Integer to "binary string"?

[Alfred Morgan]

I decided to write a version 2 of my bin function.  Since version 1 
couldn't handle negative numbers I added an assertion otherwise it would 
get stuck in an infinite loop.  I also changed the way s added a '0' or 
'1' since Dan didn't like my "short-circuiting" and/or method.  I like 
it this way much better anyway because it's more petite.

def bin(i):
     assert i >= 0, "Can't be used for negative numbers."
     s = ''
     while i:
         s = '01'[i & 1] + s
         i >>= 1
     return s or '0'

-alfred


Dan Bishop wrote:
> Gustaf Liljegren <gustafl at algonet.se> wrote in message news:<Xns92E5D08902B0Fgustafl at 195.100.94.182>...
> 
>>I'm writing a short program to help me understand certain concepts of the 
>>character encoding UTF-16. The script should be able to encode integers to 
>>strings of ones and zeros, to illustrate what happens during serialization.
>>
>>I was hunting for a function to convert integers to these strings and vice 
>>versa, but found none. Then I hunted on Google and found:
> 
> 
> A binary to integer conversion can be done with int(bitString, 2). 
> There's a standard function to convert the other way, but right now I
> don't remember what it's called.
> 
> 
>>def bin(i):
>>     s = ''
>>     while i:
>>         s = (i & 1 and '1' or '0') + s
>>         i >>= 1
>>     return s or '0'
>>
>>This works, but I don't understand it. Can anyone explain what happens on 
>>the three last rows? Also, can you show how to write a similar function for 
>>decoding?
> 
> 
> The code is (imho) obfuscated, so I'll rewrite it to increase clarity
> by replacing the and's and or's with if's and else's, using the
> short-circuiting property of "and" and "or".
> 
> def bin(i):
>    s = ''
>    while i:
>       if i & 1:
>          s = '1' + s
>       else:
>          s = '0' + s
>       i >>= 1
>    if s:
>       return s
>    else:
>       return '0'
> 
> 
> 
>>if i & 1:
> 
> 
> & is a bitwise AND operator, and the bitmask 1 corresponds to the last
> bit of the number.  If this bit is nonzero (i.e., 1), a '1' is added
> to the s; otherwise, a '0' is added.  (Why is it added to the front
> instead of the end?  We'll see later.)
> 
> 
>>i >>= 1
> 
> 
> Right shift by one bit; throw away the last bit of of i.
> 
> The recursive definition of an int-to-binary string conversion is
> 
> (all but the last bit of i) + str(last bit of i)
> = bin(i >> 1) + str(i & 1)
> 
> This iterative version calculates these terms in reverse order, which
> is why the string s is built in reverse order.
> 
> 
>>return s or '0'
> 
> 
> If s is true (i.e., len(s) > 0), then s is returned.  Otherwise, '0'
> is returned.