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Im writing prolog to print out each element of a list, as long as a counter is > 0.
printLN([],_).
printLN(_,-1).
printLN([[x,y] | T], N) :-
write(x), write(y), nl,
N2 is N-1,
printLN(T, N2).
This should print the 3 pairs.
printLN([[1,2],[3,4],[5,6]], 2).
This should print the 3 pairs.
printLN([[1,2],[3,4],[5,6]], 8).
This should print first pair.
printLN([[1,2],[3,4],[5,6]], 0).
I keep getting false though. Anyone know why this may be the case?
As well, how could i execute something like:
getPartialList(N) :-
getList(LIST), printLN(LIST, N).
where getList returns a list of pairs like:
getList([[1,2],[3,4],[5,6]]).
I keep getting false though. Anyone know why this may be the case?
Two errors:
a) write(x) should be written write(X) where X is a logical variable (in capitals).
b) Your third clause keeps Calling recursively even if N < 0. To correct this, put a test N > 0 into this clause.
printLN([],_).
printLN(_, -1) :- !.
printLN([[X,Y] | T], N) :-
N > 0,
write(X), write(Y), nl,
N2 is N-1,
printLN(T, N2).
I want to write predicate which can count all encountered number:
count(1, [1,0,0,1,0], X).
X = 2.
I tried to write it like:
count(_, [], 0).
count(Num, [H|T], X) :- count(Num, T, X1), Num = H, X is X1 + 1.
Why doesn't work it?
Why doesn't work it?
Prolog is a programming language that often can answer such question directly. Look how I tried out your definition starting with your failing query:
?- count(1, [1,0,0,1,0], X).
false.
?- count(1, Xs, X).
Xs = [], X = 0
; Xs = [1], X = 1
; Xs = [1,1], X = 2
; Xs = [1,1,1], X = 3
; ... .
?- Xs = [_,_,_], count(1, Xs, X).
Xs = [1,1,1], X = 3.
So first I realized that the query does not work at all, then I generalized the query. I replaced the big list by a variable Xs and said: Prolog, fill in the blanks for me! And Prolog did this and reveals us precisely the cases when it will succeed.
In fact, it only succeeds with lists of 1s only. That is odd. Your definition is too restricted - it correctly counts the 1s in lists where there are only ones, but all other lists are rejected. #coder showed you how to extend your definition.
Here is another one using library(reif) for
SICStus|SWI. Alternatively, see tfilter/3.
count(X, Xs, N) :-
tfilter(=(X), Xs, Ys),
length(Ys, N).
A definition more in the style of the other definitions:
count(_, [], 0).
count(E, [X|Xs], N0) :-
if_(E = X, C = 1, C = 0),
count(E, Xs, N1),
N0 is N1+C.
And now for some more general uses:
How does a four element list look like that has 3 times a 1 in it?
?- length(L, 4), count(1, L, 3).
L = [1,1,1,_A], dif(1,_A)
; L = [1,1,_A,1], dif(1,_A)
; L = [1,_A,1,1], dif(1,_A)
; L = [_A,1,1,1], dif(1,_A)
; false.
So the remaining element must be something different from 1.
That's the fine generality Prolog offers us.
The problem is that as stated by #lurker if condition (or better unification) fails then the predicate will fail. You could make another clause for this purpose, using dif/2 which is pure and defined in the iso:
count(_, [], 0).
count(Num, [H|T], X) :- dif(Num,H), count(Num, T, X).
count(Num, [H|T], X) :- Num = H, count(Num, T, X1), X is X1 + 1.
The above is not the most efficient solution since it leaves many choice points but it is a quick and correct solution.
You simply let the predicate fail at the unification Num = X. Basically, it's like you don't accept terms which are different from the only one you are counting.
I propose to you this simple solution which uses tail recursion and scans the list in linear time. Despite the length, it's very efficient and elegant, it exploits declarative programming techniques and the backtracking of the Prolog engine.
count(C, L, R) :-
count(C, L, 0, R).
count(_, [], Acc, Acc).
count(C, [C|Xr], Acc, R) :-
IncAcc is Acc + 1,
count(C, Xr, IncAcc, R).
count(C, [X|Xr], Acc, R) :-
dif(X, C),
count(C, Xr, Acc, R).
count/3 is the launcher predicate. It takes the term to count, the list and gives to you the result value.
The first count/4 is the basic case of the recursion.
The second count/4 is executed when the head of the list is unified with the term you are looking for.
The third count/4 is reached upon backtracking: If the term doesn’t match, the unification fails, you won't need to increment the accumulator.
Acc allows you to scan the entire list propagating the partial result of the recursive processing. At the end you simply have to return it.
