count successive occurrences of number in Prolog

Question

Hello I am trying to make a program in Prolog that given a list it counts the occurrences of each successive element in the list as follows:

count(1,[1,1,1,2,2,2         


        

Answer 1

If we skip usage of "is" we can have a solution like:

precondition(Clause):-
  Clause =.. [_|ARGS],
  ( maplist(var,ARGS) -> true; Clause ).

count( [], [] ).

count( [X], [(X,1)] ) :- !.

count( [H|Q], [(H,1),(HR,NR)|QR] ) :-
   count( Q, [(HR,NR)|QR] ),
   H \= HR,
   !.

count( [H|Q], [(H,NR)|QR] ) :-
   precondition( succ(N,NR) ),
   count( Q, [(H,N)|QR] ), 
   succ(N,NR).

that allows not only the usual query:

[debug]  ?- count([1,1,1,2,2,2,3,1,1],R).
R = [ (1, 3), (2, 3), (3, 1), (1, 2)].

but also the reverse one:

[debug]  ?- count(X, [ (1, 3), (2, 3), (3, 1), (1, 2)] ).
X = [1, 1, 1, 2, 2, 2, 3, 1, 1].