ca, D8BAD3BC, http://web.em.ca/~tomg/contact.html

**David Bolen** · Jul 18 '05, 12:38 AM

Re: hex to signed integer

Tom Goulet <tomg@em.ca> writes:
[color=blue]
> Tim Roberts wrote:[color=green]
> > Tom Goulet <tomg@em.ca> wrote:[/color]
>[color=green][color=darkred]
> >>I want to convert a string of hexadecimal characters to the signed
> >>integer they would have been before the <print> statement converted
> >>them. How do I do this in such a way that is compatible with Python
> >>versions 1.5.2 through 2.4, and not machine-dependent?[/color][/color]
>[color=green]
> > if hexbit[0] < '8':
> > temp = int(hexbit,16)
> > else:
> > temp = int(long(hexbit ,16)-2**32)[/color]
>
> The <int> function takes only one argument in Python 1.5.2.[/color]

Yes, the base argument was added in later around Python 2.0.

An equivalent operation to int(value,base) in Python 1.5.2 and any of
the later versions (at least through 2.3 - 2.4 doesn't exist yet)
would be the string.atoi(val ue,base) function.

However, as indicated by the above code, both int() and string.atoi()
limit their result to a signed integer, so depending on the
hexadecimal string it might overflow and result in an exception. So
for any sized hexadecimal string, use string.atol(val ue,base) instead.

For example:

Python 2.2.3 (#42, May 30 2003, 18:12:08) [MSC 32 bit (Intel)] on win32
Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
>>> import string
>>> print string.atoi('20 ',16)[/color][/color][/color]
32[color=blue][color=green][color=darkred]
>>> print string.atoi('FF FFFFFF',16)[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "c:\Python\2.2\ lib\string.py", line 225, in atoi
return _int(s, base)
ValueError: int() literal too large: FFFFFFFF[color=blue][color=green][color=darkred]
>>> print string.atol('FF FFFFFF',16)[/color][/color][/color]
4294967295[color=blue][color=green][color=darkred]
>>> print string.atol('%0 8X' % -1,16)[/color][/color][/color]
4294967295[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

One note - if you end up converting the result of string.atol() with
str(), under Python 1.5.2 it will have a trailing "L" but will not
have that under any later Python release. Converting it to a string
with repr() will have the trailing "L" under all Python releases.

-- David

**Steven Taschuk** · Jul 18 '05, 12:38 AM

Re: hex to signed integer

Quoth Bengt Richter:
[...][color=blue][color=green][color=darkred]
> >>> -2**31[/color][/color]
> -2147483648L
> Oops, is that a wart/buglet BTW?[/color]

That it's a long and not an int? Certainly not a bug, arguably a
wart, and in any case it's 2's-complement's fault, not Python's:
-2**31 is equivalent to -(2**31), and the inner expression doesn't
fit into an int (on suitable machines).

I suppose long arithmetic could produce ints when possible, but it
seems unlikely to be worth the trouble.

--
Steven Taschuk "The world will end if you get this wrong."
staschuk@telusp lanet.net -- "Typesettin g Mathematics -- User's Guide",
Brian Kernighan and Lorrinda Cherry

hex to signed integer

hex to signed integer

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