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I'm new in Prolog and trying to do some programming with Lists
I want to do this :
?- count_occurrences([a,b,c,a,b,c,d], X).
X = [[d, 1], [c, 2], [b, 2], [a, 2]].
and this is my code I know it's not complete but I'm trying:
count_occurrences([],[]).
count_occurrences([X|Y],A):-
occurrences([X|Y],X,N).
occurrences([],_,0).
occurrences([X|Y],X,N):- occurrences(Y,X,W), N is W + 1.
occurrences([X|Y],Z,N):- occurrences(Y,Z,N), X\=Z.
My code is wrong so i need some hits or help plz..
Here's my solution using bagof/3 and findall/3:
count_occurrences(List, Occ):-
findall([X,L], (bagof(true,member(X,List),Xs), length(Xs,L)), Occ).
An example
?- count_occurrences([a,b,c,b,e,d,a,b,a], Occ).
Occ = [[a, 3], [b, 3], [c, 1], [d, 1], [e, 1]].
How it works
bagof(true,member(X,List),Xs) is satisfied for each distinct element of the list X with Xs being a list with its length equal to the number of occurrences of X in List:
?- bagof(true,member(X,[a,b,c,b,e,d,a,b,a]),Xs).
X = a,
Xs = [true, true, true] ;
X = b,
Xs = [true, true, true] ;
X = c,
Xs = [true] ;
X = d,
Xs = [true] ;
X = e,
Xs = [true].
The outer findall/3 collects element X and the length of the associated list Xs in a list that represents the solution.
Edit I: the original answer was improved thanks to suggestions from CapelliC and Boris.
Edit II: setof/3 can be used instead of findall/3 if there are free variables in the given list. The problem with setof/3 is that for an empty list it will fail, hence a special clause must be introduced.
count_occurrences([],[]).
count_occurrences(List, Occ):-
setof([X,L], Xs^(bagof(a,member(X,List),Xs), length(Xs,L)), Occ).
Note that so far all proposals have difficulties with lists that contain also variables. Think of the case:
?- count_occurrences([a,X], D).
There should be two different answers.
X = a, D = [a-2]
; dif(X, a), D = [a-1,X-1].
The first answer means: the list [a,a] contains a twice, and thus D = [a-2]. The second answer covers all terms X that are different to a, for those, we have one occurrence of a and one occurrence of that other term. Note that this second answer includes an infinity of possible solutions including X = b or X = c or whatever else you wish.
And if an implementation is unable to produce these answers, an instantiation error should protect the programmer from further damage. Something along:
count_occurrences(Xs, D) :-
( ground(Xs) -> true ; throw(error(instantiation_error,_)) ),
... .
Ideally, a Prolog predicate is defined as a pure relation, like this one. But often, pure definitions are quite inefficient.
Here is a version that is pure and efficient. Efficient in the sense that it does not leave open any unnecessary choice points. I took #dasblinkenlight's definition as source of inspiration.
Ideally, such definitions use some form of if-then-else. However, the traditional (;)/2 written
( If_0 -> Then_0 ; Else_0 )
is an inherently non-monotonic construct. I will use a monotonic counterpart
if_( If_1, Then_0, Else_0)
instead. The major difference is the condition. The traditional control constructs relies upon the success or failure of If_0 which destroys all purity. If you write ( X = Y -> Then_0 ; Else_0 ) the variables X and Y are unified and at that very point in time the final decision is made whether to go for Then_0 or Else_0. What, if the variables are not sufficiently instantiated? Well, then we have bad luck and get some random result by insisting on Then_0 only.
Contrast this to if_( If_1, Then_0, Else_0). Here, the first argument must be some goal that will describe in its last argument whether Then_0 or Else_0 is the case. And should the goal be undecided, it can opt for both.
count_occurrences(Xs, D) :-
foldl(el_dict, Xs, [], D).
el_dict(K, [], [K-1]).
el_dict(K, [KV0|KVs0], [KV|KVs]) :-
KV0 = K0-V0,
if_( K = K0,
( KV = K-V1, V1 is V0+1, KVs0 = KVs ),
( KV = KV0, el_dict(K, KVs0, KVs ) ) ).
=(X, Y, R) :-
equal_truth(X, Y, R).
This definition requires the following auxiliary definitions:
if_/3, equal_truth/3, foldl/4.
If you use SWI-Prolog, you can do :
:- use_module(library(lambda)).
count_occurrences(L, R) :-
foldl(\X^Y^Z^(member([X,N], Y)
-> N1 is N+1,
select([X,N], Y, [X,N1], Z)
; Z = [[X,1] | Y]),
L, [], R).
