convert char to byte representation

[Mike Meyer]

[Format recovered from top posting.]

Larry Bates <larry.bates at websafe.com> writes:
> Grant Edwards wrote:
>> On 2005-10-10, Larry Bates <larry.bates at websafe.com> wrote:
>>>>I am trying to xor the byte representation of every char in a string with
>>>>its predecessor. But I don't know how to convert a char into its byte
>>>>representation. This is to calculate the nmea checksum for gps data.
>> 
>>>ord(c) gives you decimal representation of a character.
>> 
>> While ord(c) is what the OP needs, it doesn't give a decimal
>> represention -- which I guess would be a string like "65" for
>> the ASCII characer "A".  What ord() gives you is an integer
>> object with the value of the character [which the hardware
>> stores in binary on all of the platforms I'm aware of].
>
> I've always read it written that the number that is returned by
> ord(c) is the "decimal" (not hex, not octal) representation of
> the ASCII/UNICODE character that is stored in memory location
> pointed to by variable c.  While the result is an integer (as
> it couldn't really be anything else), I believe that most
> character charts list the number that ord() returns as the
> "decimal representation" of that character (as they normally
> also show the octal and hex values as well).

The value returned by ord is a *number*. That number has a decimal
representation. It also has a hex representation and an octal
representation. These are all strings, and they all represent the same
number. You can't print a number - you can only print characters. So
Python (indeed, most languages) translate the number into a string of
characters that represent the number, using the decimal
representation.  You can use the hex and oct builtins to ask for the
hex and octal representations of that number and print those strings
if you want.

> Probably an "old school" answer on my part.

The decimal representation of ' ' is '32'. Python doesn't think that
that's what ord(' ') returns:

>>> ord(' ') == '32'
False

So I'd say it was "wrong" rather than old school.

On the other hand, if you check ord(' ') against numbers, it doeesn't
care what representation you use, so long as they represent the same
number:

>>> ord(' ') == 0x20
True
>>> ord(' ') == 040
True
>>> ord(' ') == 32
True

Of course you can't read a number any more than you can write one, so
Python kindly translates strings representing numbers into numbers
when itt reads them. This process is often referred to as "reading a
number", but what's actually read is characters.

        <mike
-- 
Mike Meyer <mwm at mired.org>			http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.