Question

I’m trying to write a program in Prolog, which will insert an element into a certain position, so e.g.

?- ins(a, [1,2,3,4,5], 3, X).
X = [1,2,a,3,4,5].

I have the following code:

ins(X,[H|T],P,OUT) :-
   length([T3],P),
   concatenate(X,[H],T),
   ins(...). 

The problem is that it is inserting element X in given index from back (I even know where the problem is -> the length([T3],P) which is obviously the length of the list from back not from head) . I was trying to remember how much elements did I cut off and insert X when “number of cut off elements” = P, but I can’t really write that in Prolog. Any ideas?

Answer 1

% ins(Val,List,Pos,Res)

ins(Val,[H|List],Pos,[H|Res]):- Pos > 1, !, 
                                Pos1 is Pos - 1, ins(Val,List,Pos1,Res). 
ins(Val, List, 1, [Val|List]).

The predicate fails if Pos = 0 or Pos > length(List) + 1