prolog

I want to build list of numbers that are in range of two given numbers.

For example: `betweenRange(1,5,X)`

will give the answer: `X=[1,2,3,4,5]`.

any idea how to do that?

I've tried something like:

``````elementsBetween(N1, N2, [N1|_]):-
N2 =:= N1.
elementsBetween(N1, N2, List):-
N2 > N1, N2New is N2-1,
elementsBetween(N1, N2New, [N2|List]).
``````

but its not working, some problem with backtracking after the recursion.

Solution

• ``````betweenToList(X,X,[X]) :- !.
betweenToList(X,Y,[X|Xs]) :-
X =< Y,
Z is X+1,
betweenToList(Z,Y,Xs).
``````

Output:

``````?- betweenToList(1,5,X).
X = [1, 2, 3, 4, 5].

?- betweenToList(1,2,X).
X = [1, 2].

?- betweenToList(1,8,X).
X = [1, 2, 3, 4, 5, 6, 7, 8].

?- betweenToList(1,1,X).
X = [1].

?- betweenToList(1,0,X).
false.
``````

Same logic by decreasing `Y` you can use `reverse/2` (Easy to implement):

``````betweenDecYAux(X,X,[X]) :- !.
betweenDecYAux(X,Y,[Y|Ys]) :-
X =< Y,
Z is Y-1,
betweenDecYAux(X,Z,Ys).

betweenDecY(X,Y,R) :-
betweenDecYAux(X,Y,L),
reverse(L, R). % reverse [c,b,a] to [a,b,c]
``````

Output:

``````?- betweenDecY(1,6,X).
X = [1, 2, 3, 4, 5, 6].

?- betweenDecY(2,8,X).
X = [2, 3, 4, 5, 6, 7, 8].

?- betweenDecY(1,0,X).
false.
``````