How to define member (X,L) using concatenating relationship? could anyone explain the method?
concatenate([],L,L).
concatenate([X|L],L2,[X|CL]):-concatenate(L,L2,CL).
It is given concatenating goals
Try the behaviour of concatenate/3 in your Prolog interpreter (the command line, AKA REPL) for this specific query:
?- L=[1,2,3], concatenate(_,[X|_],L).
This pattern it's actually what you need... but the details of adding a predicate member/2 will depend on the actual Prolog implementation.
I'd call it member/2 predicate, not a member function,
it sounds like the C++ term.
Here's concat_member/2:
concat_member( X, Xs ) :- concat( _ , [ X | _ ], Xs ).
Related
I've recently started learning Prolog and I've a question about predicates and functions. How can I write a function which will check if objects in a predicate is in another predicate For instance:
vertex(a).
edge(l,k,-1).
edge(k,l,4).
edge(a,z,-2).
checkEdges(edge(X,Y,_)) :- vertex(X),vertex(Y)
P.S How can I make this function to print a message if elements are not vertices?
Something like
checkEdges(edge(X,Y,_)) :-
( vertex(X), vertex(Y) ->
true
; write('not vertices'),nl ).
?
I have to write a predicate to do work like following:
?- cat(north,south,X).
X = northsouth
?- cat(alley,'91',Y).
X = alley91
?-cat(7,uthah,H).
Bad Input
H = H
Please Help..
atom_concat_redefined(A1, A2, A3) :-
( nonvar(A1) -> atom_chars(A1, Chs1) ; true ),
( nonvar(A2) -> atom_chars(A2, Chs2) ; true ),
( nonvar(A1), nonvar(A2) -> true ; atom_chars(A3, Chs3) ),
append(Chs1, Chs2, Chs3),
atom_chars(A1, Chs1),
atom_chars(A2, Chs2),
atom_chars(A3, Chs3).
This definition produces the same errors in a standard conforming implementation like SICStus or GNU - there should be no other differences, apart from performance. To compare the errors use the goal:
?- catch(atom_concat_redefined(A,B,abc+1), error(E,_), true).
E = type_error(atom,abc+1).
Note the underscore in error(E,_), which hides the implementation defined differences. Implementations provide additional information in this argument, in particular, they would reveal that atom_chars/2 or atom_concat/3 produced the error.
atom_codes/2 it's the ISO approved predicate to convert between an atom and a list of codes. When you have 2 lists corresponding to first two arguments, append/3 (alas, not ISO approved, but AFAIK available in every Prolog), will get the list corresponding to third argument, then, convert that list to atom...
Note that, while append/3 is a 'pure' Prolog predicate, and can work with any instantiation pattern, atom_codes/2 requires at least one of it's argument instantiated. Here is a SWI-Prolog implementation of cat/3, 'working' a bit more generally. I hope it will inspire you to read more about Prolog...
ac(X,Xs) :- when((ground(X);ground(Xs)), atom_codes(X,Xs)).
cat(X,Y,Z) :- maplist(ac, [X,Y,Z],[Xs,Ys,Zs]), append(Xs,Ys,Zs).
edit
as noted by #false I was wrong about append/3. Now I'll try to understand better what append/3 does... wow, a so simple predicate, so behaviour rich!
Given this program, why am I forced to define every atom in the predicate, even if they're anonymous. Why is it that undefined variables in a dict predicate aren't thought of as anonymous?
funt2(X) :-
X = point{x:5, y:6}.
evalfunt(point{x:5, y : 6}) :-
write('hello world!').
evalfunt(point{x:_, y : _} ) :-
write('GoodBye world!').
Why can't I just say
evalfunt(point{x:5}) :-
write('GoodBye world!').
^that won't match, by the way.
I may as well just use a structure if I have to define every possible value in the dict to use dicts.
What's the motivation here? Can I do something to make my predicate terse? I'm trying to define a dict with 30 variables and this is a huge roadblock. It's going to increase my program size by a magnitude if I'm forced to define each variables (anonymous or not).
Dict is just a complex data type, like tuple, which has data AND structure. If you have, for example two facts:
fact(point{x:5, y:6}).
fact(point{x:5}).
