Prolog sort list using permutation

Question

Writing sort function, sort(A,B) in Prolog using the built in permutation Prolog function. The sort function holds if B is a sorted version of A.

% sorted holds if list is sorted
sorted([]).
sorted([A]).
sorted([A,B|T]) :- A=<B, sorted([B|T]).

% sort list holds if A is sorted list of B
sort(A,B) :- permutation(A,B), sorted(B).

The problem is: when there are duplicate values in L, R does not include these duplicates.

Output:

?- sort([1,4,2,5,4,4,2], X).
X = [1, 2, 4, 5].

How do I change the sort function so that it doesn't drop duplicates?

CapelliC · Accepted Answer · 2013-06-11 06:37:54Z

1

permutation sort doesn't remove duplicates: testing your code

?- sort_([1,4,2,5,4,4,2], X).
X = [1, 2, 2, 4, 4, 4, 5] .

(I renamed to sort_ to avoid the builtin name clash).

But sort/2 does remove duplicates. You can use msort/2

?- msort([1,4,2,5,4,4,2], X).
X = [1, 2, 2, 4, 4, 4, 5].

answered Jun 11, 2013 at 6:37

CapelliC

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