Negative hex to int

[andreas.lydersen]

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.---------------------------

The problem is negative values. If the unit returns the hex value 'e7',
it means -25, but python says it's 231:
---------------------------231
---------------------------

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

Thanks!

Andreas Lydersen

 

[John Machin]

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.25
---------------------------

The problem is negative values. If the unit returns the hex value 'e7',
it means -25, but python says it's 231:
---------------------------231

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 =?iso-8859-2?Q?Mu=B3a?=]

The problem is negative values. If the unit returns the hex value 'e7',
it means -25, but python says it's 231:

def u2(x):
if x & 0x80: # MSB set -> neg.
return -((~x & 0xff) + 1)
else:
return x

w.

 

[John Machin]

def u2(x):
if x & 0x80: # MSB set -> neg.
return -((~x & 0xff) + 1)

Holy obfuscation, Batman!

 

[Ben Finney]

The problem is negative values. If the unit returns the hex value
'e7', it means -25, but python says it's 231:

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_reading(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.
-1

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

 

[James Stroud]

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.

125

25

---------------------------

The problem is negative values. If the unit returns the hex value 'e7',
it means -25, but python says it's 231:
---------------------------

231
---------------------------

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

Thanks!

Andreas Lydersen

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]

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::

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.

def value_from_reading(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.

thing & 0x7F looks like lowest 7 bits to me.

-1

Looks like TWOS complement to me.

-128

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.

 

[Ben Finney]

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.

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.

 

[Dennis Lee Bieber]

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

You've no doubt had lots of response by now, but... Here's a little
thing that works with differing "word" sizes...
.... mask = int("1" * wordlength, 2)
.... if value > (mask >> 1):
.... return -((~value + 1) & mask)
.... else:
.... return value
....
# 256, being 9 bits, obviously becomes a 0 when trimmed to 8-LSB

Thanks!

Andreas Lydersen

--
Wulfraed Dennis Lee Bieber KD6MOG
(e-mail address removed) (e-mail address removed)
HTTP://wlfraed.home.netcom.com/
(Bestiaria Support Staff: (e-mail address removed))
HTTP://www.bestiaria.com/