One thing that should make solving the problem easier would be to design a helper predicate to increment the count.
Imagine a predicate that takes a list of pairs [SomeAtom,Count] and an atom whose count needs to be incremented, and produces a list that has the incremented count, or [SomeAtom,1] for the first occurrence of the atom. This predicate is easy to design:
increment([], E, [[E,1]]).
increment([[H,C]|T], H, [[H,CplusOne]|T]) :-
CplusOne is C + 1.
increment([[H,C]|T], E, [[H,C]|R]) :-
H \= E,
increment(T, E, R).
The first clause serves as the base case, when we add the first occurrence. The second clause serves as another base case when the head element matches the desired element. The last case is the recursive call for the situation when the head element does not match the desired element.
With this predicate in hand, writing count_occ becomes really easy:
count_occ([], []).
count_occ([H|T], R) :-
count_occ(T, Temp),
increment(Temp, H, R).
This is Prolog's run-of-the-mill recursive predicate, with a trivial base clause and a recursive call that processes the tail, and then uses increment to account for the head element of the list.
Demo.
You have gotten answers. Prolog is a language which often offers multiple "correct" ways to approach a problem. It is not clear from your answer if you insist on any sort of order in your answers. So, ignoring order, one way to do it would be:
Sort the list using a stable sort (one that does not drop duplicates)
Apply a run-length encoding on the sorted list
The main virtue of this approach is that it deconstructs your problem to two well-defined (and solved) sub-problems.
The first is easy: msort(List, Sorted)
The second one is a bit more involved, but still straight forward if you want the predicate to only work one way, that is, List --> Encoding. One possibility (quite explicit):
list_to_rle([], []).
list_to_rle([X|Xs], RLE) :-
list_to_rle_1(Xs, [[X, 1]], RLE).
list_to_rle_1([], RLE, RLE).
list_to_rle_1([X|Xs], [[Y, N]|Rest], RLE) :-
( dif(X, Y)
-> list_to_rle_1(Xs, [[X, 1],[Y, N]|Rest], RLE)
; succ(N, N1),
list_to_rle_1(Xs, [[X, N1]|Rest], RLE)
).
So now, from the top level:
?- msort([a,b,c,a,b,c,d], Sorted), list_to_rle(Sorted, RLE).
Sorted = [a, a, b, b, c, c, d],
RLE = [[d, 1], [c, 2], [b, 2], [a, 2]].
On a side note, it is almost always better to prefer "pairs", as in X-N, instead of lists with two elements exactly, as in [X, N]. Furthermore, you should keep the original order of the elements in the list, if you want to be correct. From this answer:
rle([], []).
rle([First|Rest],Encoded):-
rle_1(Rest, First, 1, Encoded).
rle_1([], Last, N, [Last-N]).
rle_1([H|T], Prev, N, Encoded) :-
( dif(H, Prev)
-> Encoded = [Prev-N|Rest],
rle_1(T, H, 1, Rest)
; succ(N, N1),
rle_1(T, H, N1, Encoded)
).
Why is it better?
we got rid of 4 pairs of unnecessary brackets in the code
we got rid of clutter in the reported solution
we got rid of a whole lot of unnecessary nested terms: compare .(a, .(1, [])) to -(a, 1)
we made the intention of the program clearer to the reader (this is the conventional way to represent pairs in Prolog)
From the top level:
?- msort([a,b,c,a,b,c,d], Sorted), rle(Sorted, RLE).
Sorted = [a, a, b, b, c, c, d],
RLE = [a-2, b-2, c-2, d-1].
The presented run-length encoder is very explicit in its definition, which has of course its pros and cons. See this answer for a much more succinct way of doing it.
refining joel76 answer:
count_occurrences(L, R) :-
foldl(\X^Y^Z^(select([X,N], Y, [X,N1], Z)
-> N1 is N+1
; Z = [[X,1] | Y]),
L, [], R).
Hello is there any way to separate a list in Prolog into two other lists, the first includes everything before an element and the second everything after the element. For example
A=[1,2,3,5,7,9,0] and element=5
the two lists should be
A1=[1,2,3] and A2=[7,9,0]
I don't care about finding the element just what to do next
it's easy as
?- Elem = 5, A = [1,2,3,5,7,9,0], append(A1, [Elem|A2], A).
edit to explain a bit...
append/3 it's a relation among 3 lists.