I solved it myself:
count(_, [], 0).
count(Num, [H|T], X) :- Num \= H, count(Num, T, X).
count(Num, [H|T], X) :- Num = H, count(Num, T, X1), X is X1 + 1.
I have decided to add my solution to the list here.
Other solutions here use either explicit unification/failure to unify, or libraries/other functions, but mine uses cuts and implicit unification instead. Note my solution is similar to Ilario's solution but simplifies this using cuts.
count(_, [], 0) :- !.
count(Value, [Value|Tail],Occurrences) :- !,
count(Value,Tail,TailOcc),
Occurrences is TailOcc+1.
count(Value, [_|Tail], Occurrences) :- count(Value,Tail,Occurrences).
How does this work? And how did you code it?
It is often useful to equate solving a problem like this to solving a proof by induction, with a base case, and then a inductive step which shows how to reduce the problem down.
Line 1 - base case
Line 1 (count(_, [], 0) :- !.) handles the "base case".
As we are working on a list, and have to look at each element, the simplest case is zero elements ([]). Therefore, we want a list with zero elements to have no instances of the Value we are looking for.
Note I have replaced Value in the final code with _ - this is because we do not care what value we are looking for if there are no values in the list anyway! Therefore, to avoid a singleton variable we negate it here.
I also added a ! (a cut) after this - as there is only one correct value for the number of occurrences we do not want Prolog to backtrack and fail - therefore we tell Prolog we found the correct value by adding this cut.
Lines 2/3 - inductive step
Lines 2 and 3 handle the "inductive step". This should handle if we have one or more elements in the list we are given. In Prolog we can only directly look at the head of the list, therefore let us look at one element at a time. Therefore, we have two cases - either the value at the head of the list is the Value we are looking for, or it is not.
Line 2
Line 2 (count(Value, [Value|Tail],Occurrences) :- !, count(Value,Tail,TailOcc), Occurrences is TailOcc+1.) handles if the head of our list and the value we are looking for match. Therefore, we simply use the same variable name so Prolog will unify them.
A cut is used as the first step in our solution (which makes each case mutually exclusive, and makes our solution last-call-optimised, by telling Prolog not to try any other rules).
Then, we find out how many instances of our term there are in the rest of the list (call it TailOcc). We don't know how many terms there are in the list we have at the moment, but we know it is one more than there are in the rest of the list (as we have a match).
Once we know how many instances there are in the rest of the list (call this Tail), we can take this value and add 1 to it, then return this as the last value in our count function (call this Occurences).
Line 3
Line 3 (count(Value, [_|Tail], Occurrences) :- count(Value,Tail,Occurrences).) handles if the head of our list and the value we are looking for do not match.
As we used a cut in line 2, this line will only be tried if line 2 fails (i.e. there is no match).
We simply take the number of instances in the rest of the list (the tail) and return this same value without editing it.
Basically I want to remove the first N numbers from a list, the function that checks whether a number is prime or not seems to work well but the program itself doesn't
For example for input [2,4,5,7,6,9,11] and N = 3 I should get [4, 6, 9, 11] but I get only [4, 6, 9].
divisible(X,Y) :-
0 is X mod Y, !.
divisible(X,Y) :-
X > Y+1,
divisible(X, Y+1).
%isPrime function check whether or not the argument is a prime number
isPrime(2) :- true,!.
isPrime(X) :- X < 2,!,false.
isPrime(X) :- not(divisible(X, 2)).
%delFunction (input_list, N, output_list)
delFunction([],_,_).
delFunction(_,0,_).
delFunction([H|T], N, [H|Res]):-
not(isPrime(H)), !,
delFunction(T, N, Res).
delFunction([_|T], N, Res):-
N1 is N-1,
delFunction(T,N1,Res).
delFunction([2,4,5,7,6,9,11],3,X) -> [4,6,9] (which is not the correct answer)
Honestly, I don't know where I went wrong, the implementation idea seems pretty easy and straight forward, so is the code.
Also, when I run it it stops at [4] and I have to keep on clicking next to get me to the end of the execution (thus the result). Any idea how to fix it? I'm thinking maybe I need some cuts but not sure where.
P.S: I'd rather not use built-in functions (if there are any that would help in this scenario)
Fists of all, instead of
delFunction([],_,_).
you should write
delFunction([],_,[]).
because when the input list (the left one) is empty, you have to construct the base for the output list: an empty list; with delFunction([], _, _) you don't unify the output list with the empty list so the result end with an not unified variable.
Second. Instead of
delFunction(_,0,_).
you should write
delFunction(L,0,L).
The problem is the same: when number is zero you can "copy" the input in the output; that is, you can unify they; that is delFunction(L,0,L). With delFunction(_,0,_) you don't unify and the result ends with a not unified variable.
