I intend to describe "a rule is commutative" in my database, an immediate idea is that like the code.
certain_rule(X,Y) :- X = struct(_X,Others) , Y = some_process_base_on(_X).
certain_rule(Y,X) :- X = struct(_X,Others) , Y = some_process_base_on(_X).
But if X is unbound, it will make X bind with struct(_X,Others) directly, then some_process_base_on(_X) will be unknown. Now my solution is insert compound(X) before breakdown X.
Usually, compound/1 is not strict enough, something unexpected will be taken in, functor/3 may be a better solution but too many rules will exist since too many functors I need to compare, even if I use member/2 to find functor in list I still construct too many combination.
Is there any better way to describe 'commutative', BTW, is there any better way to screen some specified structures out?
First, note that your clauses are equivalent to writing:
certain_rule(struct(X,_), some_process_base_on(X)).
certain_rule(some_process_base_on(X), struct(X,_)).
A possible solution to screen for valid structures is to use a table of valid structures. For example:
valid_structure(-) :-
!,
fail.
valid_structure(struct(_,_)).
valid_structure(some_process_base_on(_)).
...
The first clause will screen out variables while preserving first-argument indexing for the predicate. You could then define a gatekeeper predicate:
certain_rule_gatekeeper(X, Y) :-
once((valid_structure(X); valid_structure(Y))),
certain_rule(X, Y).
To address commutativity, would a definition such as the following would work in your specific case?
certain_rule_gatekeeper(X, Y) :-
once((valid_structure(X); valid_structure(Y))),
once((certain_rule(X, Y); certain_rule(Y, X))).
Sample calls using just one of the clauses above for the certain_rule/2 predicate (thus avoiding duplicated information):
| ?- certain_rule_gatekeeper(X, Y).
no
| ?- certain_rule_gatekeeper(struct(X,_), Y).
Y = some_process_base_on(X)
yes
| ?- certain_rule_gatekeeper(Y, struct(X,_)).
Y = some_process_base_on(X)
yes
| ?- certain_rule_gatekeeper(some_process_base_on(X), Y).
Y = struct(X,_)
yes
| ?- certain_rule_gatekeeper(Y, some_process_base_on(X)).
Y = struct(X,_)
yes
Related
I am developing a path finding algorithm in Prolog, giving all nodes accessible by a path from a starting node. To avoid duplicate paths, visited nodes are kept in a list.
Nodes and neighbors are defined as below:
node(a).
node(b).
node(c).
node(d).
node(e).
edge(a,b).
edge(b,c).
edge(c,d).
edge(b,d).
neighbor(X,Y) :- edge(X,Y).
neighbor(X,Y) :- edge(Y,X).
The original algorithm below works fine:
path2(X,Y) :-
pathHelper(X,Y,[X]).
pathHelper(X,Y,L) :-
neighbor(X,Y),
\+ member(Y,L).
pathHelper(X,Y,H) :-
neighbor(X,Z),
\+ member(Z,H),
pathHelper(Z,Y,[Z|H]).
This works fine
[debug] ?- path2(a,X).
X = b ;
X = c ;
X = d ;
X = d ;
X = c ;
false.
however, when changing the order of the two clauses in the second definition, such as below
pathHelper(X,Y,L) :-
\+ member(Y,L),
neighbor(X,Y).
When trying the same here, swipl returns the following:
[debug] ?- path2(a,X).
false.
The query doesn't work anymore, and only returns false. I have tried to understand this through the tracecommand, but still can't make sense of what exactly is wrong.
In other words, I am failing to understand why the order of neighbor(X,Y)and \+ member(Y,L)is crucial here. It makes sense to have neighbor(X,Y) first in terms of efficiency, but not in terms of correctness to me.
You are now encountering the not so clean-cut borders of pure Prolog and its illogical surroundings. Welcome to the real world.
Or rather, not welcome! Instead, let's try to improve your definition. The key problem is
\+ member(Y, [a]), Y = b.
which fails while
Y = b, \+ member(Y,[a]).
succeeds. There is no logic to justify this. It's just the operational mechanism of Prolog's built-in (\+)/1.
