Prolog-GCD.pl

:- set_prolog_flag(verbose, silent).
:- initialization(main).

gcd(X, 0, X):- !.
gcd(0, X, X):- !.
gcd(X, Y, D):- X > Y, !, Z is X mod Y, gcd(Y, Z, D).
gcd(X, Y, D):- Z is Y mod X, gcd(X, Z, D).

choose(1, Num1, Num2) :- commandline(Num1, Num2).
choose(2, Num1, Num2) :- fileinput(Num1, Num2).

fileinput(Num1, Num2):-
    open('ayaya.txt', read, Str),
    read(Str, Num1),
    read(Str, Num2),
    close(Str).

commandline(Num1, Num2):-
    write("Enter your first positive number: "),
    read(Num1),
    write("Enter your second positive number: "),
    read(Num2).

main:-
    write("How would you like to proceed?\n"),
    write("\t1. Command Line Input\n"),
    write("\t2. File Input\n\n"),
    write("Enter your choice: "),
    read(Choice),
    choose(Choice, Num1, Num2),
    gcd(Num1, Num2, D),
    write("The gcd of "), write(Num1), write(" and "), write(Num2), write(" is "), write(D), write("."),
    halt.
main:-
    halt(1).