CODEWITHSUNDEEP
prolog
Phil_Charly
Phil_Charly

Reputation: 237

PROLOG:How to exclude an option

I have this piece of code which suggests for a certain animal which other animals can coexist to it under those circumstances:

suggest(X) :- bigAnimal(X),bigAnimal(Y),coexist(X,Y),write(Y),nl,fail.

This loops until fail.My issue is that it prints out also the variable X

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How can I exclude it from the output, thx in adcavance.

Upvotes: 1

Views: 975

Answers (3)

false
false

Reputation: 10102

In your program you are mixing querying via the toplevel shell and performing side-effects with write/1. In this manner you will most probably miss a lot of what Prolog can offer you.

Instead, stick to pure side-effect free relations and let all writing be done by the toplevel.

Thus, instead of suggest/1, consider a new relation, say biganimal_compatiblewith/2.

biganimal_compatiblewith(X,Y) :-
   bigAnimal(X),
   bigAnimal(Y),
   coexist(X,Y),
   dif(X,Y).     % maybe

?- biganimal_compatiblewith(X,Y).
   X = melan, Y = narty 
;  X = melman, Y = nelman
;  ... .

Now, both X and Y are printed on the toplevel. Think how useful this relation is compared to the original suggest/1. You can ask for one or the other, or you even can ask

?- biganimal_compatiblewith(X,X).

You can also reuse this relation, building more complex ones:

?- biganimal_compatiblewith(X,Y), burgervore(Y).

That is the essence of Prolog relations. You can spare out side effects for quite some time.

For another example how to avoid unnecessary side effects: What are the pros and cons of using manual list iteration vs recursion through fail

Upvotes: 3

Paulo Moura
Paulo Moura

Reputation: 18663

suggest(X) :-
    bigAnimal(X), bigAnimal(Y), X \== Y, coexist(X,Y), write(Y), nl, fail.

Upvotes: 1

Shevliaskovic
Shevliaskovic

Reputation: 1564

When I have this piece of code:

bigAnimal(elephantas).
bigAnimal(kamilopardali).

coexist(elephantas,kamilopardali).
coexist(kamilopardali,elephantas).
/* coexist has to be symmetric */


suggest(X) :- bigAnimal(X),bigAnimal(Y),coexist(X,Y),write(Y),nl,fail.

The result is as expected.

2 ?- suggest(elephantas).
kamilopardali
false.
4 ?- suggest(kamilopardali).
elephantas
false.

You shouldn't end your code with ; because that means OR. When I tried to compile that, I get an error. That's why I have to end my rule with a fullstop (.)

Upvotes: 1

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