Example 6.38. roman9.py

If you have not already done so, you can download this and other examples used in this book.

#Define exceptions
class RomanError(Exception): pass
class OutOfRangeError(RomanError): pass
class NotIntegerError(RomanError): pass
class InvalidRomanNumeralError(RomanError): pass

#Roman numerals must be less than 5000
MAX_ROMAN_NUMERAL = 4999

#Define digit mapping
romanNumeralMap = (('M',  1000),
                   ('CM', 900),
                   ('D',  500),
                   ('CD', 400),
                   ('C',  100),
                   ('XC', 90),
                   ('L',  50),
                   ('XL', 40),
                   ('X',  10),
                   ('IX', 9),
                   ('V',  5),
                   ('IV', 4),
                   ('I',  1))

#Create tables for fast conversion of roman numerals.
#See fillLookupTables() below.
toRomanTable = [ None ]  # Skip an index since Roman numerals have no zero
fromRomanTable = {}

def toRoman(n):
    """convert integer to Roman numeral"""
    if not (0 < n <= MAX_ROMAN_NUMERAL):
        raise OutOfRangeError, "number out of range (must be 1..%s)" % MAX_ROMAN_NUMERAL
    if int(n) <> n:
        raise NotIntegerError, "decimals can not be converted"
    return toRomanTable[n]

def fromRoman(s):
    """convert Roman numeral to integer"""
    if not s:
        raise InvalidRomanNumeralError, "Input can not be blank"
    if not fromRomanTable.has_key(s):
        raise InvalidRomanNumeralError, "Invalid Roman numeral: %s" % s
    return fromRomanTable[s]

def toRomanDynamic(n):
    """convert integer to Roman numeral using dynamic programming"""
    result = ""
    for numeral, integer in romanNumeralMap:
        if n >= integer:
            result = numeral
            n -= integer
            break
    if n > 0:
        result += toRomanTable[n]
    return result

def fillLookupTables():
    """compute all the possible roman numerals"""
    #Save the values in two global tables to convert to and from integers.
    for integer in range(1, MAX_ROMAN_NUMERAL + 1):
        romanNumber = toRomanDynamic(integer)
        toRomanTable.append(romanNumber)
        fromRomanTable[romanNumber] = integer

fillLookupTables()

Example 6.39. Output of romantest9.py against roman9.py

.............
----------------------------------------------------------------------
Ran 13 tests in 0.791s

OK