Negative hex to int

[andreas.lydersen@gmail.com]

Hi!

While communicating with a monitoring unit, I get some hex values
representing degrees celcius from its probes. The values can be
something like '19' or '7d'. To convert it to int, I do the following:
---------------------------
Python 2.4.2 (#1, Sep 28 2005, 10:25:47)
[GCC 3.4.3 20041212 (Red Hat 3.4.3-9.EL4)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
>>> int('7d', 16)[/color][/color][/color]
125[color=blue][color=green][color=darkred]
>>> int('19', 16)[/color][/color][/color]
25[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]
---------------------------

The problem is negative values. If the unit returns the hex value 'e7',
it means -25, but python says it's 231:
---------------------------[color=blue][color=green][color=darkred]
>>> int('e7', 16)[/color][/color][/color]
231
---------------------------

Does anyone have a clue a to what I need to do?

Thanks!

Andreas Lydersen

[John Machin]

Re: Negative hex to int

On 15/06/2006 9:09 AM, andreas.lyderse n@gmail.com wrote:[color=blue]
> Hi!
>
> While communicating with a monitoring unit, I get some hex values
> representing degrees celcius from its probes. The values can be
> something like '19' or '7d'. To convert it to int, I do the following:
> ---------------------------
> Python 2.4.2 (#1, Sep 28 2005, 10:25:47)
> [GCC 3.4.3 20041212 (Red Hat 3.4.3-9.EL4)] on linux2
> Type "help", "copyright" , "credits" or "license" for more information.[color=green][color=darkred]
>>>> int('7d', 16)[/color][/color]
> 125[color=green][color=darkred]
>>>> int('19', 16)[/color][/color]
> 25
> ---------------------------
>
> The problem is negative values. If the unit returns the hex value 'e7',
> it means -25, but python says it's 231:
> ---------------------------[color=green][color=darkred]
>>>> int('e7', 16)[/color][/color]
> 231
> ---------------------------
>[/color]

The Da Vinci code it aint :-)

|>> readings = ['19', 'e7']
|>> for reading in readings:
.... intval = int(reading, 16)
.... if intval >= 128:
.... intval -= 256
.... print intval
....
25
-25

[Wojciech Mu³a]

Re: Negative hex to int

andreas.lyderse n@gmail.com wrote:[color=blue]
> The problem is negative values. If the unit returns the hex value 'e7',
> it means -25, but python says it's 231:
> ---------------------------[color=green][color=darkred]
>>>> int('e7', 16)[/color][/color]
> 231
> ---------------------------
>
> Does anyone have a clue a to what I need to do?[/color]

def u2(x):
if x & 0x80: # MSB set -> neg.
return -((~x & 0xff) + 1)
else:
return x
[color=blue][color=green][color=darkred]
>>> u2(int('e7', 16))[/color][/color][/color]
-25[color=blue][color=green][color=darkred]
>>> u2(int('12', 16))[/color][/color][/color]
18[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

w.

[John Machin]

Re: Negative hex to int

On 15/06/2006 9:24 AM, Wojciech Muła wrote:[color=blue]
> andreas.lyderse n@gmail.com wrote:[color=green]
>> The problem is negative values. If the unit returns the hex value 'e7',
>> it means -25, but python says it's 231:
>> ---------------------------[color=darkred]
>>>>> int('e7', 16)[/color]
>> 231
>> ---------------------------
>>
>> Does anyone have a clue a to what I need to do?[/color]
>
> def u2(x):
> if x & 0x80: # MSB set -> neg.
> return -((~x & 0xff) + 1)[/color]

Holy obfuscation, Batman!
[color=blue]
> else:
> return x
>[color=green][color=darkred]
>>>> u2(int('e7', 16))[/color][/color]
> -25[color=green][color=darkred]
>>>> u2(int('12', 16))[/color][/color]
> 18
>
> w.[/color]

[Ben Finney]

Re: Negative hex to int

andreas.lyderse n@gmail.com writes:
[color=blue]
> The problem is negative values. If the unit returns the hex value
> 'e7', it means -25, but python says it's 231:[/color]

Python is right. There is no "negative bit" in Python numbers, now
that unification of 'long' and 'int' is complete; numbers can grow
indefinitely large.

If you want a special interpretation of the value, you'll have to
calculate it.

Example assuming you want a one's-complement interpretation: :

def value_from_read ing(num):
""" Gets the integer value from the reading string.
num is a positive hexadecimal number as a string.
Numbers greater than 0x7F are interpreted as negative,
with magnitude greater than 0x7F being the negative value.
Only the lowest 15 bits are used for the magnitude.
[color=blue][color=green][color=darkred]
>>> value_from_read ing('00')[/color][/color][/color]
0[color=blue][color=green][color=darkred]
>>> value_from_read ing('05')[/color][/color][/color]
5[color=blue][color=green][color=darkred]
>>> value_from_read ing('7f')[/color][/color][/color]
127[color=blue][color=green][color=darkred]
>>> value_from_read ing('80')[/color][/color][/color]
-128[color=blue][color=green][color=darkred]
>>> value_from_read ing('e7')[/color][/color][/color]
-25[color=blue][color=green][color=darkred]
>>> value_from_read ing('ff')[/color][/color][/color]
-1[color=blue][color=green][color=darkred]
>>> value_from_read ing('100')[/color][/color][/color]
-128[color=blue][color=green][color=darkred]
>>> value_from_read ing('fff')[/color][/color][/color]
-1

"""
num_base = 16
negative_thresh old = 0x7F
int_val = int(num, num_base)
if int_val > negative_thresh old:
magnitude = (int_val & negative_thresh old)
int_val = -(1 + negative_thresh old - magnitude)
return int_val

Adjust for whatever algorithm you want to use. Consult a book of
algorithms if you want a better implementation than my off-the-cuff
brute-force approach.

