Re: how to make this fail driven loop succesfull ?

Roelof,

You can make your failure-drive loop finish as 'true' like this:

siblings(Father,Mother) :- is_father(X, Father), is_mother(X, Mother), write(X), nl, fail ; true.

Reading from left-to-right, when it has found all the possible values
for X and failed to find any more with backtracking, it will try the
right-hand branch of the ';' and finally succeed with the 'true'.


On 08/03/2014 21:00, Roelof Wobben wrote:
> Hello, 
>
>
> I have these facts: 
>
>
> male(roelof). 
>
> male(mans).
>
> male(jan).
>
>
> female(chantal). 
>
> female(tamara). 
>
> female(marie).
>
> female(gerda). 
>
>
> parent(roelof,tamara).
>
> parent(mans, roelof). 
>
> parent(jan, chantal). 
>
> parent(marie, roelof). 
>
> parent(chantal, tamara).
>
> parent(mans, gerda).
>
> parent(marie, gerda). 
>
>
> is_mother(Child,Mother):- 
>
> parent(Mother, Child),
>
> female(Mother).
>
>
>
> is_father(Child, Father):- 
>
> parent(Father, Child),
>
> male(Father). 
>
>
>
> siblings(Father,Mother) :- 
>
> is_father(X, Father), 
>
> is_mother(X, Mother),
>
>  write(X),
>
>  nl, 
>
> fail.
>
>
> When I do siblings(mans,marie) it produces:
>
>
> roelof
>
> gerda
>
> no
>
>
> How can I change this to make the output like this:
>
>
> roelof
>
> gerda
>
> yes. 
>
>
> I have tried to add siblings(_) but with no success 
>
>
> Roelof 		 	   		  
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