Roman Numeral Conversion in Every Language

Published on 01 November 2018 (Updated: 02 May 2020)

Roman Numeral Conversion in Every Language

Roman numerals are the numbers that were used in ancient Rome, which employed combinations of letters from the Latin alphabet (I, V, X, L, C, D and M).

The following table shows the letter to decimal mapping:

Letter Decimal
I 1
V 5
X 10
L 50
C 100
D 500
M 1000

Stringing together these digits yields a value that is the sum of their respective mappings. However, there is a catch. Roman numerals must appears in order of greatest to least. If a smaller value appears before a larger one, the smaller value is subtracted from the total.

As a result, a string like XV would evaluate to 15 while XIV would evaluate to 14.

Of course, there are other limitations, but we’ll ignore those for simplicity.


Create a file called Roman Numeral Conversion using whatever naming convention is appropriate for the choice language.

Using the table above, write a sample program which accepts a Roman numeral on the command line and outputs its decimal value on standard output. Be careful to appropriately handle invalid input such as XT. More on that in the testing section.

Please make sure your program is executable. In other words, the solution should be able to be called in the appropriate environment with a string of roman numerals (i.e. ./roman-numeral-conversion XXVI).


The following table contains appropriate examples for testing a Roman Numeral Conversion program:

Description Input Output
No Input   “Usage: please provide a string of roman numerals”
Invalid Input “XT” “Error: invalid string of roman numerals”
Empty Input ”” 0
Single I “I” 1
Single V “V” 5
Single X “X” 10
Single L “L” 50
Single C “C” 100
Single D “D” 500
Single M “M” 1000
Addition “XXV” 25
Subtraction “XIV” 14


Further Reading