Prolog: script doesn't halt - prolog

I'm currently trying to learn prolog. I hope you can help..
I have three rules:
reverse - retrieves the reverse of a list
startswith - checks if the second list is a prefix of the first list
suffix - checks if the first list is a suffix of the second list
reverse([H|T], Y) :- append(Z, [H], Y), reverse(T, Z).
reverse([], Y) :- Y = [].
startswith(_, []).
startswith([Xh|Xt], [Yh|Yt]) :- Xh=Yh, startswith(Xt, Yt).
suffix(X, Y) :- reverse(X, XR), reverse(Y, YR), startswith(YR,XR).
reverse and startswith seem to work as they should.
But suffix doesn't stop calculating. I cannot understand why?


In general, it's a bad idea to pass variables that have yet to be unified to predicates that you will unify later. Essentially, append(Z,[H],Y) is spiraling off into prolog never-never land because it's unifying Z over and over based on your version of Prolog's append.
Change
reverse([H|T], Y) :- append(Z, [H], Y), reverse(T, Z).
To
reverse([H|T], Y) :- reverse(T, Z), append(Z, [H], Y).
So that you unify Z before you pass it to append

Related

Prolog - Find second and second last elements in list

I am new to prolog and currently stuck trying to understand how to implement this.
I need a predicate to find the second and the second last elements of a list using recursion, so for example:
second_secondLast([1,2], X, Y). must return X=2, Y=1.
second_secondLast([1,2,3], X, Y). must return X=2, Y=2.
second_secondLast([1], X, Y). must print 'Error' and return false.
First, I have the error-checking clauses:
second_secondLast([], X, Y) :- print("Error"), !, fail.
second_secondLast([_], X, Y) :- print("Error"), !, fail.
Next, I tried something like this:
second_secondLast([Y,X],X,Y) :- !.
second_secondLast(L, X, Y) :-
second(L,X),
secondLast(L,Y).
second([_,S|_], X) :- X = S.
secondLast([P,_], Y) :- Y = P.
secondLast([F|R], Y) :- secondLast(R, Y).
However, the output using [1,2,3] is X=Y, Y=2.
I'm not sure if it is possible to force the output to be X=2 instead, or if there is a better method to do this.

First of all, the output X=Y, Y=2. has nothing to do with your program, it is an idiosyncracy of swipl (and maybe other interactive environments for Prolog implementations).
I think, your program looks fine, but you are asking for possible improvements.
second([_,S|_], S). is a more elegant version of your second([_,S|_], X) :- X = S..
Likewise, secondLast([P,_], P). is more elegant than your secondLast([P,_], Y) :- Y = P..
I would also prefer secondLast([_|R], Y) :- secondLast(R, Y). to your
secondLast([F|R], Y) :- secondLast(R, Y)..
Your error-checking clauses look fine to me.
You could also get rid of the predicate second and alter the definition of second_secondLast by using
second_secondLast([H,X|T], X, Y):-
secondLast([H,X|T], Y).
instead of your
second_secondLast(L, X, Y) :-
second(L,X),
secondLast(L,Y).
That change would also make it a bit more efficient.
Another possibility is to use
second_secondLast(L, X, Y):-
L= [_,X|_],
secondLast(L, Y).
Then you could also get rid of the predicate secondLast and alter the above clause to
second_secondLast(L, X, Y):-
L= [_,X|_],
append(_, [Y,_], L).
.
There is always a ton of possibilities...

prolog - check if item is somewhere on the right/left/between in a list

I have to figure out how to find out if the item is in between of two other items in the list. My thinking is that to have a somewhereleft and somewhereright functions and then use it in a way somewherebetween(X,Y,Z,L) :- somewhereleft(X,Y,L), somewhereright(Y,Z,L). I have implemented function to find out if the item is to the right right(X,Y, [X, Y|_]). right(X, Y, [_|T]) :-right(X, Y, T). and to the left left(X, Y, L) :- right(Y, X, L).; however, cannot wrap my head around finding out the recursive somewhereleft/right functions.

append/3 may be your friend
in_order(X, Y, Z, Lst) :-
append(_, [X|T1], Lst),
append(_, [Y|T2], T1),
append(_, [Z|_], T2).
for example
?- in_order(a,b,c,[a,e,b,d,c]).
true .
but
?- in_order(a,b,c,[c,b,a,b,c,b,a]).
true ;
false.

Write a Prolog predicate next(X,List,List1)

Prolog predicate next(X, List,List1), that returns in List1 the next element(s) from List that follows X, e.g., next(a,[a,b,c,a,d],List1), will return List1=[b,d].
I have tried following:
next(X, [X,Y|List], [Y|List1]) :- % X is the head of the list
next(X, [Y|List], List1).
next(X, [Y|List], List1) :- % X is not the head of the list
X \== Y,
next(X, List, List1).
next(_,[], []).

