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Challenge 28: Roman Numeral Parser

curriculum/challenges/english/blocks/daily-coding-challenges-python/68adce01c0e1144d0a90295c.md

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--description--

Given a string representing a Roman numeral, return its integer value.

Roman numerals consist of the following symbols and values:

SymbolValue
I1
V5
X10
L50
C100
D500
M1000
  • Numerals are read left to right. If a smaller numeral appears before a larger one, the value is subtracted. Otherwise, values are added.

--hints--

parse_roman_numeral("III") should return 3.

js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(parse_roman_numeral("III"), 3)`)
}})

parse_roman_numeral("IV") should return 4.

js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(parse_roman_numeral("IV"), 4)`)
}})

parse_roman_numeral("XXVI") should return 26.

js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(parse_roman_numeral("XXVI"), 26)`)
}})

parse_roman_numeral("XCIX") should return 99.

js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(parse_roman_numeral("XCIX"), 99)`)
}})

parse_roman_numeral("CDLX") should return 460.

js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(parse_roman_numeral("CDLX"), 460)`)
}})

parse_roman_numeral("DIV") should return 504.

js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(parse_roman_numeral("DIV"), 504)`)
}})

parse_roman_numeral("MMXXV") should return 2025.

js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(parse_roman_numeral("MMXXV"), 2025)`)
}})

--seed--

--seed-contents--

py
def parse_roman_numeral(numeral):

    return numeral

--solutions--

py
def parse_roman_numeral(numeral):
    roman_map = {
        "I": 1,
        "V": 5,
        "X": 10,
        "L": 50,
        "C": 100,
        "D": 500,
        "M": 1000
    }

    total = 0
    for i in range(len(numeral)):
        current = roman_map[numeral[i]]
        next_val = roman_map[numeral[i + 1]] if i + 1 < len(numeral) else 0

        if current < next_val:
            total -= current
        else:
            total += current

    return total