Program implements quicksort algorithm.

Source: PROLOG programming for artificial intelligence, 3rd Edition, Harlow, 2001, ISBN 0-201-40375-7.

Program source code: quicksort_2.pl

% Figure 9.3  A more efficient implementation of quicksort using difference-pair
% representation for lists. 
 
 
% quicksort( List, SortedList): sort List with the quicksort algorithm
 
quicksort( List, Sorted)  :-
  quicksort2( List, Sorted - [] ).
 
% quicksort2( List, SortedDiffList): sort List, result is represented as difference list
 
quicksort2( [], Z - Z).
 
quicksort2( [X | Tail], A1 - Z2)  :-
   split( X, Tail, Small, Big),
   quicksort2( Small, A1 - [X | A2] ),
   quicksort2( Big, A2 - Z2).
 
split( X, [], [], []).
 
split( X, [Y|Tail], [Y|Small], Big)  :-
   gt( X, Y), !,
   split( X, Tail, Small, Big).
 
split( X, [Y|Tail], Small, [Y|Big])  :-
   split( X, Tail, Small, Big).
 
gt(A,B):-A > B.
 
%  conc(L1,L2,L3): list L3 is th econcatenation of lists L1 and L2
 
conc( [], L, L).
 
conc( [X | L1], L2, [X | L3])  :-
  conc( L1, L2, L3).