natural(1).
natural(N) :- natural(M), N is M+1.
% myloop fails as a goal
myloop(N) :- N > 0, natural(I),
write(I), nl,
I = N,
!, fail.
%myloop2 succeeds as a goal
myloop2(N) :- N > 0, natural(I),
write(I), nl,
I = N, !.
myloop2(_).
myevenloop(N) :- N > 0, natural(I),
J is I mod 2, J = 0,
write(I), nl,
I = N,
!, fail.
% myevenloop(3) goes on infinite loop.
myevenloop2(N) :- N > 0, natural(I),
J is I mod 2, J = 0,
write(I), nl,
I >= N,
!, fail.
% myevenloop2(3) gets to 4.
% loop body should always succeed as a goal to be able to reach loop
% end condition, e.g.:
printeven(N) :- K is N mod 2, K = 0,
write(N), nl.
printeven(_).
myevenloop3(N) :- N > 0, natural(I),
printeven(I),
I = N, !.
myevenloop3(_).
% myloop fails as a goal: outer loop never reaches end condition!
nestedloop(N,M):-
N>0,
natural(I),
write(I),nl,
myloop(M),nl,
I=N,
!,fail.
nestedloop2(N,M):-
N>0,
natural(I),
write(I),nl,
myloop2(M),nl,
I=N,!.
nestedloop2(_,_).