prolog convert numbers into roman numerals

Question

i have this code that converts integers into roman numerals i need to add a function that compares an integer with a roman numeral input and show if it's try or false, for example: roman(v,5). true

toroman(0).
toroman(N) :- N < 4, put("I"), M is N - 1, toroman(M).
toroman(N) :- N = 4, put("I"), put("V").
toroman(N) :- N = 5, put("V").
toroman(N) :- N < 9, put("V"), M is N - 5, toroman(M).
toroman(N) :- N = 9, put("I"), put("X").
toroman(N) :- N < 40, put("X"), M is N - 10, toroman(M).
toroman(N) :- N < 50, put("X"), put("L"), M is N - 40, toroman(M).
toroman(N) :- N < 90, put("L"), M is N - 50, toroman(M).
toroman(N) :- N < 100, put("X"), put("C"), M is N - 90, toroman(M).
toroman(N) :- N < 400, put("C"), M is N - 100, toroman(M).
toroman(N) :- N < 500, put("C"), put("D"), M is N - 400, toroman(M).
toroman(N) :- N < 900, put("D"), put("D"), M is N - 500, toroman(M).
toroman(N) :- N < 1000, put("C"), put("M"), M is N - 900, toroman(M).
toroman(N) :- N < 4000, put("M"), M is N - 1000, toroman(M).



roman(N) :- toroman(N).

Answer 1

Try to formulate the problem differently: Write a grammar (dcg) to relate an integer and a list of characters denoting roman numerals. Here is a start:

:- use_module(library(clpfd)).

roman(0) --> "".
roman(N0) --> "I", { 1 #=< N0, N0 #=< 3, N1 #= N0-1}, roman(N1).

You can use it like so:

?- phrase(roman(3), L).
   L = "III"
;  false.

or

?- phrase(roman(N), "II").
   N = 2
;  false.

or, if you don't know what to ask, simply ask the most general question:

?- phrase(roman(N), L).
   N = 0, L = []
;  N = 1, L = "I"
;  N = 2, L = "II"
;  N = 3, L = "III"
;  false.

To get answers compactly like L = "III", use :- set_prolog_flag(double_quotes,chars). See this answer for more.