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.

int('7d', 16) 125 int('19', 16) 25 ---------------------------

The problem is negative values. If the unit returns the hex value 'e7',
it means -25, but python says it's 231:
--------------------------- int('e7', 16)

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

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

Thanks!

Andreas Lydersen

[John Machin]

On 15/06/2006 9:09 AM, an**************@gmail.com wrote:

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.

int('7d', 16) 125 int('19', 16) 25
---------------------------

The problem is negative values. If the unit returns the hex value 'e7',
it means -25, but python says it's 231:
--------------------------- int('e7', 16)

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 Mu³a]

an**************@gmail.com wrote:

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

int('e7', 16) 231
---------------------------

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

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

u2(int('e7', 16)) -25 u2(int('12', 16)) 18

w.

[John Machin]

On 15/06/2006 9:24 AM, Wojciech Muła wrote:

an**************@gmail.com wrote:

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

> int('e7', 16) 231
---------------------------

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

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

Holy obfuscation, Batman!
else:
return x

u2(int('e7', 16)) -25 u2(int('12', 16))

18

w.

[Ben Finney]

an**************@gmail.com writes:

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.

value_from_reading('00') 0 value_from_reading('05') 5 value_from_reading('7f') 127 value_from_reading('80') -128 value_from_reading('e7') -25 value_from_reading('ff') -1 value_from_reading('100') -128 value_from_reading('fff')

-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.

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

[James Stroud]

an**************@gmail.com wrote:

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.

int('7d', 16)
125
int('19', 16)
25

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

The problem is negative values. If the unit returns the hex value 'e7',
it means -25, but python says it's 231:
---------------------------
int('e7', 16)

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]

On 15/06/2006 10:31 AM, Ben Finney wrote:

an**************@gmail.com writes:

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::

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.

>>> value_from_reading('00') 0 >>> value_from_reading('05') 5 >>> value_from_reading('7f') 127 >>> value_from_reading('80') -128 >>> value_from_reading('e7') -25 >>> value_from_reading('ff') -1

Looks like TWOS complement to me.

>>> value_from_reading('100') -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.

>>> value_from_reading('fff')

-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.

[Ben Finney]

John Machin <sj******@lexicon.net> writes:

On 15/06/2006 10:31 AM, Ben Finney wrote:

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.

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