Third. In the is-prime clause
delFunction([_|T], N, Res):-
N1 is N-1,
delFunction(T,N1,Res).
you should check that N is greater than zero
delFunction([_|T], N, Res):-
N > 0,
N1 is N-1,
delFunction(T,N1,Res).
or, if you prefer, you should add a cut (!) in the zero clause
delFunction(L,0,L) :- !.
Just to avoid that Prolog, via backtracking, can give multiple undesired answers.
--- EDIT ---
As Guy Coder, I avoid cuts like a plague; so I propose the following solution to avoid unwanted repetitions.
delFunction([], _, []).
delFunction([H | T], 0, [H | T]).
delFunction([H | T], N, [H | Res]) :-
N > 0,
not(isPrime(H)),
delFunction(T, N, Res).
delFunction([H | T], N, Res):-
N > 0,
isPrime(H),
N1 is N-1,
delFunction(T, N1, Res).
Here is my code.
equals2(X,Y,N,I):- X is Y,I is N+1; I is N.
elemNum(X,[],0).
elemNum(X,[Y|Ys],N) :- elemNum(X,Ys,N1),equals2(X,Y,N1,I),N is I.
lemNum first argument is element from array, second is array. It counts the number of elements in array.
Then in console
| ?- elemNum(1,[1,2,3,1,1],N),N<2.
N = 1 ?
yes
I am sure than my function elemNum works just fine. How its possible that in console this assertion returns 1?
Thanks for help
Non sure to understand what do you want ... but I suppose that you want count the number of element in the list (second argument of elemNum/3) that are equals to the first argument.
If so, you should modify equals2/4 as follows
equals2(X,Y,N,I):- X is Y,I is N+1; X \== Y, I is N.
or better (IMHO) split it in 2 different clauses
equals2(X,X,N,I):- I is N+1.
equals2(X,Y,N,N):- X \== Y.
With your equal2/4, the second or case (I is N) is executed (in backtracking) even when X is equal to Y so elemNum(1,[1,2,3,1,1],N) unifiy N with 3, 2, 2 again, 1, 2, 1, 1 again and 0.
Regarding elemNum/3, works but you can semplify it (avoiding a warning) as
elemNum(_,[],0).
elemNum(X,[Y|Ys],I) :- elemNum(X,Ys,N1), equals2(X,Y,N1,I).
or you can rewrite it, avoiding the use of equals2/4 as
elemNum(_, [], 0).
elemNum(X, [X | Ys], I) :- elemNum(X, Ys, I0), I is I0+1.
elemNum(X, [Y | Ys], I) :- X \== Y, elemNum(X, Ys, I).
I was just introduced to Prolog and am trying to write a predicate that finds the Max value of a list of integers. I need to write one that compares from the beginning and the other that compares from the end. So far, I have:
max2([],R).
max2([X|Xs], R):- X > R, max2(Xs, X).
max2([X|Xs], R):- X <= R, max2(Xs, R).
I realize that R hasn't been initiated yet, so it's unable to make the comparison. Do i need 3 arguments in order to complete this?
my_max([], R, R). %end
my_max([X|Xs], WK, R):- X > WK, my_max(Xs, X, R). %WK is Carry about
my_max([X|Xs], WK, R):- X =< WK, my_max(Xs, WK, R).
my_max([X|Xs], R):- my_max(Xs, X, R). %start
other way
%max of list
max_l([X],X) :- !, true.
%max_l([X],X). %unuse cut
%max_l([X],X):- false.
max_l([X|Xs], M):- max_l(Xs, M), M >= X.
max_l([X|Xs], X):- max_l(Xs, M), X > M.
Ignoring the homework constraints about starting from the beginning or the end, the proper way to implement a predicate that gets the numeric maximum is as follows:
list_max([P|T], O) :- list_max(T, P, O).
list_max([], P, P).
list_max([H|T], P, O) :-
( H > P
-> list_max(T, H, O)
; list_max(T, P, O)).
A very simple approach (which starts from the beginning) is the following:
maxlist([],0).
maxlist([Head|Tail],Max) :-
maxlist(Tail,TailMax),
Head > TailMax,
Max is Head.
maxlist([Head|Tail],Max) :-
maxlist(Tail,TailMax),
Head =< TailMax,
Max is TailMax.
As you said, you must have the variables instantiated if you want to evaluate an arithmetic expression. To solve this, first you have to make the recursive call, and then you compare.
Hope it helps!
