From: trans <transfire@...>
Date: 2009-09-21T00:12:49+09:00
Subject: Re: Roman numeral conversion



On Sep 18, 1:26 pm, Brian Tickler <btick...@yahoo.com> wrote:
> First time poster here...
>
> I realize that rubyquiz.com is no longer really active, but some of the
> code there is still used by people googling for example code (roman
> numeral conversion in my case for a class I am taking), and I wanted to
> point out that many of the solutions listed for quiz #22 are evaluating
> roman to arabic numerals incorrectly.
>
> If you run many of the solutions, especially the ones that use this
> table:
>
>         [ 1000, 'M'  ],
>         [  900, 'CM' ],
>         [  500, 'D'  ],
>         [  400, 'CD' ],
>         [  100, 'C'  ],
>         [   90, 'XC' ],
>         [   50, 'L'  ],
>         [   40, 'XL' ],
>         [   10, 'X'  ],
>         [    9, 'IX' ],
>         [    5, 'V'  ],
>         [    4, 'IV' ],
>         [    1, 'I'  ]
>         ]
>
> ...and then you try to evaluate the string value XD (for example), it
> will convert this string to 510.  This should not happen; XD is not a
> legal roman numeral, DX would be 510, and CDXC would be 490.  The
> problem seems to be that in roman to arabic numeral conversion, the
> solutions just add the raw values together.  This means that converters
> using this kind of conversion will accept any combination of roman
> numeral values from the table above...regardless of order.
>
> A crazier example:
>
> If you evaluate XIV, you get 14, by finding X and then IV in the table
> above and then adding their corresponding values together.
>
> If you put in IXIVI, it will *also* evaluate to 14 by adding IX + IV + I
> and accept it as a valid roman numeral.
>
> I don't have a solution yet, but I thought I would at least point this
> out in case someone is adding a to_roman method to Integer or something
> and wants it to be 100% robust.

english/roman.rb checks out.

T.