So in the tag prolog someone wanted to solve the "the giant cat army riddle" by Dan Finkel (see video / Link for description of the puzzle).
Since I want to improve in answer set programming I hereby challenge you to solve the puzzle more efficient than me. You will find my solution as answer. I'll accept the fastest running answer (except if it's using dirty hacks).
Rules:
hardcoding the length of the list (or something similar) counts as dirty hack.
The output has to be in the predicate r/2, where it's first argument is the index of the list and the second its entry.
Time measured is for the first valid answer.
num(0..59).
%valid operation pairs
op(N*N,N):- N=2..7.
% no need to add operations that start with 14
op(Ori,New):- num(Ori), New = Ori+7, num(New), Ori!=14.
op(Ori,New):- num(Ori), New = Ori+5, num(New), Ori!=14.
%iteratively create new numbers from old numbers
l(0,0).
{l(T+1,New) : op(Old,New)} = 1 :- l(T,Old), num(T+1), op(Old,_).
%no number twice
:- 2 #sum {1,T : l(T,Value)}, num(Value).
%2 before 10 before 14
%linear encoding
reached(T,10) :- l(T,10).
reached(T+1,10) :- reached(T,10), num(T+1).
:- reached(T,10), l(T,2).
:- l(T,14), l(T+1,_).
%looks nicer, but quadratic
%:- l(T2,2), l(T10,10), T10<T2.
%:- l(T14,14), l(T10,10), T14<T10.
%we must have these three numbers in the list somewhere
:- not l(_,2).
:- not l(_,10).
:- not l(_,14).
#show r(T,V) : l(T,V).
#show.
Having a slightly more ugly encoding improves grounding a lot (which was your main problem).
I restricted op/2 to not start with 14, as this should be the last element in the list
I do create the list iteratively, this may not be as nice, but at least for the start of the list it already removed impossible to reach values via grounding. So you will never have l(1,33) or l(2,45) etc...
Also list generation stops when reaching the value 14, as no more operation is possible/needed.
I also added a linear scaling version of the "before" section, although it is not really necessary for this short list (but a cool trick in general if you have long lists!) This is called "chaining".
Also note that your show statement is non-trivial and does create some constraints/variables.
I hope this helps, otherwise feel free to ask such questions also on our potassco mailing list ;)
My first attempt is to generate a permutation of numbers and force successor elements to be connected by one of the 3 operations (+5, +7 or sqrt). I predefine the operations to avoid choosing/counting problems. Testing for <60 is not necessary since the output of an operation has to be a number between 0 and 59. The generated List l/2 is forwarded to the output r/2 until the number 14 appears. I guess there is plenty of room to outrun my solution.
num(0..59).
%valid operation pairs
op(N*N,N):- N=2..7.
op(Ori,New):- num(Ori), New = Ori+7, num(New).
op(Ori,New):- num(Ori), New = Ori+5, num(New).
%for each position one number
l(0,0).
{l(T,N):num(N)}==1:-num(T).
{l(T,N):num(T)}==1:-num(N).
% following numbers are connected with an operation until 14
:- l(T,Ori), not op(Ori,New), l(T+1,New), l(End,14), T+1<=End.
% 2 before 10 before 14
:- l(T2,2), l(T10,10), T10<T2.
:- l(T14,14), l(T10,10), T14<T10.
% output
r(T,E):- l(T,E), l(End,14), T<=End.
#show r/2.
first Answer:
r(0,0) r(1,5) r(2,12) r(3,19) r(4,26) r(5,31) r(6,36) r(7,6)
r(8,11) r(9,16) r(10,4) r(11,2) r(12,9) r(13,3) r(14,10) r(15,15)
r(16,20) r(17,25) r(18,30) r(19,37) r(20,42) r(21,49) r(22,7) r(23,14)
There are multiple possible lists with different length.
Related
I am trying to implement a basic function but I'm out of practice with Haskell and struggling so would really appreciate some help. My question is specifically how to select a section of a list by index. I know how to in other languages but have been struggling
[ x | x <- graph, x!! > 5 && x!! <10 ]
I have been fiddling around with basic list comprehension similar to what is above, and while I know that isn't right I was hoping a similarly simple solution would be available.
If anyone wants more information or felt like helping on the further question I have included more information below, thanks!
type Node = Int
type Branch = [Node]
type Graph= [Node]
next :: Branch -> Graph -> [Branch]
This is the individual question for the "next" function
This is the general set up information but most importantly that the graph is represented as a flattened adjacency matric
Apologies for the two pictures but it seemed the best way to convey the information.
As pointed out in the comments !! does not give you the index of a value in the way it seems you expect. It is just an infix for getting an element of a list.
There is no way to get the index of x like this in Haskell since the x object doesn't keep track of where it is.
