ActiveState Code

Recipe 222109: radix.str(N,R) -- reverse function to int(S,R) and long(S,R)

The function produces an arbitrary radix string representation of integer numbers.

Python 
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# radix.py

"""
Defines str(number,radix) -- reverse function to int(str,radix) and long(str,radix)

number -- signed int or long,
radix  -- 2 to 36

Usage:
import radix
str_repr = radix.str( number, radix )

print radix.str( 10, 16 ), radix.str( 1570137287, 36 ) # a python

"""


import string


def str( number, radix ):
   """str( number, radix ) -- reverse function to int(str,radix) and long(str,radix)"""

   if not 2 <= radix <= 36:
      raise ValueError, "radix must be in 2..36"

   abc = string.digits + string.letters

   result = ''

   if number < 0:
      number = -number
      sign = '-'
   else:
      sign = ''

   while True:
      number, rdigit = divmod( number, radix )
      result = abc[rdigit] + result
      if number == 0:
         return sign + result

   # never here because number >= 0, radix > 0, we repeat (number /= radix)


if __name__ == '__main__':
   src = 'qwertyuioplkjhgfdsazxcvbnm0987654321'
   dst = 79495849566202193863718934176854772085778985434624775545L

   num = int( src, 36 )
   assert num == dst
   res = str( num, 36 )
   assert res == src
   print "%s radix 36 is\n%d decimal" % (src, dst)


# EOF

Discussion

I've discovered that a function returning an arbitrary radix string from a number is missing in the language. So I implemented it to restore the symmetry.

Moon aka Sun

Comments

  1. 1. At 12:09 a.m. on 17 sep 2003, Moon aka Sun (the author) said:

    --> Category: Algorithms. I'm sorry, it belongs rather to the Algorithms Category.

    See also here: Numeric base converter that accepts arbitrary digits

    http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/111286

  2. 2. At 12:10 a.m. on 17 sep 2003, Raymond Hettinger said:

    A few nits. Nicely written.

    Here are a few nits to take it to industrial strength:

    • rename the function to base() or some such to avoid overwriting __builtin__.str()

    • only build abc once and do it outside the function

    • use ''.join() on a list instead of quadratic time string addition.

    • make divmod, abc, and result.append into locals for faster lookup in the loop.

      import string abc = string.digits + string.letters

      def base(number, radix): """base(number, radix) inverse function to int(str,radix) and long(str,radix) """

      if not 2 <= radix <= 36: raise ValueError, "radix must be in 2..36"

      result = [] addon = result.append if number < 0: number = -number addon('-') elif number == 0: addon('0')

      _divmod, _abc = divmod, abc while number: number, rdigit = _divmod(number, radix) addon(_abc[rdigit])

      result.reverse() return ''.join(result)

  3. 3. At 11:16 a.m. on 15 oct 2007, Richard Nichols III said: 
    added an optional minimim-width parameter.
note: '0' * -1 == ''

import string
abc = string.digits + string.letters

def base(number, radix, width=1):
   """base(number, radix)
         inverse function to int(str,radix) and long(str,radix)
   """

   if not 2 &lt;= radix &lt;= 36:
      raise ValueError, "radix must be in 2..36"

   result = []
   addon = result.append
   if number &lt;
      number = -number
      addon('-')

   _divmod, _abc = divmod, abc
   while number:
      number, rdigit = _divmod(number, radix)
      addon(_abc[rdigit])

   addon("0"*(width-len(result)))

   result.reverse()
   return ''.join(result)
  4. 4. At 11:53 p.m. on 17 jun 2008, Damon McCormick said:

    Minus sign should be at the beginning. The code directly above puts the minus sign at the end instead of the beginning.

    Here's the version I ended up with, which also allows you to use whatever characters you like as the digits:

    def toRadixStr (number, radix, width=1,
          digits=string.digits+string.letters):
   """Inverse function to int(str,radix) and long(str,radix)."""
   digitCount = len(digits)
   if not 2 &lt;= radix &lt;= digitCount:
      raise ValueError, "radix must be in 2..%d" % digitCount
   result = []
   # convert some globals to locals for speed
   _divmod, _digits = divmod, digits
   append = result.append
   # check for a negative number
   if number &lt; 0:
       number = -number
       isNegative = True
   else:
       isNegative = False
   # append digits in reverse order
   while number:
      (number, remainder) = _divmod(number, radix)
      append(_digits[remainder])
   # append leading 0s
   leading0s = digits[0] * (width-len(result)) # note: "0" * -1 == ''
   append(leading0s)
   # append sign if necessary and reverse
   if isNegative:
      append('-')
   result.reverse()
   return ''.join(result)
  5. 5. At 12:38 a.m. on 18 jun 2008, Damon McCormick said:

    typo. string.letters should be string.uppercase (or string.lowercase).

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