I was trying to implement permutation to cycles in Haskell without using Monad. The problem is as follow: given a permutation of numbers [1..n], output the correspondence disjoint cycles. The function is defined like
permToCycles :: [Int] -> [[Int]]
For the input:
permToCycles [3,5,4,1,2]
The output should be
[[3,4,1],[5,2]]
By the definition of cyclic permutation, the algorithm itself is straightforward. Since [3,5,4,1,2] is a permutation of [1,2,3,4,5], we start from the first element 3 and follow the orbit until we get back to 3. In this example, we have two cycles 3 -> 4 -> 1 -> 3. Continue to do so until we traverse all elements. Thus the output is [[3,4,1],[5,2]].
Using this idea, it is fairly easy to implement in any imperative language, but I have trouble with doing it in Haskell. I find something similar in the module Math.Combinat.Permutations, but the implementation of function permutationToDisjointCycles uses Monad, which is not easy to understand as I'm a beginner.
I was wondering if I could implement it without Monad. Any help is appreciated.
UPDATE: Here is the function implemented in Python.
def permToCycles(perm):
pi_dict = {i+1: perm[i]
for i in range(len(perm))} # permutation as a dictionary
cycles = []
while pi_dict:
first_index = next(iter(pi_dict)) # take the first key
this_elem = pi_dict[first_index] # the first element in perm
next_elem = pi_dict[this_elem] # next element according to the orbit
cycle = []
while True:
cycle.append(this_elem)
# delete the item in the dict when adding to cycle
del pi_dict[this_elem]
this_elem = next_elem
if next_elem in pi_dict:
# continue the cycle
next_elem = pi_dict[next_elem]
else:
# end the cycle
break
cycles.append(cycle)
return cycles
print(permToCycles([3, 5, 4, 1, 2]))
The output is
[[3,4,1],[5,2]]
I think the main obstacle when implementing it in Haskell is how to trace the marked (or unmarked) elements. In Python, it can easily be done using a dictionary as I showed above. Also in functional programming, we tend to use recursion to replace loops, but here I have trouble with thinking the recursive structure of this problem.
Let's start with the basics. You hopefully started with something like this:
permutationToDisjointCycles :: [Int] -> [[Int]]
permutationToDisjointCycles perm = ...
We don't actually want to recur on the input list so much as we want to use an index counter. In this case, we'll want a recursive helper function, and the next step is to just go ahead and call it, providing whatever arguments you think you'll need. How about something like this:
permutationToDisjointCycles perm = cycles [] 0
where
cycles :: [Int] -> Int -> [[Int]]
cycles seen ix = ...
Instead of declaring a pi_dict variable like in Python, we'll start with a seen list as an argument (I flipped it around to keeping track of what's been seen because that ends up being a little easier). We do the same with the counting index, which I here called ix. Let's consider the cases:
cycles seen ix
| ix >= length perm = -- we've reached the end of the list
| ix `elem` seen = -- we've already seen this index
| otherwise = -- we need to generate a cycle.
That last case is the interesting one and corresponds to the inner while loop of the Python code. Another while loop means, you guessed it, more recursion! Let's make up another function that we think will be useful, passing along as arguments what would have been variables in Python:
| otherwise = let c = makeCycle ix ix in c : cycles (c ++ seen) (ix+1)
makeCycle :: Int -> Int -> [Int]
makeCycle startIx currentIx = ...
Because it's recursive, we'll need a base case and recursive case (which corresponds to the if statement in the Python code which either breaks the loop or continues it). Rather than use the seen list, it's a little simpler to just check if the next element equals the starting index:
makeCycle startIx currentIx =
if next == start
then -- base case
else -- recursive call, where we attach an index onto the cycle and recur
where next = perm !! i
I left a couple holes that need to be filled in as an exercise, and this version works on 0-indexed lists rather than 1-indexed ones like your example, but the general shape of the algorithm is there.
As a side note, the above algorithm is not super efficient. It uses lists for both the input list and the "seen" list, and lookups in lists are always O(n) time. One very simple performance improvement is to immediately convert the input list perm into an array/vector, which has constant time lookups, and then use that instead of perm !! i at the end.
The next improvement is to change the "seen" list into something more efficient. To match the idea of your Python code, you could change it to a Set (or even a HashSet), which has logarithmic time lookups (or constant with a hashset).
The code you found Math.Combinat.Permutations actually uses an array of Booleans for the "seen" list, and then uses the ST monad to do imperative-like mutation on that array. This is probably even faster than using Set or HashSet, but as you yourself could tell, readability of the code suffers a bit.
Related
I'm currently working on an assignment. I have a function called gamaTipo that converts the values of a tuple into a data type previously defined by my professor.
The problem is: in order for gamaTipo to work, it needs to receive some preceding element. gamaTipo is defined like this: gamaTipo :: Peca -> (Int,Int) -> Peca where Peca is the data type defined by my professor.
