CODEWITHSUNDEEP
prolog
CompilerSaysNo
CompilerSaysNo

Reputation: 415

List check that elements are different

I would like to write a predicate different_from(Xs,X) which is a check that succeeds if and only if X is different from all the elements of the list Xs.

So the query

different_from([3,2,5],4)

should succeed but this following query should fail:

different_from([3,2,5],2)

Since the predicate is a check it should not instantiate either of its arguments.

thank you in advance for any help you can provide.

Upvotes: 1

Views: 1205

Answers (1)

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726479

This is a very simple rule consisting of two clauses:

  • The first clause says that different_from is successful when the list is empty,
  • The second clause says that different_from is successful when the first element of the list does not match the element being searched, and also when different_from for the tail is successful as well.

Here is the same thing written in Prolog syntax:

different_from([], _).
different_from([H|T], E) :- E \= H, different_from(T, E).

Upvotes: 2

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