Negative hex to int

[John Machin]

On 15/06/2006 10:31 AM, Ben Finney wrote:
> andreas.lydersen at 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.