To fix this we can make a list of objects that do keep track of where they were. This can be achieved with zip.
zip [0..] graph
This creates a list of tuples each containing their index and the value in graph.
So you can write your list comprehensions as
[ x | (index, x) <- zip [0..] graph, index > 5, index < 10 ]
Now this is not going to be terribly fast since it still needs to go through every element of the list despite the fact that we know no element after the 11th will be used. For speed we would want to use a combination of take and drop.
drop 5 (take 10 graph)
However if we wanted to do some other selections (e.g. all even indexes), we can still go back to the list comprehension.
In this case, you could drop 5 <&> take 4. As in drop 5 x & take 4. Drop skips the first few elements and take leaves out all but the first few left after the drop.
So, my assignment is to get the centered average of a list, much like a few of the other posts on here like this one (https://codereview.stackexchange.com/questions/108404/how-to-get-a-centered-mean-excluding-max-and-min-value-of-a-list-in-python) and a few others. However, my professor has told us we are not allowed to use min, max, or sort to solve this. So what I have right now is this, it is still a work in progress:
def centered_average(nums):
high=0
low=0
a=0
b=0
for i in range(len(nums)):
if nums[i]>a:
a=nums[i]
high=a
for i in range(len(nums)):
if nums[i]<b:
b=nums[i]
low=b
total=sum(nums)
average=(total-high-low)/(len(nums)-2)
print(average)
My problem is that I can't get low to be recognized as the lowest number in the list. For example, if I input [1,2,3,4,5] as the list, my function should return 5 as the high, 1 as the low, and 3 as the centered average since 2+3+4 is 9/3=3. However, what I have right there returns the low as 0. I think it is because of the (lens(nums) since it would think the first number is a 0. I'm not sure how I should fix this.
Note: I am still a beginner at this stuff so I know what I have might not be the best or that the error could be simple to fix, but I am still in the process of learning so any help and advice would be much appreciated.
The problem is your starting the running minimum (and running maximum) as 0.
Start the running minimum as float("inf") (as everything is guaranteed to be less than that). Start the running maximum as float("-inf") (as everything is guaranteed to be greater than that).
Or, start both as the first element of the list (which is either a true minimum/maximum, or there's another element that is lower/higher than it).
Feel free to cut immediately past the first two paragraphs, they are mostly waffle explaining the situation.
I'm working on a task for my University course, while I don't want any help solving the actual problem (I feel like that's "cheating" as it were) I would like help finding a way to extend the lengths of lists shown in prolog when tracing. For example, in the task you have to make a path finder through a maze with coloured "edges" between nodes which are each assigned a unique letter from the alphabet. The edges are "two way" and there is a "start" node that connects via a red edge to the "m" node also. The goal is to reach the "g" node in the middle while going along edges from the start in a repeating order of [red,brown,yellow].
Anyway, I think my algorithm finds a correct route at the bottom of the recursion, but going through the tracer is possible thousands of steps (I was holding return for about 2 minutes before it finished). Currently it doesn't "return" the generated list of steps (and while I'm sure some of you would be able to tell me how to do so, I'd rather you didn't because it's important I learn the actual prolog myself I feel) so the only time I see what's in the list of route steps is in the trace. SO here is the problem:
path(k, [red, brown, yellow], [[start, red], [m, brown], [e, yellow], [h, red], [r, brown], [p, yellow], [n|...], [...|...]|...], [start, m, e, h, r, p, n, j|...], g)
The final list holds the route I want to know if it's valid, however:
[start, m, e, h, r, p, n, j|...]
Cuts off at j, I want the trace to show FULL lists, otherwise I will have to go back through 100s of lines of trace trying to find the broken up and "correct" nodes in the path, with lots of backtracking mixed in i.e. really hard, and really easy to make a mistake. Also the I'm using program only keeps 30 or so lines prior (no idea if this is normal but I am using a SWI-Prolog(Multi-threaded, version 7.2.3) from the official site). Which means I'd have to go through everything past the first time it reaches the j node which would take a huge amount of time.
So as I say, this could be solved by having the list be unified (or whatever it is called) as a "return" (or whatever it is called) but I don't want an answer like that spoon fed to me and would rather figure it out on my own. So if you know how to do so, please refrain from telling me and just still with the way to increase the maximum displayed list with trace thank you.
I appreciate the help, sorry for the hoops I'm asking people to jump through.
to prevent these kinds of outputs [_|...] adding the code below ;
:- set_prolog_flag(toplevel_print_options,
[quoted(true), portrayed(true), max_depth(0)]).
I'm trying to write a function that will take an array or vector and have its values taken to a "power of" and then display it's values. I'm not too familiar with arrays but simply put I'm trying to create something like
n = {2^1, 3^1, 5^1,2^2,3^2,5^2,....}
the "power of" is going to be looped.
