Prolog - [GNU Prolog] counter

Author

[GNU Prolog] counter

gai luron

2004-05-04, 2:59 pm

Hi,
I need to generate unique keys, so I want to use a counter, using
assert/retract predicates. But, what's wrong with my code?

assert(counter(0)).
next_key(X) :-
retract(counter(Y)),
X is Y+1,
assert(counter(X)).

thanks

--
nico

Gregor Rot

2004-05-04, 2:59 pm

gai luron wrote:
> Hi,
> I need to generate unique keys, so I want to use a counter, using
> assert/retract predicates. But, what's wrong with my code?
>
> assert(counter(0)).
> next_key(X) :-
> retract(counter(Y)),
> X is Y+1,
> assert(counter(X)).
>
> thanks
>
> --
> nico
>

use:

next_key :-

you don't need a parameter here...

br,
Greg

Bart Demoen

2004-05-04, 2:59 pm

Gregor Rot wrote:
> gai luron wrote:
>
>
> use:
>
> next_key :-
>
> you don't need a parameter here...
>
> br,
> Greg

Sure he needs the parameter - he is interested in the value of the next key.
What's wrong is the fact

assert(counter(0)).

In any other Prolog system I tried, this seems forbidden, because assert
is a builtin predicate.

In GNU Prolog, you can define it, and assert/1 it is not a builtin:

| ?- assert(foo).
uncaught exception: error(existence_error(procedure,assert/1),top_level/0)

but the fact does not have the desired effect.

You just want to set the counter to 0 initially, right.
There are (at least) three ways to do that:

:- dynamic counter/1.
counter(0).

or by defining:

init_counter :- asserta(counter(0)).
and calling it at some point in your program
(note the use of asserta/1 which GNU knows )

or by putting in you file:

:- asserta(counter(0)).

Note the :- if you don't know what it does, see manual.

Redefine next_key as:

next_key(X) :-
retract(counter(Y)),
X is Y+1,
asserta(counter(X)).

Cheers

Bart Demoen

gai luron

2004-05-04, 2:59 pm

Bart Demoen wrote:

> init_counter :- asserta(counter(0)).
> and calling it at some point in your program
> (note the use of asserta/1 which GNU knows )

Yes! You're right. In my version I was redefining the predicate assert/1...
thanks a lot.
Best.

--
nico

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