binary representation of an integer

[Nick Craig-Wood]

eliben <eliben at gmail.com> wrote:
>  I'm interested in converting integers to a binary representation,
>  string. I.e. a desired function would produce:
> 
>  dec2bin(13) => "1101"
> 
>  The other way is easily done in Python with the int() function.
> 
>  Perl has a very efficient way to do dec2bin, because its pack/unpack
>  have a B format for binary representations, so it's done with:
> 
>  sub dec2bin {
>      my $str = unpack("B32", pack("N", shift));
>      $str =~ s/^0+(?=\d)//;   # otherwise you'll get leading zeros
>      return $str;
>  }
> 
>  Python's pack/unpack don't have the binary format for some reason, so
>  custom solutions have to be developed. One suggested in the ASPN
>  cookbook is:
>  http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/111286
>  However, it is very general and thus inefficient.
> 
>  What would be the quickest way to do this ? I think that for dec2bin
>  conversion, using hex() and then looping with a hex->bin lookup table
>  would be probably much faster than the general baseconvert from the
>  recipe.
> 
>  What do you think?

Something like this...

>>> hex_to_binary = {
...     "0" : "0000",
...     "1" : "0001",
...     "2" : "0010",
...     "3" : "0011",
...     "4" : "0100",
...     "5" : "0101",
...     "6" : "0110",
...     "7" : "0111",
...     "8" : "1000",
...     "9" : "1001",
...     "a" : "1010",
...     "b" : "1011",
...     "c" : "1100",
...     "d" : "1101",
...     "e" : "1110",
...     "f" : "1111",
... }

>>> def to_binary(inp):
...     out = []
...     for hex_digit in ("%x" % inp):
...             out.append(hex_to_binary[hex_digit])
...     out = "".join(out).lstrip("0")
...     if out == "":
...             out = "0"
...     return out
...
>>> to_binary(0)
'0'
>>> to_binary(10)
'1010'
>>> to_binary(100)
'1100100'
>>> to_binary(1000)
'1111101000'
>>> to_binary(10000000000000000000)
'1000101011000111001000110000010010001001111010000000000000000000'

But don't try this ;-)

>>> to_binary(-1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in to_binary
KeyError: '-'

-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick