Shift Confusion

[John Machin]

On 23 Feb 2005 22:06:54 -0800, "Kamilche" <klachemin at comcast.net>
wrote:

>I'm trying to pack two characters into a single byte, and the shifting
>in Python has me confused.
>
>Essentially, it should be possible to use a 'packed string' format in
>Python, where as long as the characters you're sending are in the ASCII
>range 0 to 127, two will fit in a byte.

It should be possible, but only in a realm where phlogiston and
perpetual motion machines exist.

To hold one ASCII character 0 <= ord(c) < 128, you need log2(128) == 7
bits. There are 8 bits in a byte. Therefore you can hold only 8/7.0 ==
1.14... ASCII characters in a standard 8-bit byte. If there is such a
thing as a 14-bit byte, that's news to me.

Other things you are doing wrong:

1. Using "L".lower() as a variable name. Some fonts make it extremely
hard to work out what is what in "l1l1l1l1l1l1ll1l1l" -- can you read
that???

2. Hmmm:
 >>> "chr((y << 1) | x))".upper()
'CHR((Y << 1) | X))'

OK so that's a one, not the length of your string, as augmented.
Either would be be wrong. Shifting a character left ONE bit (or,
changing the emphasis, one BIT) and then ORing in another character
would be vaguely reasonable only if you were writing a hash function.

3. Supposing this modern alchemy had worked: when unpacking, how would
you know whether the string was originally (say) seven characters long
or 8 characters long?

4. Your unpacking routine appears to be trying to unpack two 4-bit
items (nibbles) out of a byte, but is not doing (temp & 0xf0) >> 4 for
the top nibble as one might expect ..... aaahhh!!?? are you trying to
emulate packed decimal???

5. Not writing down a clear statement of what you are trying to do,
followed by examples of input and expected output. This latter goes by
fancy names like "test-driven development"; when I started programming
it was known as "common sense".

6. Not using Python interactively to explore what's going on:

>>> ord('x')
120
>>> ord('y')
121
>>> (120 << 1)
240
>>> (120 << 1) | 121
249
>>>

HTH,
John