Then the query
fact(point{x:_}).
will match the second one, but not the first one.
And the query
fact(point{x:_, y:_}).
Will match the first one, but not the second.
Now, if you want to match facts of the form fact(point{x:_, y:_, z:_}) only by one specific field, you can always write a helper rule:
matchByX(X, P) :- fact(P), P=point{x:X, y:_, z:_}.
So having facts:
fact(point{x:5, y:6, z:1}).
fact(point{x:1, y:2, z:3}).
fact(point{x:2, y:65, z:4}).
and quering
matchByX(1, P).
will return:
P = point{x:1, y:2, z:3}
UPDATE:
Moreover, in SWI-Prolog 7 version the field names can be matched as well, so it can be written in much more generic way, even for facts with different structures:
fact(point{x:5, y:6, z:1}).
fact(point{x:1, y:2}).
fact(point{x:2}).
fact(point{x:2, y:2}).
matchByField(F, X, P) :- fact(P), P.F = X.
So query:
?- matchByField(x, 2, P).
P = point{x:2} ;
P = point{x:2, y:2}.
I was able to accomplish what I needed by doing the following
checkiffive(Y) :-
get_dict(x, Y, V), V=5.
You need to use the built in methods for unifying values from a dict.
Described in chapter 5.4 of the SWI prolog reference
http://www.swi-prolog.org/download/devel/doc/SWI-Prolog-7.1.16.pdf
I'd like to know why I get an error with my SWI Prolog when I try to do this:
(signal(X) = signal(Y)) :- (terminal(X), terminal(Y), connected(X,Y)).
terminal(X) :- ((signal(X) = 1);(signal(X) = 0)).
I get the following error
Error: trabalho.pro:13: No permission to modify static procedure
'(=)/2'
It doesn't recognize the "=" in the first line, but the second one "compiles". I guess it only accepts the "=" after the :- ? Why?
Will I need to create a predicate like: "equal(x,y) :- (x = y)" for this?
Diedre - there are no 'functions' in Prolog. There are predicates. The usual pattern
goes
name(list of args to be unified) :- body of predicate .
Usually you'd want the thing on the left side of the :- operator to be a predicate
name. when you write
(signal(X) = signal(Y))
= is an operator, so you get
'='(signal(X), signal(Y))
But (we assume, it's not clear what you're doing here) that you don't really want to change equals.
Since '=' is already in the standard library, you can't redefine it (and wouldn't want to)
What you probably want is
equal_signal(X, Y) :- ... bunch of stuff... .
or
equal_signal(signal(X), signal(Y)) :- ... bunch of stuff ... .
This seems like a conceptual error problem. You need to have a conversation with somebody who understands it. I might humbly suggest you pop onto ##prolog on freenode.net or
some similar forum and get somebody to explain it.
Because = is a predefined predicate. What you actually write is (the grounding of terms using the Martelli-Montanari algorithm):
=(signal(X),signal(Y)) :- Foo.
You use predicates like functions in Prolog.
You can define something like:
terminal(X) :- signal(X,1);signal(X,0).
where signal/2 is a predicate that contains a key/value pair.
And:
equal_signal(X,Y) :- terminal(X),terminal(Y),connected(X,Y).
I have some basic formulas like
female(camilla).
female(diana).
...
parent(person, child).
...
and predicate language/2 that specifies which predicates will I be using:
langugage(female, 1).
language(parent, 2).
What I have to do is to create predicate called body_lit/1 that returns whole clauses specified in language like this:
?- body_lit(X).
X = parent(charles, harry) ;
X = parent(diana, harry) ;
...
X = female(camilla) ;
X = female(diana) ;
...
I know I need to use call/n function and functor/3. I know how functor/3 works but I cant seem to figure out how to return the whole clause instead of just the name of the predicate.
I presume that body_lit is supposed to return all answers to predicates given in language with their arities.
body_lit(X) :- language(N,A), functor(F,N,A), call(F), X=F.
language/2 selects a predicate, functor/3 constructs an actual call from the predicate name and its arity, call/1 executes the call, and finally X=F records the call as the output argument. The last unification could have been removed:
body_lit(X) :- language(N,A), functor(X,N,A), call(X).