I then plan to sort the array, and display 1500th term.
this problem corresponds to prime number sequence only divisible by 2 , 3 & 5;
I'm trying to find a more time efficient way than just if statements and mod operators.
If I remember correctly this is the Ugly Numbers problem I've faced some years ago in the UVa.
The idea to solve this problem is to use a priority queue with the numbers 2, 3 and 5 as initial values. At each step remove the topmost value t and insert the values 2*t, 3*t and 5*t in the priority queue, repeat this steps till the 1500th term is found.
See this forum for more info: http://online-judge.uva.es/board/viewtopic.php?t=93
I am having trouble with recursive searching of list and creation of list of lists from the result..
The knowledge base contains team name, number of wins and zone they are in, all associated withe their team number. I am passing list of team numbers in Teams and I am searching for a matching pair with findMinMax/3. The result I need is...
List of lists of paired teams (ex. X = [[gonzaga, washington], [iowa, oklahoma], …])
And 1 unmatched team (resulted from odd number of teams) or 0 (in case of even)
I figured out everything else and can get up to the part [gonzaga, washington], but failing at recursive portion...
findPair(Teams,[HL|TL],Rest) :-
findMinMax(Teams,Min,Max),
delete(Teams,Min,TeamsNoMin),
delete(TeamsNoMin,Max,Rest),
createPair(Min,Max,Pair), %Pair = "["Min_team","Max_team"]"
append(HL,[Pair],TL),
findPair(Rest,TL,[]).
A general recursive scheme
Here I'll try to show you how we usually perform recursion in Prolog. It's not simple to get for beginners because the lists are built "backwards": nothing gets really built until we hit the end of the list.
The reason for this "build backwards" principle is that once a variable is set, you can't set it to another value, so for example it'd be hard to say that the result is [1] at the first step of the recursion and then becomes [1, 2]. Instead, what we say in Prolog is that the result head is 1 and that the result tail is the result of the recursive call (yeah read it twice if it got messy : d). So as long as we do not hit a base case (a case where no recursion is performed), we don't bind variables definitely (ie we always let a part of the term unbound).
For a predicate rec/2: rec(Input, Result) producing a result list from an input list by linking their elements with somepredicate/2, we'd write:
rec([InputHead|InputTail], [ResultHead|ResultTail]) :-
somepredicate(InputHead, ResultHead),
rec(InputTail, ResultTail).
to represent that.
Here you can see that we stated that the head of the result is ResultHead and that its tail is calculated thanks to the call rec(InputTail, ResultTail).
Now that's fine but we need to stop at some point, when the list is empty, for example. We'd write that as follows:
rec([], []).
which means: when the input list is empty, so is the result list.
An application to your predicate
Now, to fix your problem, you first have to fix the recursive clause:
findPair(Teams,[HL|TL],Rest) :-
findMinMax(Teams,Min,Max),
delete(Teams,Min,TeamsNoMin),
delete(TeamsNoMin,Max,Rest),
createPair(Min,Max,Pair), %Pair = "["Min_team","Max_team"]"
append(HL,[Pair],TL),
findPair(Rest,TL,[]).
would become
findPair(Teams, [Pair|Tail], LeftOver) :-
findMinMax(Teams, Min, Max),
delete(Teams, Min, TeamsNoMin),
delete(TeamsNoMin, Max, Rest),
createPair(Min, Max, Pair), %Pair = "["Min_team","Max_team"]"
findPair(Rest, Tail, LeftOver).
Important to note: now Rest has become two separate variables. The last argument of findPair/3 doesn't get changed anymore, since in the recursive call we do not know anything about it yet, so we can't bind it, and the in-predicate Rest is therefore now independant and just represents the teams that have not been handled yet and are therefore of interest for the tail of our result list (and for LeftOver).
Now we have to handle the base cases:
when there are no teams left
findPair([], [], []).
Here we say that when Teams is empty, so are the Result and LeftOver.
when there is one team left
findPair([Last], [], [Last]).
Here we say that when Teams has only one element, LeftOver is equal to Teams and Result is empty.
Resulting code is:
findPair([], [], []).
findPair([Last], [], [Last]).
findPair(Teams, [Pair|Tail], LeftOver) :-
findMinMax(Teams, Min, Max),
delete(Teams, Min, TeamsNoMin),
delete(TeamsNoMin, Max, Rest),
createPair(Min, Max, Pair), %Pair = "["Min_team","Max_team"]"
findPair(Rest, Tail, LeftOver).
To make your clauses exclusive you could replace Teams with [Not, Empty|AtAll] to ensure the last clause is used only with lists of length 2 or more, or just add a guard such as Teams = [_, _|_], at the start of the clause.
Hope it helped and do not hesitate to ask for clarifications in comments :)