Prolog Remove the elements between the first and the last elements in a list

Question

I am trying to keep only the first element and the last element for a list which contains only consecutive integers.

For example:

?- remove([1,2,3,4,5],          


        

Answer 1

To solve this problem you have to handle different cases, here the solution then the comment:

last([E],E).
last([_|T],E):-
    last(T,E).

remove([],[]).
remove([A],[A]).
remove([A,B],[A,B]).
remove([A|B],[A,Last]):-
    last(B,Last).

findElementsR([A,B],LT,LOT,LO):-
    B =< A,
    append(LT,[A],LT1),
    remove(LT1,LRem),
    append(LOT,[LRem,[B]],LO).

findElementsR([A,B|T],LT,LOT,LO):-
    B =< A,
    append(LT,[A],LT1),
    remove(LT1,LRem),
    append(LOT,[LRem],LOT1),
    findElementsR([B|T],[],LOT1,LO).

findElementsR([A,B],LT,LOT,LO):-
    B is A+1, %consecutive
    append(LT,[A,B],LTO),
    remove(LTO,RM),
    append(LOT,[RM],LO).

findElementsR([A,B|T],LT,LOT,LO):-
    B is A+1, %consecutive
    append(LT,[A],LTO),
    findElementsR([B|T],LTO,LOT,LO).

findElements(L,LO):-
    findElementsR(L,[],[],LO).

?- findElements([3,1,2,3,2,3,4,5,2,3,4],L).
L = [[3], [1, 3], [2, 5], [2, 4]]
false

So, first of all, i've defined last/2 that simply, given a list as an input, returns the last element

?- last([1,2,3],L).
L = 3

Then with remove/3 i get the list composd by th first and the last element.

findElements/2 is used to call findElementsR/4 (to make it tail recursive). findElements/4 finds a list of consecutive elements and then calls remove/2 to get the first and the last.