Project Euler Problem 89 Solution
The rules for writing Roman numerals allow for many ways of writing each number (see About Roman Numerals…). However, there is always a “best” way of writing a particular number.
For example, the following represent all of the legitimate ways of writing the number sixteen:
IIIIIIIIIIIIIIII VIIIIIIIIIII VVIIIIII XIIIIII VVVI XVI
The last example being considered the most efficient, as it uses the least number of numerals.
The 11K text file, roman.txt (right click and ‘Save Link/Target As…’), contains one thousand numbers written in valid, but not necessarily minimal, Roman numerals; that is, they are arranged in descending units and obey the subtractive pair rule (see About Roman Numerals… for the definitive rules for this problem).
Find the number of characters saved by writing each of these in their minimal form.
Note: You can assume that all the Roman numerals in the file contain no more than four consecutive identical units.
#!/usr/bin/env python import os def subtractive(roman): result = roman replacements = [ ("VIIII", "IX"), ("IIII", "IV"), ("LXXXX", "XC"), ("XXXX", "XL"), ("DCCCC", "CM"), ("CCCC", "CD"), ] for old, new in replacements: result = result.replace(old, new) return result current = 0 improved = 0 for line in open(os.path.join(os.path.dirname(__file__), 'roman.txt')): roman = line.strip() current += len(roman) improved += len(subtractive(roman)) print(current - improved)
$ time python3 minimal.py real 0m0.022s user 0m0.020s sys 0m0.000s
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