It's general enough to solve any concatenation on proper lists - when not there are circular arguments.
The comparison it's a plain unification, that take place on second argument. That must be a list beginning with Elem. Prolog list constructor syntax is [Head|Tail]. To make unification succeed, Elem must match the Head.
Here's an alternative method, illustrating how to handle it with list recursion:
split([E|T], E, [], T).
split([X|T], E, [X|LL], LR) :-
X \== E,
split(T, E, LL, LR).
Or better, if your Prolog supports dif/2:
split([E|T], E, [], T).
split([X|T], E, [X|LL], LR) :-
dif(X, E),
split(T, E, LL, LR).
Examples:
| ?- split([1,2,3,4,5], 3, L, R).
L = [1,2]
R = [4,5] ? ;
no
| ?- split([1,2,3,4,5], 5, L, R).
L = [1,2,3,4]
R = [] ? ;
(1 ms) no
| ?- split([1,2,3,4,5], 1, L, R).
L = []
R = [2,3,4,5] ? ;
no
| ?-
It is a sort of specialized twist on append/3 as CapelliC showed.
Well, for the last few hours, I've been trying to swap the second item of a given list with its penultimate item (the second last). Give the list [a,b,c,d,e,f], I want to get [a,e,c,d,b,f]. For example:
correct(List1,X,List2)
?-correct([a,y,b,c,d,e,x,f],x,List2).
List2[a,x,b,c,d,e,y,f].
List1 is the list i got to swap second and penultimate (second last) element.
X is the penultimate element.
List2 is the new list with the swapped elements.
The solutions posted by mbratch and CapelliC both fail for the following base case:
?- correct([a,y], X, List2).
false.
The following solution takes care of this base case and doesn't rely on list predicates that may or may not be available. It traverses the list once and is more efficient than the other two solutions:
correct([PreLast, Second], Second, [Second, PreLast]) :-
!.
correct([First, Second, Last], Second, [First, Second, Last]) :-
!.
correct([First, Second| InRest], PreLast, [First, PreLast| OutRest]) :-
correct_aux(InRest, Second, PreLast, OutRest).
correct_aux([PreLast, Last], Second, PreLast, [Second, Last]) :-
!.
correct_aux([Other| InRest], Second, PreLast, [Other| OutRest]) :-
correct_aux(InRest, Second, PreLast, OutRest).
Sample queries:
?- correct([a,b], X, List).
X = b,
List = [b, a].
?- correct([a,b,c], X, List).
X = b,
List = [a, b, c].
?- correct([a,b,c,d], X, List).
X = c,
List = [a, c, b, d].
?- correct([a,b,c,d,e], X, List).
X = d,
List = [a, d, c, b, e].
This will work for lists of length 4 or greater:
correct( [H1|[H2|T1]], X, [H1|[X|T2]] ) :-
reverse(T1, [HR|[X|TR]]),
reverse([HR|[H2|TR]], T2).
| ?- correct( [1,2,3,4,5,6], X, L ).
L = [1,5,3,4,2,6]
X = 5
(1 ms) yes
| ?-
You can include the shorter cases, if that's the intention, by adding two more predicates, bringing the solution to:
correct( [A,X], X, [X,A] ).
correct( [A,X,B], X, [A,X,B] ).
correct( [H1|[H2|T1]], X, [H1|[X|T2]] ) :-
reverse(T1, [HR|[X|TR]]),
reverse([HR|[H2|TR]], T2).
another available builtin is append/2:
3 ?- [user].
correct(L, X, R) :- append([[A,B],C,[X,E]], L), append([[A,X],C,[B,E]], R).
|:
% user://2 compiled 0.02 sec, 2 clauses
true.
4 ?- correct( [1,2,3,4,5,6], X, L ).
X = 5,
L = [1, 5, 3, 4, 2, 6] ;
I like mbratch one (+1), maybe this solution is more intuitive.
I have these two programs and they're not working as they should. The first without_doubles_2(Xs, Ys)is supposed to show that it is true if Ys is the list of the elements appearing in Xs without duplication. The elements in Ys are in the reversed order of Xs with the first duplicate values being kept. Such as, without_doubles_2([1,2,3,4,5,6,4,4],X) prints X=[6,5,4,3,2,1] yet, it prints false.
without_doubles_2([],[]).
without_doubles_2([H|T],[H|Y]):- member(H,T),!,
delete(H,T,T1),
without_doubles_2(T1,Y).
without_doubles_2([H|T],[H|Y]):- without_doubles_2(T,Y).
reverse([],[]).
reverse([H|T],Y):- reverse(T,T1), addtoend(H,T1,Y).
addtoend(H,[],[H]).
addtoend(X,[H|T],[H|T1]):-addtoend(X,T,T1).
without_doubles_21(X,Z):- without_doubles_2(X,Y),
reverse(Y,Z).
