在Prolog中删除列表的第一个元素

Question

my problem is how to remove first elements of a list until an element (first occurrence). The list can have repeated elements.

Eg

?- L = [a, b, c, a, c, d], remove_elements(c, L, Result).
Result = [c, a, c, d].

What I have so far is this:

remove_elements(Element, [], []).
remove_elements(Element, [Element|Tail], [Tail]).
remove_elements(Element, [Head|Tail], [Tail]) :-
    Head \== Element,
    remove_elements(Element, Tail, Tail).

I cannot use any library.

If the Element is in the first position, the solution is correct but if it's in the middle it fails.

Thanks

Answer 1

Just little corrections, be careful with unification.

remove_elements(Element, [], []).
remove_elements(Element, [Element|Tail], [Element|Tail]).
remove_elements(Element, [Head|Tail], List) :- 
                Head \== Element, remove_elements(Element, Tail, List).

Terms S and T are unifiable if:

• S a T are identical constants
• S a T are (free) variables (S and T are made equal (they co-refer))
• S is a variable, T is a term ≠ variable (term T is substituted to variable S)
• T is a variable, S is a term ≠ variable (term S is substituted to variable T)
• S a T are compound terms (with identical main functor, corresponding arguments are unifiable)

Examples:

?- f(X,a(b,c)) = f(d,a(Y,c)).

X = d, Y = b.

?- f(X,a(b,c)) = f(Y,a(Y,c)).

X = Y = b

?- f(c,a(b,c)) = f(Y,a(Y,c)).

false.