Happily, we can improve upon this. Enter non_member/2.
non_member(_X, []).
non_member(X, [E|Es]) :-
dif(X, E),
non_member(X, Es).
Now,
?- non_member(Y, [a]).
dif(Y,a).
Mark this answer, it says: Yes, Y is not an element of [a], provided Y is different from a. Think of the many solutions this answer includes, like Y = 42, or Y = b and infinitely many more such solutions that are not a. Infinitely many solutions captured in nine characters!
Now, both non_member(Y, [a]), Y = b and Y = b, non_member(Y, [a]) succeed. So exchanging them has only influence on runtime and space consumption. If we are at it, note that you check for non-memberness in two clauses. You can factor this out. For a generic solution to this, see closure/3. With it, you simply say: closure(neighbor, A,B).
Also consider the case where you have only edge(a,a). Your definition fails here for path2(a,X). But shouldn't this rather succeed?
And the name path2/2 is not that fitting, rather reserve this word for an actual path.
The doubt you have is related to how prolog handle negation. Prolog uses negation as failure. This means that, if prolog has to negate a goal g (indicate it with not(g)), it tries to prove g by executing it and then, if the g fails, not(g) (or \+ g, i.e. the negation of g) succeeds and viceversa.
Keep in mind also that, after the execution of not(g), if the goal has variables, they will not be instantiated. This because prolog should instantiate the variables with all the terms that makes g fail, and this is likely an infinite set (for example for a list, not(member(A,[a]) should instantiate the variable A with all the elements that are not in the list).
Let's see an example. Consider this simple program:
test:-
L = [a,b,c],
\+member(A,L),
writeln(A).
and run it with ?- trace, test. First of all you get a Singleton variable in \+: A warning for the reason i explained before, but let's ignore it and see what happens.
Call:test
Call:_5204=[a, b]
Exit:[a, b]=[a, b]
Call:lists:member(_5204, [a, b])
Exit:lists:member(a, [a, b]) % <-----
Fail:test
false
You see at the highlighted line that the variable A is instantiated to a and so member/2 succeeds and so \+ member(A,L) is false.
So, in your code, if you write pathHelper(X,Y,L) :- \+ member(Y,L), neighbor(X,Y)., this clause will always fail because Y is not sufficiently instantiated. If you swap the two terms, Y will be ground and so member/2 can fail (and \+member succeeds).
Assuming I have some facts like the following
person(jessica,19,usa).
person(james,18,uk).
person(eric,34,italy).
person(jake,24,france).
how can I create a predicate that creates a large list of pairs of all the names and their corresponding country like so:
?-filter(L).
L=[(jessica,usa),(james,uk),(eric,italy),(jake,france)]
The best solution is this one:
?- bagof((P,C), Age^person(P,Age,C), People).
People = [(jessica, usa), (james, uk), (eric, italy), (jake, france)].
This gives you the same result as findall/3, because findall/3 implicitly assumes existential quantification on all variables not present in the template ((P,C) is the template). In your case you only have one, the age variable. Notice what happens if you don't include that:
?- bagof((P,C), person(P,_,C), People).
People = [(james, uk)] ;
People = [(jessica, usa)] ;
People = [(jake, france)] ;
People = [(eric, italy)].
What happened here? The value of the second parameter was the same across each solution because we didn't inform bagof/3 that we didn't care what it was bound to or even if it was bound to just one thing. This property of bagof/3 and setof/3 (but not findall/3) sometimes turns out to be surprisingly useful, so I tend to prefer using bagof/3 over findall/3 if I only need to mark a variable or two.
It's more obvious if we add another person the same age to the database:
person(janet,18,australia).
?- bagof((P,C), person(P,Age,C), People).
Age = 18,
People = [(james, uk), (janet, australia)] .
?- bagof((P,C), person(P,_,C), People).
People = [(james, uk), (janet, australia)] ;
Assuming person/3 is ground and terminates, you can implement it without setof as:
notin(_, []).
notin(X, [Y|Ys]) :-
dif(X,Y),
notin(X,Ys).
lt_list(_, []).
lt_list(X, [Y|Ys]) :-
X #< Y,
lt_list(X,Ys).
f( [ Name-Location | Rest], Acc) :-
person(Name, _, Location),
lt_list( Name-Location, Acc ),
f(Rest, [Name-Location | Acc]).
f( [], Acc) :-
\+ (person(Name,_,Location), notin(Name-Location,Acc)).
