Python Cookbook

Module: Roman Numerals

Credit: Paul M. Winkler

There are many algorithms for creating Roman numerals. Example 3-2 presents the easiest-to-read algorithm that I’ve been able to find for this purpose: it establishes a mapping between integer values and Roman numerals, then counts how many of each value can fit into the input integer. The code uses two tuples for the mapping, instead of a dictionary, because it needs to go through them sequentially and doesn’t care about random access. Thus, a dictionary would be more hindrance than help.

Example 3-2. Roman numerals

def int_to_roman(input): """ Convert an integer to a Roman numeral. """ if not isinstance(input, type(1)): raise TypeError, "expected integer, got %s" % type(input) if not 0 < input < 4000: raise ValueError, "Argument must be between 1 and 3999" ints = (1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1) nums = ('M', 'CM', 'D', 'CD','C', 'XC','L','XL','X','IX','V','IV','I') result = [] for i in range(len(ints)): count = int(input / ints[i]) result.append(nums[i] * count) input -= ints[i] * count return ''.join(result) def roman_to_int(input): """ Convert a Roman numeral to an integer. """ """ if not isinstance(input, type("")): raise TypeError, "expected string, got %s" % type(input) input = input.upper( ) nums = {'M':1000, 'D':500, 'C':100, 'L':50, 'X':10, 'V':5, 'I':1} sum = 0 for i in range(len(input)): try: value = nums[input[i]] # If the next place holds a larger number, this value is negative if i+1 < len(input) ...