--
\ "Remorse: Regret that one waited so long to do it." -- Henry |
`\ L. Mencken |
_o__) |
Ben Finney

[James Stroud]

Re: Negative hex to int

andreas.lyderse n@gmail.com wrote:[color=blue]
> Hi!
>
> While communicating with a monitoring unit, I get some hex values
> representing degrees celcius from its probes. The values can be
> something like '19' or '7d'. To convert it to int, I do the following:
> ---------------------------
> Python 2.4.2 (#1, Sep 28 2005, 10:25:47)
> [GCC 3.4.3 20041212 (Red Hat 3.4.3-9.EL4)] on linux2
> Type "help", "copyright" , "credits" or "license" for more information.
>[color=green][color=darkred]
>>>>int('7d', 16)[/color][/color]
>
> 125
>[color=green][color=darkred]
>>>>int('19', 16)[/color][/color]
>
> 25
>
> ---------------------------
>
> The problem is negative values. If the unit returns the hex value 'e7',
> it means -25, but python says it's 231:
> ---------------------------
>[color=green][color=darkred]
>>>>int('e7', 16)[/color][/color]
>
> 231
> ---------------------------
>
> Does anyone have a clue a to what I need to do?
>
> Thanks!
>
> Andreas Lydersen
>[/color]


py> t = lambda x: int(x, 16) - ((int(x, 16) >> 7) * 256)
py> t('e7')
-25


--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/

[John Machin]

Re: Negative hex to int

On 15/06/2006 10:31 AM, Ben Finney wrote:[color=blue]
> andreas.lyderse n@gmail.com writes:
>[color=green]
>> The problem is negative values. If the unit returns the hex value
>> 'e7', it means -25, but python says it's 231:[/color]
>
> Python is right. There is no "negative bit" in Python numbers, now
> that unification of 'long' and 'int' is complete; numbers can grow
> indefinitely large.
>
> If you want a special interpretation of the value, you'll have to
> calculate it.
>
> Example assuming you want a one's-complement interpretation: :[/color]

Given that the OP had to ask the question at all, it is doubtful that he
knows what "one's-complement" means. He may well not be alone -- see later.
[color=blue]
>
> def value_from_read ing(num):
> """ Gets the integer value from the reading string.
> num is a positive hexadecimal number as a string.
> Numbers greater than 0x7F are interpreted as negative,
> with magnitude greater than 0x7F being the negative value.
> Only the lowest 15 bits are used for the magnitude.[/color]

thing & 0x7F looks like lowest 7 bits to me.
[color=blue]
>[color=green][color=darkred]
> >>> value_from_read ing('00')[/color][/color]
> 0[color=green][color=darkred]
> >>> value_from_read ing('05')[/color][/color]
> 5[color=green][color=darkred]
> >>> value_from_read ing('7f')[/color][/color]
> 127[color=green][color=darkred]
> >>> value_from_read ing('80')[/color][/color]
> -128[color=green][color=darkred]
> >>> value_from_read ing('e7')[/color][/color]
> -25[color=green][color=darkred]
> >>> value_from_read ing('ff')[/color][/color]
> -1[/color]

Looks like TWOS complement to me.
[color=blue][color=green][color=darkred]
> >>> value_from_read ing('100')[/color][/color]
> -128[/color]

Same result as '80'?
In any case the OP gave no indication that he was getting more than two
hex digits. His desired interpretation of 'e7' as -25 strongly indicates
that he's getting only 2 hex digits.
[color=blue][color=green][color=darkred]
> >>> value_from_read ing('fff')[/color][/color]
> -1
>
> """
> num_base = 16
> negative_thresh old = 0x7F
> int_val = int(num, num_base)
> if int_val > negative_thresh old:
> magnitude = (int_val & negative_thresh old)
> int_val = -(1 + negative_thresh old - magnitude)
> return int_val
>
> Adjust for whatever algorithm you want to use. Consult a book of
> algorithms if you want a better implementation than my off-the-cuff
> brute-force approach.[/color]

[Ben Finney]

Re: Negative hex to int

John Machin <sjmachin@lexic on.net> writes:
[color=blue]
> On 15/06/2006 10:31 AM, Ben Finney wrote:[color=green]
> > If you want a special interpretation of the value, you'll have to
> > calculate it.
> >
> > Example assuming you want a one's-complement interpretation: :[/color]
>
> Given that the OP had to ask the question at all, it is doubtful
> that he knows what "one's-complement" means. He may well not be
> alone -- see later.[/color]

You've pointed out something useful: a quick attempt (by me, in this
case) to describe the algorithm isn't worth much, less so if it's
already well-documented. Hopefully the original poster can consult a
more authoritative reference on the topic. Fortunately, that was also
one of my points :-)

The implementation itself seems to do the job the OP asked. I hope
it's useful in some form.

--
\ "If society were bound to invent technologies which could only |
`\ be used entirely within the law, then we would still be sitting |
_o__) in caves sucking our feet." -- Gene Kan, creator of Gnutella |
Ben Finney