When we query f, we get our solutions:
?- f(Xs,[]).
Xs = [jessica-usa, james-uk, jake-france, eric-italy] ;
false.
I used X-Y instead of (X,Y) for better readability. The predicate notin describes an element that is not contained in a list and lt_list describes an element that is smaller than anything in the list by the standard term order.
The idea is that the first rule generates persons that I have not seen yet. Using the term order makes sure that we don't generate all permutations of the list (try replacing lt_list by notin to see what happens). The second rule makes sure we only terminate if there are no more solutions to generate. Be aware that the rule contains negation, which can have some unwanted side-effects. Most of them are filtered out by only looking at ground terms, but I have not thought well, how bad the impact is in this solution.
I'm working on some prolog that I'm new to.
I'm looking for an "or" operator
registered(X, Y), Y=ct101, Y=ct102, Y=ct103.
Here's my query. What I want to write is code that will:
"return X, given that Y is equal to value Z OR value Q OR value P"
I'm asking it to return X if Y is equal to all 3 though. What's the or operator here? Is there one?
Just another viewpoint. Performing an "or" in Prolog can also be done with the "disjunct" operator or semi-colon:
registered(X, Y) :-
X = ct101; X = ct102; X = ct103.
For a fuller explanation:
Predicate control in Prolog
you can 'invoke' alternative bindings on Y this way:
...registered(X, Y), (Y=ct101; Y=ct102; Y=ct103).
Note the parenthesis are required to keep the correct execution control flow. The ;/2 it's the general or operator. For your restricted use you could as well choice the more idiomatic
...registered(X, Y), member(Y, [ct101,ct102,ct103]).
that on backtracking binds Y to each member of the list.
edit I understood with a delay your last requirement. If you want that Y match all 3 values the or is inappropriate, use instead
...registered(X, ct101), registered(X, ct102), registered(X, ct103).
or the more compact
...findall(Y, registered(X, Y), L), sort(L, [ct101,ct102,ct103]).
findall/3 build the list in the very same order that registered/2 succeeds. Then I use sort to ensure the matching.
...setof(Y, registered(X, Y), [ct101,ct102,ct103]).
setof/3 also sorts the result list
Disclaimer: This is informal and non-assessed coursework to do in my own time. I have tried it myself, failed and am now looking for some guidance.
I am trying to implement a version of the member/2 function which will only return members for a list once.
For example:
| ?- member(X, [1,2,3,1]).
X = 1 ? ;
X = 2 ? ;
X = 3 ? ;
X = 1 ? ;
I would like it to only print out each number a maximum of once.
| ?- once_member(X, [1,2,3,1]).
X = 1 ? ;
X = 2 ? ;
X = 3 ? ;
no
We have been told to do this with the cut '!' operator but I have looked over the notes for my course for cut and more online and yet still can't make it click in my head!
So far I have managed to get:
once_member(E, [E | L]) :- !.
once_member(E, [_, L]) :-
once_member(E, L).
Which returns 1 and then nothing else, I feel like my cut is in the wrong place and preventing a backtrack for each possible match but I'm really not sure where to go with it next.
I have looked in my course notes and also at: http://www.cs.ubbcluj.ro/~csatol/log_funk/prolog/slides/5-cuts.pdf and Programming in Prolog (Google Books)
Guidance on how to logically apply the cut would be most useful, but the answer might help me figure that out myself.
We have also been told to do another method which uses '\+' negation by failure but hopefully this may be simpler once cut has twigged for me?
Remove redundant answers and stay pure!
We define memberd/2 based on if_/3 and (=)/3:
memberd(X, [E|Es]) :-
if_(X = E, true, memberd(X, Es)).
Particularly with meta-predicates, a different argument order may come in handy sometimes:
list_memberd(Es, X) :-
memberd(X, Es).
Sample query:
?- memberd(X, [1,2,3,1]).
X = 1 ;
X = 2 ;
X = 3 ;
false.
The solution with cut... at first it sounds quite troublesome.