What I need to do is to create a funcion that takes a list of tuples and converts it into Peca data type. The part that im strugling with is taking the preceding element of the list. i.e : let's say we have a list [(1,2),(3,4)] where the first element of the list (1,2) always corresponds to Dirt Ramp (data type defined by professor). I have to create a function convert :: [(Int,Int)] -> [Peca] where in order to calculate the element (3,4) i need to first translate (1,2) into Peca, and use it as the previous element to translate (3,4)
Here's what I've tried so far:
updateTuple :: [(Int,Int)] -> [Peca]
updateTuple [] = []
updateTuple ((x,y):xs) = let previous = Dirt Ramp
in (gamaTipo previous (x,y)): updateTuple xs
Although I get no error messages with this code, the expected output isn't correct. I'm also sorry if it's not easy to understand what I'm asking, English isn't my native tongue and it's hard to express my self. Thank you in advance! :)
If I understand correctly, your program needs to have a basic structure something like this:
updateTuple :: [(Int, Int)] -> [Peca]
updateTuple = go initialValue
where
go prev (xy:xys) =
let next = getNextValue prev xy
in prev : (go next xys)
go prev [] = prev
Basically, what’s happening here is:
updateTuple is defined in terms of a helper function go. (Note that ‘helper function’ isn’t standard terminology, it’s just what I’ve decided to call it).
go has an extra argument, which is used to store the previous value.
The implementation of go can then make use of the previous value.
When go recurses, the recursive call can then pass the newly-calculated value as the new ‘previous value’.
This is a reasonably common pattern in Haskell: if a recursive function requires an extra argument, then a new function (often named go) can be defined which has that extra argument. Then the original function can be defined in terms of go.
positions :: Eq a => a -> [a] -> [Int]
positions x xs = [i | (x',i) <- zip xs [0..], x == x']
i need to create a Test function for the positions function, which passes the quickcheck.
Does someone has an idea?
A possible test could perform the following operations:
randomly generate xs, ys :: [Int] and y :: Int
define list = xs ++ y : ys
test length xs `elem` positions y list
You might also want to write tests for missing elements.
That being said, it's weird to craft tests from the code. One should design test using the specification used to write the code instead. Otherwise, if the code has some quirks, they end up in tests as well: instead of checking for what the code should do, we check for what the code does, which can be pointless.
Good question! I had a bit of trouble with this too when I started using QuickCheck, but as I used it more I started to get a feel for what sort of properties you should test. Often, a good place to start is to think about what sort of relationships should hold between the input(s) and output. Your function finds the indices of an element in a list; now what properties should hold in this case? Here's two which I can think of:
The list of indices should have the same number of elements as there are occurrences of the value in the list.
Each index should correspond to the correct value.
And here's an outline of how these properties could be tested:
Generate a random list, apply positions to it, then count the number of indices returned and check that it matches with the number of occurrences of the searched value.
Generate a random list, then apply positions to it and check that the element at each of the returned indices is the value which was searched for.
I also think that #chi's answer makes a good point in saying that you should design tests from the specification rather than the code; this ties in with what I was saying above, in that a specification can help you find relationships between input(s) and output which may not be immediately apparent from the code.
Is there anyway I could check the number of recursion the program is at. For example, I want to stop the recursion after 2 times. Is there anyway to do that in haskell.
Yes, there is but...
Stopping recursion is normally done when some final state is achieved, something like "I've run out of data to process" or "I have reached the base case." When I see something as arbitrary as "after 2 times" I want to ask where you came up with 2.
However in the interest of answering the question asked:
You need to pass in a counter to the recursive function and bail out when you have done the required number of cycles. For cases like this where the number of cycles is not a matter external to the function, it is typically the case that one creates an auxilary function to introduce it.
myFunction :: Value -> Value
myFunction init = recurse 2 init
where
recurse :: Int -> Value -> Value
recurse 0 result = result
recurse n intermediate = recurse (n-1) (someFun intermediate)
I'm trying to write a function in Haskell that calculates all factors of a given number except itself.
The result should look something like this:
factorlist 15 => [1,3,5]
I'm new to Haskell and the whole recursion subject, which I'm pretty sure I'm suppoused to apply in this example but I don't know where or how.
My idea was to compare the given number with the first element of a list from 1 to n div2
with the mod function but somehow recursively and if the result is 0 then I add the number on a new list. (I hope this make sense)
I would appreciate any help on this matter
Here is my code until now: (it doesn't work.. but somehow to illustrate my idea)
factorList :: Int -> [Int]
factorList n |n `mod` head [1..n`div`2] == 0 = x:[]
There are several ways to handle this. But first of all, lets write a small little helper:
isFactorOf :: Integral a => a -> a -> Bool
isFactorOf x n = n `mod` x == 0
That way we can write 12 `isFactorOf` 24 and get either True or False. For the recursive part, lets assume that we use a function with two arguments: one being the number we want to factorize, the second the factor, which we're currently testing. We're only testing factors lesser or equal to n `div` 2, and this leads to:
createList n f | f <= n `div` 2 = if f `isFactorOf` n
then f : next
else next
| otherwise = []
where next = createList n (f + 1)
So if the second parameter is a factor of n, we add it onto the list and proceed, otherwise we just proceed. We do this only as long as f <= n `div` 2. Now in order to create factorList, we can simply use createList with a sufficient second parameter:
factorList n = createList n 1
The recursion is hidden in createList. As such, createList is a worker, and you could hide it in a where inside of factorList.
