How to shuffle a list? - haskell

How can I sample without replacement from a set of numbers ([1, 2, 3]) until I hit x?
My plan was to shuffle the list [1, 2, 3] and chop it at x:
-- chopAt 3 [2, 3, 1] == [2, 3]
-- chopAt 3 [2, 1, 3] == [2, 1, 3]
-- chopAt 3 [3, 1, 2] == [3]
chopAt _ [] = []
chopAt x (y:ys)
| x /= y = y : chopAt x ys
| otherwise = [y]
However I could not figure out how to shuffle the list (or understand Monads yet).
-- sample without replacement from [1, 2, 3] until one hits a 3
-- x <- shuffle [1, 2, 3]
-- print (chopAt 3 x)
main = do
-- shuffle [1, 2, 3]
print (chopAt 3 [1, 3, 2])

Use random and maybe even MonadRandom to implement your shuffles. A few good answers exist here
But that's really operational. Here's what's going on behind the scenes.
I.
Randomness is one of the first places in Haskell that you encounter and have to handle impurity---which seems offensive, because shuffles and samples seem so simple and don't feel like they ought to be bundled up with printing to a physical screen or launching nukes, but often purity == referentially transparent and referentially transparent randomness would be useless.
random = 9 -- a referentially transparent random number
So we need a different idea about randomness to make it pure.
II.
A typical "cheat" in scientific code used to enhance reproducibility—super important—is to fix your random seed of an experiment so that others can verify that they get exactly the same results every time your code is run. This is exactly referential transparency! Let's try it.
type Seed = Int
random :: Seed -> (Int, Seed)
random s = (mersenneTwisterPerturb s, splitSeed s)
where mersenneTwisterPerturb is a pseudorandom mapping from Seeds to Int and splitSeed is a pseudorandom mapping from Seeds to Seeds. Note that both of these functions are totally deterministic (and referentially transparent), so random is as well, but we can create an infinite, lazy pseudorandom stream like so
randomStream :: Seed -> [Int]
randomStram s = mersenneTwisterPerturb s : randomStream (splitSeed s)
Again, this stream is deterministic based on the Seed value, but an observer who sees only the stream and not the seed should be unable to predict its future values.
III.
Can we shuffle a list using a random stream of integers? Sure we can, by using modular arithmetic.
shuffle' :: [Int] -> [a] -> [a]
shuffle' (i:is) xs = let (firsts, rest) = splitAt (i `mod` length xs) xs
in (head rest) : shuffle' is (firsts ++ tail rest)
Or, to make it more self-contained, we can precompose our stream generating function to get
shuffle :: Seed -> [a] -> [a]
shuffle s xs = shuffle' (randomStream s) xs
another "seed consuming" referentially transparent "random" function.
IV.
So this seems to be a repeating trend. In fact, if you browse the module System.Random you'll see lots of functions like what we wrote above (I've specialized some type classes)
random :: (Random a) => StdGen -> (a, StdGen)
randoms :: (Random a) => StdGen -> [a]
where Random is the type class of things which can be generated randomly and StdGen is a kind of Seed. This is already enough actual working code to write the necessary shuffling function.
shuffle :: StdGen -> [a] -> [a]
shuffle g xs = shuffle' (randoms g) xs
and there's an IO function newStdGen :: IO StdGen which will let us build a random seed.
main = do gen <- newStdGen
return (shuffle gen [1,2,3,4,5])
But you'll notice something annoying: we need to keep varying the gen if we want to make different random permutations
main = do gen1 <- newStdGen
shuffle gen1 [1,2,3,4,5]
gen2 <- newStdGen
shuffle gen2 [1,2,3,4,5]
-- using `split :: StdGen -> (StdGen, StdGen)`
gen3 <- newStdGen
let (_, gen4) = split gen3
shuffle gen3 [1,2,3,4,5]
let (_, gen5) = split gen4
shuffle gen4 [1,2,3,4,5]
This means you'll either have to do lots of StdGen bookkeeping or stay in IO if you want different random numbers. This "makes sense" because of referential transparency again---a set of random numbers have to be random with respect to each other so you need to pass information from each random event on to the next.
It's really annoying, though. Can we do better?
V.
Well, generally what we need is a way to have a function take in a random seed then output some "randomized" result and the next seed.
withSeed :: (Seed -> a) -> Seed -> (a, Seed)
withSeed f s = (f s, splitSeed s)
The result type withSeed f :: Seed -> (a, Seed) is a fairly general result. Let's give it a name
newtype Random a = Random (Seed -> (a, Seed))
And we know that we can create meaningful Seeds in IO, so there's an obvious function to convert Random types to IO
runRandom :: Random a -> IO a
runRandom (Random f) = do seed <- newSeed
let (result, _) = f seed
return result
And now it feels like we've got something useful---a notion of a random value of type a, Random a is just a function on Seeds which returns the next Seed so that later Random values won't all be identical. We can even make some machinery to compose random values and do this Seed-passing automatically
sequenceRandom :: Random a -> Random b -> Random b
sequenceRandom (Random fa) (Random fb) =
Random $ \seed -> let (_aValue, newSeed) = fa seed in fb newSeed
but that's a little silly since we're just throwing away _aValue. Let's compose them such that the second random number actually depends materially on the first random value.
bindRandom :: Random a -> (a -> Random b) -> Random b
bindRandom (Random fa) getRb =
Random $ \seed -> let (aValue, newSeed) = fa seed
(Random fb) = getRb aValue
in fb newSeed
We also ought to note that we can do "pure" things to Random values, for instance, multiplying a random number by 2:
randomTimesTwo :: Random Int -> Random Int
randomTimesTwo (Random f) = Random $ \seed -> let (value, newSeed) = f seed
in (value*2, newSeed)
which we can abstract out as a Functor instance
instance Functor Random where
fmap f (Random step) = Random $ \seed -> let (value, newSeed) = step seed
in (f value, newSeed)
and now we can create cool random effects like Brownian motion
brownianMotion :: Random [Int]
brownianMotion =
bindRandom random $ \x ->
fmap (\rest -> x : map (+x) rest) brownianMotion
VI.
And this gets to the heart of the whole matter that I've been writing up to. Randomness can exist in the IO monad perfectly well, but it can also exist on its own as a simpler Random monad. We can write the instance immediately.
instance Monad Random where
return x = Random (\seed -> (x, seed))
rx >>= f = bindRandom rx f
And since it's a monad, we get free do notation
brownianMotion' = do x <- random
rest <- brownianMotion'
return $ x : map (+x) rest
and you could even get fancy and call runRandom a monad homomorphism, but that's a very different topic.
So, to recap
randomness in a referentially transparent language needs Seeds
carting Seeds are is annoying
there's a common pattern to "lifting" and "binding" random values which routes the Seeds around naturally
that pattern forms a monad
And the really short answer is that you probably want to be using random and maybe even MonadRandom to implement your shuffles. They'll come in handy for "sampling" generally.
Cheers!

