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Recipe 2.5 Working with Roman Numerals

2.5.1 Problem

You want to convert between regular numbers and Roman numerals. You need to do this with items in outlines, page numbers on a preface, and copyrights for movie credits.

2.5.2 Solution

Use the Roman module from CPAN:

use Roman;
$roman = roman($arabic);                        # convert to roman numerals
$arabic = arabic($roman) if isroman($roman);    # convert from roman numerals

2.5.3 Discussion

The Roman module provides both Roman and roman for converting Arabic ("normal") numbers to their Roman equivalents. Roman produces uppercase letters, whereas roman gives lowercase ones.

The module only deals with Roman numbers from 1 to 3999, inclusive. The Romans didn't represent negative numbers or zero, and 5000 (which 4000 is represented in terms of) uses a symbol outside the ASCII character set.

use Roman;
$roman_fifteen = roman(15);                         # "xv"
print "Roman for fifteen is $roman_fifteen\n";
$arabic_fifteen = arabic($roman_fifteen);
print "Converted back, $roman_fifteen is $arabic_fifteen\n";

Roman for fifteen is xv
Converted back, xv is 15

Or to print the current year:

use Time::localtime;
use Roman;
printf "The year is now %s\n", Roman(1900 + localtime->year);

The year is now MMIII

Now, if you happen to have Unicode fonts available, you'll find that code points U+2160 through U+2183 represent Roman numerals, including those beyond the typical ASCII values.

use charnames ":full";

2003 is figs/U216F.giffigs/U216F.giffigs/U2162.gif

However, the Roman module doesn't yet have an option to use those characters.

Believe it or not, there's even a CPAN module that lets you use Roman numerals in arithmetic.

use Math::Roman qw(roman);
print $a  = roman('I'); #  I
print $a += 2000;       #  MMI
print $a -= "III";      #  MCMXCVIII
print $a -= "MCM";      #  XCVIII

2.5.4 See Also

The Encyclopaedia Britannica article on "Mathematics, History Of"; the documentation with the CPAN modules Roman and Math::Roman; Recipe 6.23

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