Note that one could easily define factorList with filter or list comprehensions:
factorList' n = filter (`isFactorOf` n) [1 .. n `div` 2]
factorList'' n = [ x | x <- [1 .. n`div` 2], x `isFactorOf` n]
But in this case you wouldn't have written the recursion yourself.
Further exercises:
Try to implement the filter function yourself.
Create another function, which returns only prime factors. You can either use your previous result and write a prime filter, or write a recursive function which generates them directly (latter is faster).
#Zeta's answer is interesting. But if you're new to Haskell like I am, you may want a "simple" answer to start with. (Just to get the basic recursion pattern...and to understand the indenting, and things like that.)
I'm not going to divide anything by 2 and I will include the number itself. So factorlist 15 => [1,3,5,15] in my example:
factorList :: Int -> [Int]
factorList value = factorsGreaterOrEqual 1
where
factorsGreaterOrEqual test
| (test == value) = [value]
| (value `mod` test == 0) = test : restOfFactors
| otherwise = restOfFactors
where restOfFactors = factorsGreaterOrEqual (test + 1)
The first line is the type signature, which you already knew about. The type signature doesn't have to live right next to the list of pattern definitions for a function, (though the patterns themselves need to be all together on sequential lines).
Then factorList is defined in terms of a helper function. This helper function is defined in a where clause...that means it is local and has access to the value parameter. Were we to define factorsGreaterOrEqual globally, then it would need two parameters as value would not be in scope, e.g.
factorsGreaterOrEqual 4 15 => [5,15]
You might argue that factorsGreaterOrEqual is a useful function in its own right. Maybe it is, maybe it isn't. But in this case we're going to say it isn't of general use besides to help us define factorList...so using the where clause and picking up value implicitly is cleaner.
The indentation rules of Haskell are (to my tastes) weird, but here they are summarized. I'm indenting with two spaces here because it grows too far right if you use 4.
Having a list of boolean tests with that pipe character in front are called "guards" in Haskell. I simply establish the terminal condition as being when the test hits the value; so factorsGreaterOrEqual N = [N] if we were doing a call to factorList N. Then we decide whether to concatenate the test number into the list by whether dividing the value by it has no remainder. (otherwise is a Haskell keyword, kind of like default in C-like switch statements for the fall-through case)
Showing another level of nesting and another implicit parameter demonstration, I added a where clause to locally define a function called restOfFactors. There is no need to pass test as a parameter to restOfFactors because it lives "in the scope" of factorsGreaterOrEqual...and as that lives in the scope of factorList then value is available as well.
I was trying to write a function to get all subsequences of a list that are of size n, but I'm not sure how to go about it.
I was thinking that I could probably use the built-in Data.List.subsequences and just filter out the lists that are not of size n, but it seems like a rather roundabout and inefficient way of doing it, and I'd rather not do that if I can avoid it, so I'm wondering if you have any ideas?
I want it to be something like this type
subseqofsize :: Int -> [a] -> [[a]]
For further clarification, here's an example of what I'm looking for:
subseqofsize 2 [1,2,3,3]
[[1,2],[1,3],[2,3],[1,3],[2,3],[3,3]]
Also, I don't care about the order of anything.
I'm assuming that this is homework, or that you are otherwise doing this as an exercise to learn, so I'll give you an outline of what the solution looks like instead of spoon-feeding you the correct answer.
Anyway, this is a recursion question.
There are two base cases:
sublistofsize 0 _ = ...
sublistofsize _ [] = ...
Then there are two recursive steps:
sublistofsize n (x : xs) = sublistsThatStartWithX ++ sublistsThatDon'tStartWithX
where sublistsThatStartWithX = ...
sublistsThatDon'tStartWithX = ...
Remember that the definitions of the base cases need to work appropriately with the definitions in the recursive cases. Think carefully: don't just assume that the base cases both result in an empty list.
Does this help?
You can think about this mathematically: to compute the sublists of size k, we can look at one element x of the list; either the sublists contain x, or they don't. In the former case, the sublist consists of x and then k-1 elements chosen from the remaining elements. In the latter case, the sublists consist of k elements chosen from the elements that aren't x. This lends itself to a (fairly) simple recursive definition.
(There are very very strong similarities to the recursive formula for binomial coefficients, which is expected.)
(Edit: removed code, per dave4420's reasons :) )