As an alternative to BLUEPIXY' answer, SWI-Prolog has a builtin predicate, max_list/2, that does the search for you. You could also consider a slower method, IMO useful to gain familiarity with more builtins and nondeterminism (and then backtracking):
slow_max(L, Max) :-
select(Max, L, Rest), \+ (member(E, Rest), E > Max).
yields
2 ?- slow_max([1,2,3,4,5,6,10,7,8],X).
X = 10 ;
false.
3 ?- slow_max([1,2,10,3,4,5,6,10,7,8],X).
X = 10 ;
X = 10 ;
false.
edit
Note you don't strictly need three arguments, but just to have properly instantiated variables to carry out the comparison. Then you can 'reverse' the flow of values:
max2([R], R).
max2([X|Xs], R):- max2(Xs, T), (X > T -> R = X ; R = T).
again, this is slower than the three arguments loops, suggested in other answers, because it will defeat 'tail recursion optimization'. Also, it does just find one of the maxima:
2 ?- max2([1,2,3,10,5,10,6],X).
X = 10 ;
false.
Here's how to do it with lambda expressions and meta-predicate foldl/4, and, optionally, clpfd:
:- use_module([library(lambda),library(apply),library(clpfd)]).
numbers_max([Z|Zs],Max) :- foldl(\X^S^M^(M is max(X,S)),Zs,Z,Max).
fdvars_max( [Z|Zs],Max) :- foldl(\X^S^M^(M #= max(X,S)),Zs,Z,Max).
Let's run some queries!
?- numbers_max([1,4,2,3],M). % integers: all are distinct
M = 4. % succeeds deterministically
?- fdvars_max( [1,4,2,3],M).
M = 4. % succeeds deterministically
?- numbers_max([1,4,2,3,4],M). % integers: M occurs twice
M = 4. % succeeds deterministically
?- fdvars_max( [1,4,2,3,4],M).
M = 4. % succeeds deterministically
What if the list is empty?
?- numbers_max([],M).
false.
?- fdvars_max( [],M).
false.
At last, some queries showing differences between numbers_max/2 and fdvars_max/2:
?- numbers_max([1,2,3,10.0],M). % ints + float
M = 10.0.
?- fdvars_max( [1,2,3,10.0],M). % ints + float
ERROR: Domain error: `clpfd_expression' expected, found `10.0'
?- numbers_max([A,B,C],M). % more general use
ERROR: is/2: Arguments are not sufficiently instantiated
?- fdvars_max( [A,B,C],M).
M#>=_X, M#>=C, M#=max(C,_X), _X#>=A, _X#>=B, _X#=max(B,A). % residual goals
list_max([L|Ls], Max) :- foldl(num_num_max, Ls, L, Max).
num_num_max(X, Y, Max) :- Max is max(X, Y).
%Query will be
?-list_max([4,12,5,3,8,90,10,11],Max).
Max=90
Right now I was working with recursion in Prolog, so if it is useful for someone I will leave 'my two cents' solving it in the two ways that I have thought:
% Start
start :- max_trad([2, 4, 6, 0, 5], MaxNumber1),
max_tail([2, 4, 6, 0, 5], 0, MaxNumber2),
show_results(MaxNumber1, MaxNumber2).
% Traditional Recursion (Method 1)
max_trad([Head|Tail], Max) :- max_trad(Tail, Value), Head > Value, Max is Head.
max_trad([Head|Tail], Max) :- max_trad(Tail, Value), Head =< Value, Max is Value.
max_trad([], 0).
% Tail Recursion (Method 2)
max_tail([], PartialMax, PartialMax).
max_tail([Head|Tail], PartialMax, FinalMax) :- Head > PartialMax, max_tail(Tail, Head, FinalMax).
max_tail([_|Tail], PartialMax, FinalMax) :- max_tail(Tail, PartialMax, FinalMax).
% Show both of the results
show_results(MaxNumber1, MaxNumber2) :-
write("The max value (obtained with traditional recursion) is: "), writeln(MaxNumber1),
write("The max value (obtained with tail recursion) is: "), writeln(MaxNumber2).
The output of the above code is:
Both methods are similar, the difference is that in the second an auxiliary variable is used in the recursion to pass values forward, while in the first method, although we have one less variable, we are filling the Stack with instructions to be executed later, so if it were an exaggeratedly large list, the second method is appropriate.
maximum_no([],Max):-
write("Maximum No From the List is:: ",Max).
maximum_no([H|T],Max):-
H>Max,
N = H,
maximum_no(T,N).
maximum_no(L,Max):-
maximum_no(L,Max).
The maximum number in a list in Prolog ?
max([],A):-print(A),!.
max([Head | Tail] , A):-A =< Head ,A1 is Head , max(Tail,A1) ; max(Tail,A).
max(L,M):-
member(M,L),
findall(X,(member(X,L),X>M),NL),
length(NL,0).