Assuming that the first argument will be instantiated, a solution is trivial:
once_member(X,L):-
member(X,L),!.
but this will not have the behavior you want if the first arg is not instantiated.
If we know the domain of the lists elements (for example numbers between 1 and 42) we could instantiate the first argument:
once_member(X,L):-
between(1,42,X),
member_(X,L).
member_(X,L):-
member(X,L),!.
but this is veeery inefficient
at this point, I started to believe that it's not possible to do with just a cut (assuming that we dont use + or list_to_set/2
oh wait! < insert idea emoticon here >
If we could implement a predicate (like list_to_set/2 of swi-prolog) that would take a list and produce a list in which all the duplicate elements are removed we could simply use the normal member/2 and don't get duplicate results. Give it a try, I think that you will be able to write it yourself.
--------Spoilers------------
one_member(X,L):-
list_to_set(L,S),
member(X,S).
list_to_set([],[]).
list_to_set([H|T],[H|S]):-
remove_all(H,T,TT),
list_to_set(TT,S).
%remove_all(X,L,S): S is L if we remove all instances of X
remove_all(_,[],[]).
remove_all(X,[X|T],TT):-
remove_all(X,T,TT),!.
remove_all(X,[H|T],[H|TT]):-
remove_all(X,T,TT).
As you see we have to use a cut in remove_all/3 because otherwise the third clause can be matched by remove_all(X,[X|_],_) since we do not specify that H is different from X. I believe that the solution with not is trivial.
Btw, the solution with not could be characterized as more declarative than the solution with cut; the cut we used is typically called a red cut since it alters the behavior of the program. And there are other problems; note that, even with the cut, remove_all(1,[1,2],[1,2]) would succeed.
On the other hand it's not efficient to check twice for a condition. Therefore, the optimal would be to use the if-then-else structure (but I assume that you are not allowed to use it either; its implementation can be done with a cut).
On the other hand, there is another, easier implementation with not: you should not only check if X is member of the list but also if you have encountered it previously; so you will need an accumulator:
-------------Spoilers--------------------
once_member(X,L):-
once_member(X,L,[]).
once_member(X,[X|_T],A):-
\+once_member(X,A).
once_member(X,[H|T],A):-
once_member(X,T,[H|A]).
once_member(X, Xs) :-
sort(Xs, Ys),
member(X, Ys).
Like almost all other solutions posted, this has some anomalies.
?- X = 1, once_member(X, [A,B]).
X = A, A = 1
; X = B, B = 1.
?- X = 1, once_member(X, [A,A]).
X = A, A = 1.
Here's an approach that uses a cut in the definition of once_member/2 together with the classic member/2 predicate:
once_member(X,[H|T]) :-
member(H,T),
!,
once_member(X,T).
once_member(H,[H|_]).
once_member(X,[_|T]) :-
once_member(X,T).
Applied to the example above:
?- once_member(X,[1,2,3,1]).
X = 2 ;
X = 3 ;
X = 1 ;
no
Note: Despite the odd-appearing three clause definition, once_member/2 is last-call/tail-recursive optimization eligible due to the placement of the cut ahead of its first self-invocation.
I have got some values H, and I would like to find the maximum one using \+, how can i do it?
maxValue(X) :-
Get(Id, X),
\+( Get(Id, Y), X < Y ).
don't have a clue....please help, thanks!
Using negation is one way to find the maximum. And it really works.
Here is an example:
p(2).
p(1).
p(3).
?- p(X), \+ (p(Y), Y > X).
X = 3
But the complexity will be O(n*n) where n is
the number of facts. But the maximum can be
determined in O(n). So maybe the following is
more efficient for large fact bases:
:- dynamic(the_max/1).
update_max(X) :-
the_max(Y), X>Y, !, retract(the_max(Y)), assertz(the_max(X)).
update_max(_).
find_max(X) :-
assertz(the_max(0)),
(p(Y), update_max(Y), fail; true),
retract(the_max(X)).
?- find_max(X).
X = 3
But watch out, when you use it from multiple threads,
you need to adapt it a little, i.e. make the_max
thread local.
Best Regards
See also these questions/answers:
Prolog query to find largest element in database?
Max out of values defined by prolog clauses