Thread: Backtracking Problem

Could someone please guide me through backtracking in Prolog-C using the simple example below?

Here is my Prolog file:

Code:

    likes( john, mary ).

    likes( john, emma ).

    likes( john, ashley ).

Here is my C file:

Code:

    #include...

    term_t tx;
    term_t tv;
    term_t goal_term;
    functor_t goal_functor;

    int main( int argc, char** argv )
    {
        argv[0] = "libpl.dll";
        PL_initialise( argc, argv );
        PlCall( "consult( swi( 'plwin.rc' ) )" );
        PlCall( "consult( 'likes.pl' )" );

        tv = PL_new_term_ref( );
        PL_put_atom_chars( tv, "john" );
        tx = PL_new_term_ref( );
        goal_term = PL_new_term_ref( );
        goal_functor = PL_new_functor( PL_new_atom( "likes" ), 2 );
        PL_cons_functor( goal_term, goal_functor, tv, tx );

        PlQuery q( "likes", ??? );
        while ( q.next_solution( ) )
         {
          char* solution;
          PL_get_atom_chars( tx, &solution );
          cout << solution << endl;
         }

        PL_halt( PL_toplevel() ? 0 : 1 );
    }

What should I replace ??? with?

Or is this the right approach to get all the backtracking results generated and printed?

Thank you,

I have included the complete programs for Hanoi, Factorial, and Backtracking below. I hope beginners will benefit from them. If anybody is not sure about VS2008 settings for interfacing C++ and Prolog, I would gladly provide them with.

Code:

    #include <iostream>
    #include <fstream>
    #include <string>
    #include <math.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <stdafx.h>
    using namespace std;
    #include "Windows.h"
    #include "ctype.h"
    #include "SWI-cpp.h"
    #include "SWI-Prolog.h"
    #include "SWI-Stream.h"
 
    term_t t;
    predicate_t p;
 
    term_t tf;
    term_t tx;
 
    term_t tv;
    term_t tu;
    predicate_t pred;
 
    term_t goal_term;
    functor_t goal_functor;
 
    int main( int argc, char** argv )
    {
        argv[ 0 ] = "libpl.dll";
        PL_initialise( argc, argv );
        PlCall( "consult( swi( 'plwin.rc' ) )" );
        PlCall( "consult( 'myPrologFile.pl' )" );
 

        cout << "Enter your hanoi number: ";
        int n;
        cin >> n;
        cout << "Calculating hanoi of " << n << endl;
        PL_put_integer( t, n );
        p = PL_predicate( "hanoi", 1, NULL );
        PL_call_predicate( NULL, PL_Q_NORMAL, p, t );
 

        cout << "Enter your factorial number: ";
        int nf;
        cin >> nf;
        cout << "Calculating factorial of " << nf << endl;
        tf = PL_new_term_ref( );
        PL_put_integer( tf, nf );
        tx = PL_new_term_ref( );
        goal_term = PL_new_term_ref( );
        goal_functor = PL_new_functor( PL_new_atom( "factorial" ), 2 );
        PL_cons_functor( goal_term, goal_functor, tf, tx );
        int fact;
        if ( PL_call( goal_term, NULL ) )
        {
            PL_get_integer( tx, &fact );
            cout << fact << endl;
        }
        else
        {
            PL_fail;
        }


        cout << "Backtracking . . .";
        tv = PL_new_term_ref( );
        PL_put_atom_chars( tv, "john" );
        pred = PL_predicate( "likes", 2, NULL );
        tu = PL_new_term_ref( );
        qid_t qid = PL_open_query( NULL, PL_Q_NODEBUG, pred, tv );
        while ( int i = PL_next_solution( qid ) )
        {
            char* solution;
            PL_get_atom_chars( tu, &solution );
            cout << solution << endl;
        }
        PL_close_query( qid );
 

        PL_halt( PL_toplevel( ) ? 0 : 1 );
    }

This is myPrologFile.pl

Code:

    :- use_module( library( shlib ) ).
    :- use_module( library( lists ) ).


    hanoi( N ):-
        move( N, left, center, right ).
 
    move( 0, _, _, _ ):-
        !.
    move( N, A, B, C ):-
        M is N-1,
        move( M, A, C, B ),
        inform( A, B ),
        move( M, C, B, A ).
 
    inform( X, Y ):-
        write( 'move a disk from ' ),
        write( X ),
        write( ' to ' ),
        write( Y ),
        nl.


    factorial( 1, 1 ):-
        !.
    factorial( X, Fac ):-
        X > 1,
        Y is X - 1,
        factorial( Y, New_Fac ),
         Fac is X * New_Fac.


    likes( john, mary ).
    likes( john, emma ).
    likes( john, ashley ).