The second one is how do I make this program use a string? It's supposed to delete the vowels from a string and print only the consonants.
deleteV([H|T],R):-member(H,[a,e,i,o,u]),deleteV(T,R),!.
deleteV([H|T],[H|R]):-deleteV(T,R),!.
deleteV([],[]).
Your call to delete always fails because you have the order of arguments wrong:
delete(+List1, #Elem, -List2)
So instead of
delete(H, T, T1)
You want
delete(T, H, T1)
Finding an error like this is simple using the trace functionality of the swi-prolog interpreter - just enter trace. to begin trace mode, enter the predicate, and see what the interpreter is doing. In this case you would have seen that the fail comes from the delete statement. The documentation related to tracing can be found here.
Also note that you can rewrite the predicate omitting the member check and thus the third clause, because delete([1,2,3],9001,[1,2,3]) evaluates to true - if the element is not in the list the result is the same as the input. So your predicate could look like this (name shortened due to lazyness):
nodubs([], []).
nodubs([H|T], [H|Y]) :- delete(T, H, T1), nodubs(T1, Y).
For your second question, you can turn a string into a list of characters (represented as ascii codes) using the string_to_list predicate.
As for the predicate deleting vovels from the string, I would implement it like this (there's probably better solutions for this problem or some built-ins you could use but my prolog is somewhat rusty):
%deleteall(+L, +Elems, -R)
%a helper predicate for deleting all items in Elems from L
deleteall(L, [], L).
deleteall(L, [H|T], R) :- delete(L, H, L1), deleteall(L1, T, R).
deleteV(S, R) :-
string_to_list(S, L), %create list L from input string
string_to_list("aeiou", A), %create a list of all vovels
deleteall(L, A, RL), %use deleteall to delete all vovels from L
string_to_list(R, RL). %turn the result back into a string
deleteV/2 could make use of library(lists):
?- subtract("carlo","aeiou",L), format('~s',[L]).
crl
L = [99, 114, 108].
while to remove duplicates we could take advantage from sort/2 and select/3:
nodup(L, N) :-
sort(L, S),
nodup(L, S, N).
nodup([], _S, []).
nodup([X|Xs], S, N) :-
( select(X, S, R) -> N = [X|Ys] ; N = Ys, R = S ),
nodup(Xs, R, Ys).
test:
?- nodup([1,2,3,4,4,4,5,2,7],L).
L = [1, 2, 3, 4, 5, 7].
edit much better, from ssBarBee
?- setof(X,member(X,[1,2,2,5,3,2]),L).
L = [1, 2, 3, 5].
I'm having problem constructing a list of lists in my prolog program.
I have a predicate which gives me back a single case of a row. I have to group all the cases of this row and transform them into a list of lists. I can access them just fine but when I exit, all I'll get is the first element.
Here's the code:
sudoku3to2 :- s3to2(1).
s3to2(Line) :-
Line < 9,
Line1 is Line+1,
s3getLine(Line,0,[L]),
assert(sudoku2(Y,L])),
s3to2(Line1).
s3to2(9).
s3getLine(Line,X, , ) :-
X < 9,
X1 is X + 1,
sudoku3(Line,X, ),
s3getLine(Line,X1, , ).
s3getLine(Line,9,L,L).
sudoku3/3 will return the element at the X,Y coordinate. When I get to s3getLine(Line,9,L,L) I'll start going back. I want to keep all the elements I've gathered and not just the first one. And I'm really having trouble constructing the proper predicate calls.
findall/3 is the 'list constructor' more easily understood.
It's a builtin that list all solutions found, shaping the elements with a specified pattern. Here the pattern is really just the variable we are interested to.
I use between/3 to obtaing a correctly ordered matrix, without regard to sudoku3 rules order.
sudoku3(1, 1, a).
sudoku3(1, 2, b).
sudoku3(2, 1, c).
sudoku3(2, 2, d).
mat(M) :-
W = 2,
findall(Row,
(between(1, W, R),
findall(V, (between(1, W, C), sudoku3(R, C, V)), Row)
), M).
Result:
?- mat(M).
M = [[a, b], [c, d]].
You should change W=9.
HTH