Prolog "perfect"

 Admin User, erstellt 25. Okt. 2024 
         
/**

         
 * Prolog code for finding a perfect number.

         
 *

         
 * A solution that did not make use of findall/3 and rem/2

         
 * was posted Dec 16, 1988 on comp.lang.prolog by Thomas

         
 * Sj|land. It was titled Christmas pleasure and contained

         
 * an invitation to further dig into Euclids theorem.

         
 *

         
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% between1(+Integer, +Integer, -Integer)

         
between1(Lo, Hi, _) :- Lo > Hi, !, fail.

         
between1(Lo, _, Lo).

         
between1(Lo, Hi, X) :- Lo2 is Lo+1, between1(Lo2, Hi, X).

         

         
% sumlist1(+List, -Integer)

         
sumlist1([], 0).

         
sumlist1([X|Y], R) :- sumlist1(Y, H), R is X+H.

         

         
% perfect(+Integer, -Integer)

         
perfect(Hi, X) :-

         
   between1(1, Hi, X),

         
   Y is X // 2,

         
   findall(Z, (between1(1,Y,Z), X rem Z =:= 0), L),

         
   sumlist1(L, X).

         

         
% perfect

         
perfect :-

         
   perfect(500, _).