CODEWITHSUNDEEP
prolog
user3294463
user3294463

Reputation: 17

prolog rule that takes as input a list of integers and returns “yes” if the list’s elements are all equal, otherwise returns “no”

Let's say I have a list [1, 1, 1, 1]. I need a way to check IF every element within this list are all equal return yes otherwise return no without using any of the built-in predicates. list(3,3)-->> yes , list(3,3,6)-->> no I came up with this rule but it doesn't work 

equal([E1,E2|T]):- E1=:=E2,
       equal([E2,E3|T]).

Upvotes: 1

Views: 216

Answers (2)

aschepler
aschepler

Reputation: 72356

That E3 appeared out of nowhere. I think you want

equal([]).
equal([X]).
equal([E1,E2|T]) :-
    E1 =:= E2,
    equal([E2|T]).

Upvotes: 1

false
false

Reputation: 10102

Usually, such a predicate is defined using general equality, not arithmetic equality:

equal(Xs) :-
   maplist(=(_), Xs).

Is probably the most insightful definition. This uses maplist/2 commonly defined as

maplist(_, []).
maplist(C, [E|Es]) :-
   call(C, E),
   maplist(C, Es).

Otherwise:

equal(Xs) :-
   equals_to(Xs,_).

equals_to([], _).
equals_to([E|Es], E) :-
   equals_to(Es, E).

Upvotes: 5

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