bit shifting question

[David Stockwell]

Hi,
My background is c/c++ and java. I'm learning python at this point.

My question is does python share java's peculiar mode of bit shifting, or
does python adhere closer to c's bit shifting?

in java there are 3 kinds of bit shifts:
<< (shift left)
In C, the behavior is
<< (shift left)
I tried looking around but haven't really got an answer yet. I guess I need
to write a mini python script. Please don't write any code, I'm just
looking for a website or something where I can figure this stuff out in
python.

Thanks in advance,

David
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[Michael Hudson]

Hi,
My background is c/c++ and java. I'm learning python at this point.

My question is does python share java's peculiar mode of bit shifting,
or does python adhere closer to c's bit shifting?

The right shift in Python is arithmetic, i.e. preserves the sign bit.

in java there are 3 kinds of bit shifts:
<< (shift left)

In C, the behavior is
<< (shift left)

Have you tried this recently? I believe that whether >> sign fills is
undefined by the C standard but most of the time it dos.

I tried looking around but haven't really got an answer yet. I guess
I need to write a mini python script.

No you don't! If you haven't realized that you can do

$ python
Python 2.2.2 (#1, Feb 24 2003, 19:13:11)
[GCC 3.2.2 20030222 (Red Hat Linux 3.2.2-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.-1

you have MUCH to learn about Python...

Cheers,
mwh

 

[Grant Edwards]

The right shift in Python is arithmetic, i.e. preserves the
sign bit.

Which one is the sign bit? IOW, how wide is a "word"?

 

[Erik Max Francis]

Have you tried this recently? I believe that whether >> sign fills is
undefined by the C standard but most of the time it dos.

It's implementation defined, to be more exact.

 

[Scott David Daniels]

Which one is the sign bit? IOW, how wide is a "word"?

In most cases, you should not be able to discover how wide
a "word" is. Python is trying to move to a single integer
data type which is a combination of the current "long" and
"int" where the internal representation is an implementation
detail. Unfortunately, shifting is one of those places you
can still discover the "width" with code like:

def wordwidth():
for shift in range(256):
if 1 << shift < 0:
return 1 + shift

 

[David Eppstein]

Unfortunately, shifting is one of those places you
can still discover the "width" with code like:

def wordwidth():
for shift in range(256):
if 1 << shift < 0:
return 1 + shift

Of course this will give a warning in 2.3 and fail to work in 2.4...
I wish I could get the 2.4 behavior now with a from __future__ import
but I don't see one for this, instead I find myself doing 1L<<shift a
lot.

 

[Michael Hudson]

Which one is the sign bit? IOW, how wide is a "word"?

Well, quite Perhaps I should have said "preserves sign".

Cheers,
mwh