[Tutor] Binary

[Kalle Svensson]

Sez Sheila King:
> On Fri, 11 May 2001 22:10:42 -0400 (EDT), "Michael P. Reilly"
> <arcege@dsl092-074-184.bos1.dsl.speakeasy.net>  wrote about Re: [Tutor] Binary:
> 
> :def bin(n):
> :  """Return a bit string representing the number, right-justified."""
> :  s = ''
> :  while n:
> :    if n & 01:
> :      s = '1' + s
> :    else:
> :      s = '0' + s
> :    n = n >> 1
> :  return s
> 
> OK, I tried this out, and it works, apparently.
> But I don't understand the code.
> 
> What does this line do?
> 
> if n & 01:

n & 01 is the bitwise and of n and 1.  This is 1 if the least significant
bit of n is 1, else 0.

> and what about this line?
> 
> n = n >> 1

All the bits in n are shifted one step to the right:
>>> bin(13)
'1101'
>>> bin(13 >> 1)
'110'
>>> bin(13 >> 2)
'11'
>>> bin(13 >> 3)
'1'
>>> bin(13 >> 4)
''

> I thought, for a minute, someone was using C or C++ to "extend" Python, until I
> looked more carefully.

You could replace
if n & 01:
with
if n % 2:
and
n = n >> 1
with
n = n / 2
but that would be slower.

Peace,
  Kalle
-- 
Email: kalle@gnupung.net     | You can tune a filesystem, but you
Web: http://www.gnupung.net/ | can't tune a fish. -- man tunefs(8)
PGP fingerprint: 0C56 B171 8159 327F 1824 F5DE 74D7 80D7 BF3B B1DD
 [ Not signed due to lossage.  Blame Microsoft Outlook Express. ]