Prolog Recursion in lists, last but one element

Question

The question is to find the last but one character in a list, e.g.

?- last_but_one(X, [a,b,c,d]).
X = c.

My code is:

last         


        

Answer 1

another solution :

• firstly, the list must be of length >= 2,
• also the last length element of the list = 1

code :

last_but_one(R,[X|Rest]):-
   (  Rest=[_], R=X
   ;  last_but_one(R,Rest)
   ). 

Test :

| ?- last_but_one(Elem,List).
List = [Elem,_A] ? ;
List = [_A,Elem,_B] ? ;
List = [_A,_B,Elem,_C] ? ;
List = [_A,_B,_C,Elem,_D] ? ;
List = [_A,_B,_C,_D,Elem,_E] ? ;
List = [_A,_B,_C,_D,_E,Elem,_F] ? ;
List = [_A,_B,_C,_D,_E,_F,Elem,_G] ? ;
List = [_A,_B,_C,_D,_E,_F,_G,Elem,_H] ? 
yes

Hope this idea help you