Re: RHS unbound variable

On 6 Feb, 18:20, LudovicoVan <ju...@xxxxxxxxxxxxx> wrote:
On 6 Feb, 14:50, LudovicoVan <ju...@xxxxxxxxxxxxx> wrote:
On 6 Feb, 14:08, Jose Luis <ingenieros.informat...@xxxxxxxxx> wrote:

Hi all,

Maybe, it is a so simple issue, but I am not sure if it is possible
for prolog resolve convert(C, 50) and convert(50, F) with a rule as

convert(Celsius, Fahrenheit) :-
    Celsius is (Fahrenheit - 32) * 5 / 9.

I have read the solution in the Bratko book using { }, but it doesn't
work in SWI-Prolog

convert(Celsius, Fahrenheit) :-
    {Celsius is (Fahrenheit - 32) * 5 / 9}.

Any suggestions?

Thanking you in advance

Hi Jose,

You might simply have two different clauses, checking with the var
predicate whether Celsius vs Farenheit is unbound, and performing the
calculation is the proper direction depending on the case:

    conv(C, F) :- var(C), !, C is ...
    conv(C, F) :- var(F), !, F is ...

FWIW, that's not really correct, in case both C and F are
instanciated. Here is the amended code:

    conv(C, F) :- var(C), !, C is (F - 32) * 5/9.
    conv(C, F) :- F is C * 9/5 - 32.

Sorry: F is C * 9/5 + 32.

-LV

Though the cut is no more green...

-LV



Otherwise, in SWI, you could use Clpqr as follows (although I wouldn't
know how portable it is):

    :- use_module(library(clpr)).

     conv(C, F) :- { C =:= (F - 32) * 5/9 }, !.

(I have added the cuts to avoid unneeded backtracking.)

HTH,

-LV
.

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