CODEWITHSUNDEEP
databaseprolog
Anonymous
Anonymous

Reputation: 1373

Prolog Database: Why is program only running once?

I got this code that has to run a database as long as N > Order. My code only runs once :/ ?

display(N) :-
    w(Order,_,Word,Class),
    N > Order -> (write(Word), write(' '), write(Class)),
    nl, fail .

Thanks in advance!

Upvotes: 2

Views: 129

Answers (2)

thanos
thanos

Reputation: 5858

your problem is the way you use ->

First of all, the code is interpreted as:

display(N) :-
    (  w(Order,_,Word,Class),
       N > Order ) 
    ) -> 
      (  write(Word), 
         write(' '), 
         write(Class)
      ),
    nl, fail .

-> destroys choice points meaning that it will not try to call w/3 again.

You could (probably) make it work like this:

display(N) :-
    (w(Order,_,Word,Class),
    N > Order )-> (write(Word), write(' '), write(Class)),
    nl, fail .

but in the end it's really ugly code and, as you have seen, prone to bugs. A better way is to use forall/2:

display2(N) :-
    forall(w(Order, _, Word, Class),
           (N > Order ->
           writef("%t %t \n", [Word,Class]); true)).

still, this will examine the whole database and print if N>Order. It is a bit unclear from your description if this is the desired behaviour but if you want to stop at the first element that is larger you could do something like:

display2(N) :-
    catch((forall(w(Order, _, Word, Class),
           (N > Order ->
           writef("%t %t \n", [Word,Class]); throw(end))), end, true)).

not the most declarative way to do it but I'm not sure what's the best way to model it without knowing what w/4 is (I assumed that it is some prolog clauses but it could be a predicate accessing a DB though the ODBC layer)

Upvotes: 4

CapelliC
CapelliC

Reputation: 60034

I think you forgot the 'else' branch, and the precedence of (->)/2, that's higher that (,)/2, inhibits the intended order. Try

display(N) :-
    w(Order,_,Word,Class),
    ( N > Order -> write(Word), write(' '), write(Class), nl ; true ),
    fail .

Upvotes: 3

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