CODEWITHSUNDEEP
prolog
John Sall
John Sall

Reputation: 1141

How to print only once in Prolog while in a recursive call

I have the following code I've been working on for an expert system:

in(switzterland, 'prairie dog').
in(austria,'wild dog').
in(czechia, 'giant panda').
in(america, 'red kangaroo').


linked(switzterland, austria).
linked(austria, czechia).
linked(czechia, america).

tour(X, Y) :-
    linked(X, Y),
    in(X, Z), habitant(Z, I),
    format('~s~t~14|~s~t~31|', ['Animal Name: ',Z]),
    format('~s~t~8|~s~t~23|', ['Habitant: ',I]),
    format('~s~t~8|~s~t~25|', ['Region: ',X]), nl,
    in(Y, U), habitant(U, T),
    format('~s~t~14|~s~t~31|', ['Animal Name: ',U]),
    format('~s~t~8|~s~t~23|', ['Habitant: ',T]),
    format('~s~t~8|~s~t~25|', ['Region: ',Y]), nl,!.
tour(X, Y) :-
    format('~s~t~14|~s~s~s\n~s', ['Dear customer, your tour is from: ',X,
       ' to ', Y,'Through your visit, you\'ll be able to see the following animals, please enjoy.']),nl,nl,
    in(X, Z), habitant(Z, I),
    format('~s~t~14|~s~t~31|', ['Animal Name: ',Z]),
    format('~s~t~8|~s~t~23|', ['Habitant: ',I]),
    format('~s~t~8|~s~t~25|', ['Region: ',X]), nl,
    linked(X, F), tour(F, Y).

output is :

Dear customer, your tour is from: switzterland to america
Through your visit, you'll be able to see the following animals, please enjoy.

Animal Name:  prairie dog      Habitant: grasslands     Region: switzterland     
Dear customer, your tour is from: austria to america
Through your visit, you'll be able to see the following animals, please enjoy.

Animal Name:  wild dog         Habitant: grassland      Region: austria          
Animal Name:  giant panda      Habitant: open forest    Region: czechia          
Animal Name:  red kangaroo     Habitant: woodlands      Region: america

You can see that 'Dear customer ....' is being repeated twice, or every time there's a recursive call to the second tour, it will print again. I only wants that to be printed one time.

Upvotes: 0

Views: 235

Answers (1)

Paulo Moura
Paulo Moura

Reputation: 18663

You need two predicates, the first one (e.g. tour/2) printing the 'Dear customer ....' message and calling the second predicate (e.g. find_tour/2) that computes the tour(s). For example:

tour(X, Y) :-
    format('~s~t~14|~s~s~s\n~s', ['Dear customer, your tour is from: ',X,
       ' to ', Y,'Through your visit, you\'ll be able to see the following animals, please enjoy.']),nl,nl,
    find_tour(X, Y).

find_tour(X, Y) :-
    linked(X, Y),
    in(X, Z), habitant(Z, I),
    format('~s~t~14|~s~t~31|', ['Animal Name: ',Z]),
    format('~s~t~8|~s~t~23|', ['Habitant: ',I]),
    format('~s~t~8|~s~t~25|', ['Region: ',X]), nl,
    in(Y, U), habitant(U, T),
    format('~s~t~14|~s~t~31|', ['Animal Name: ',U]),
    format('~s~t~8|~s~t~23|', ['Habitant: ',T]),
    format('~s~t~8|~s~t~25|', ['Region: ',Y]), nl,!.
find_tour(X, Y) :-
    in(X, Z), habitant(Z, I),
    format('~s~t~14|~s~t~31|', ['Animal Name: ',Z]),
    format('~s~t~8|~s~t~23|', ['Habitant: ',I]),
    format('~s~t~8|~s~t~25|', ['Region: ',X]), nl,
    linked(X, F),
    find_tour(F, Y).

Upvotes: 1

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