How to arrange a list in ascending order in prolog?

Question

i want to sort my list in ascending order but without using builtin function is that possible?

 5  9   3   4   7   10  2   1   5   6

if yes then how?

Answer 1

You can implement the sort yourself, writing predicates. Let's implement selection sort (worst sort, but easy to understand and implement).

The first thing to implement is a predicate that, given a list, find both the minimum and the list without the minimum:

% Case where the list has only one element, pretty easy.
mini([H], H, []).

% H is the head of the list, T is the tail, M is the minimum of the list and R
% is the remaining list/elements (without the minimum).
% In the following predicate, H is NOT the minimum of the list.
mini([H|T], M, R) :- mini(T, M2, R2), M2 =< H, M = M2, R = [H | R2].

% Same as above, but when H (head) is the minimum of the list.
mini([H|T], M, R) :- mini(T, M2, R2), M2 > H, M = H, R = [M2 | R2].

Once you have this predicate, it's easy to write the sort predicate:

% Easy case where the given list is empty
mysort([], []).

% L is the list to sort, R is the resulted sorted list.
% This predicate will first retrieve the minimum and the list without
% the minimum, sort again the rest of the list and concatenate the 
% sorted remaining list with the minimum element found.
mysort(L, R) :- mini(L, M, Rem), mysort(Rem, T2),  R = [M|T2].

Of course, this algorithm is really bad, you definitely would prefer to implement a better sorting algorithm, such as merge sort. The goal here is just to show how this can be done with Prolog.

Answer 2

We can just express our wishes, and Prolog will take it as its command:

ascending(  [], [] ).
ascending(  [A], [A] ).
ascending(   A,  [X,Y|C] ) :-
  select( X, A, B),
  ascending(    B, [Y|C] ),
          X   <     Y.

This runs very slowly though:

2 ?- ascending( [5,9,3,4,7,10,2,1,5,6], X).
false.

% Execution Aborted
3 ?- ascending( [5,9,3,4,7,10,2,1,6], X).
X = [1, 2, 3, 4, 5, 6, 7, 9, 10] ;
false.