First, whenever possible, use prolog-dif for expressing term inequality!
Second, the question you asked is vague about corner cases: In particular, it is not clear how next(E,Xs,Ys) should behave if there are multiple neighboring Es in Xs or if Xs ends with E.
That being said, here's my shot at your problem:
next(E,Xs,Ys) :-
list_item_nexts(Xs,E,Ys).
list_item_nexts([],_,[]).
list_item_nexts([E],E,[]).
list_item_nexts([I|Xs],E,Ys) :-
dif(E,I),
list_item_nexts(Xs,E,Ys).
list_item_nexts([E,X|Xs],E,[X|Ys]) :-
list_item_nexts(Xs,E,Ys).
Let's see some queries!
?- next(a,[a,b,c,a,d],List1).
List1 = [b,d] ;
false.
?- next(a,[a,a,b,c,a,d],List1).
List1 = [a,d] ;
false.
?- next(a,[a,a,b,c,a,d,a],List1).
List1 = [a,d] ;
false.
Note that above queries succeed, but leave behind useless choicepoints.
This inefficiency can be dealt with, but I suggest figuring out more complete specs first:)

This version is deterministic for the cases given by #repeat using if_/3 and (=)/3. It shows how purity and efficiency can coexist in one and the same Prolog program.
next(E, Xs, Ys) :-
xs_e_(Xs, E, Ys).
xs_e_([], _E, []).
xs_e_([X|Xs], E, Ys) :-
if_(X = E, xs_e_xys(Xs, E, Ys), xs_e_(Xs, E, Ys)).
xs_e_xys([], _E, []).
xs_e_xys([X|Xs], E, [X|Ys]) :-
xs_e_(Xs, E, Ys).
%xs_e_xys([X|Xs], E, [X|Ys]) :- % alternate interpretation
% xs_e_([X|Xs], E, Ys).

Simple prolog program returns false too early

It's been a while since I've programmed in Prolog. Today, I tried to make a simple program. It lists some facts of who belongs to the same family. If two people belong to the same family, they cannot give eachother gifts. I want to get all the people (or at least one person) to whom someone is allowed to give a gift.
family(john, jack).
family(matt, ann).
family(ann, jack).
family(jordan, michael).
family(michael, liz).
sameFamily(X, Y) :-
family(X, Y).
sameFamily(X, X) :-
false.
sameFamilySym(X, Y) :-
sameFamily(X, Y).
sameFamilySym(X, Y) :-
sameFamily(Y, X).
sameFamilyTrans(X, Z) :-
sameFamilySym(X, Y),
sameFamilySym(Y, Z).
gift(X, Y) :-
not(sameFamilyTrans(X, Y)).
Some queries if sameFamilyTrans/2 return false when they should in fact return true.
sameFamilyTrans/2 is obviously wrong. I think I need to keep a list of intermediate transitivities. Something like this:
sameFamilyTrans(X, Z, [Y|Ys]) :-
sameFamilySym(X, Y, []),
sameFamilyTrans(Y, Z, Ys).
But then I don't know how to call this.
P.S. I am using SWI-Prolog, if that makes any difference.

Yes, you were on the right track. The trick is to call the transitive closure with an empty accumulator, and check in each step whether a cycle is found (i.e., whether we have seen this member of the family before. As "false" has pointed out, the persons need to be instantiated already before going into the not, though.
So in sum, this works:
family(john, jack).
family(matt, ann).
family(ann, jack).
family(jordan, michael).
family(michael, liz).
sameFamily(X, Y) :-
family(X, Y).
sameFamilySym(X, Y) :-
sameFamily(X, Y).
sameFamilySym(X, Y) :-
sameFamily(Y, X).
sameFamilyTrans(X, Y, Acc) :-
sameFamilySym(X, Y),
not(member(Y,Acc)).
sameFamilyTrans(X, Z, Acc) :-
sameFamilySym(X, Y),
not(member(Y,Acc)),
sameFamilyTrans(Y, Z, [X|Acc]).
person(X) :- family(X, _).
person(X) :- family(_, X).
gift(X, Y) :-
person(X),
person(Y),
X \= Y,
not(sameFamilyTrans(X, Y, [])).
A bit of background: Transitive closure is not actually first-order definable (cf. https://en.wikipedia.org/wiki/Transitive_closure#In_logic_and_computational_complexity). So it can be expected that this would be a little tricky.

Negation is implemented in Prolog in a very rudimentary manner. You can essentially get a useful answer only if a negated query is sufficiently instantiated. To do this, define a relation person/1 that describes all persons you are considering. Then you can write:
gift(X,Y) :-
person(X),
person(Y),
\+ sameFamily(X,Y).
There is another issue with the definition of sameFamily/2.

Prolog: converting atom to new atom

I have a problem with predicate which works in that way that it takes list of atoms:
nopolfont([to,jest,tekśćik,'!'],L).
and in result
L = [to,jest,tekscik,'!'].
I have problem with make_swap and swap predicates. So far I have:
k(ś,s).
k(ą,a).
% etc.
swap(X,W) :- name(X,P), k(P,Y), !, name(Y,W).
swap(X,X).
make_swap(A,W)
:- atom(A),!,
name(A,L),
swap(L,NL),
name(W,NL).
nopolfont([],[]).
nopolfont([H|T],[NH|S]) :- make_swap(H,NH), nopolfont(T,S).
Is there any elegant way to do this?

This is also quite elegant:
polish_char_replacer(X, Y) :-
k(X, Y),
!.
polish_char_replacer(X, X).
nopolfont(Atoms1, Atoms2) :-
maplist(replace(polish_char_replacer), Atoms1, Atoms2).
replace(Goal, Atom1, Atom2) :-
atom_chars(Atom1, Chars1),
maplist(Goal, Chars1, Chars2),
atom_chars(Atom2, Chars2).

Probably as elegant as it can get:
k(ś,s).
k(ą,a).
swap(X,W) :- name(P,[X]), k(P,Y), !, name(Y,[W]).
swap(X,X).
list_swap([], []).
list_swap([H|T], [W|S]) :-
swap(H, W),
list_swap(T, S).
atom_swap(A,W) :-
atom(A), !,
name(A, L),
list_swap(L,S),
name(W, S).
nopolfont([],[]).
nopolfont([H|T],[NH|S]) :-
atom_swap(H,NH),
nopolfont(T,S).
Also, obviously define this, to get the expected result, but I assume this is in the % etc
k(ć, c).

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