Are you looking for permutations?
Also it seems that cropAt can be implemented via takeWhile. I personally prefer standard combinators over hand-made.

For shuffling a list, use the random-shuffle library:
import System.Random (newStdGen)
import System.Random.Shuffle (shuffle')
main = do
rng <- newStdGen
let xs = [1,2,3,4,5]
print $ shuffle' xs (length xs) rng

Bellow you may find some simple solution from beginning of my Haskell learning. If truth to be told, I'm still at the beginning or just slightly behind ;-)
import System.Random
import Control.Applicative
shuffle :: [a] -> IO [a]
shuffle [] = return []
shuffle lst = do
(e, rest) <- pickElem <$> getIx
(e:) <$> shuffle rest
where
getIx = getStdRandom $ randomR (1, length lst)
pickElem n = case splitAt n lst of
([], s) -> error $ "failed at index " ++ show n -- should never match
(r, s) -> (last r, init r ++ s)

Everyone seem to have encountered this at one point in time or another. This is my quick solution to a problem:
import System.Random
shuffle :: [a] -> IO [a]
shuffle [] = return []
shuffle xs = do randomPosition <- getStdRandom (randomR (0, length xs - 1))
let (left, (a:right)) = splitAt randomPosition xs
fmap (a:) (shuffle (left ++ right))
Note that the complexity is O(N^2), so this is pretty inefficient for larger lists. Other way is to implement Fisher-Yates shuffle by using mutable arrays (linear complexity):
import Data.Array.IO
import System.Random
swapElements_ :: (MArray a e m, Ix i) => a i e -> i -> i -> m ()
swapElements_ arr i j = do a <- readArray arr i
b <- readArray arr j
writeArray arr i b
writeArray arr j a
return ()
shuffle :: [a] -> IO [a]
shuffle xs = do let upperBound = length xs
arr <- (newListArray (1, upperBound) :: [a] -> IO (IOArray Int a)) xs
mapM_ (shuffleCycle arr) [2..upperBound]
getElems arr
where shuffleCycle arr i = do j <- getStdRandom (randomR (1, i))
swapElements_ arr i j

add this function to your code then call it like this shuffle (mkStdGen 5) [1,2,3,4,5]
import System.Random
shuffle gen [] = []
shuffle gen list = randomElem : shuffle newGen newList
where
randomTuple = randomR (0,(length list) - 1) gen
randomIndex = fst randomTuple
newGen = snd randomTuple
randomElem = list !! randomIndex
newList = take randomIndex list ++ drop (randomIndex+1) list
Or include it in your do block like that
main = do
r <- randomIO
str <- getLine
putStrLn (show (shuffle (mkStdGen r) str))

Related

How to generate test data with shuffle in QuickCeck

For a given list [1..n], where n is a random positive integer, I want to generate the test data with 2 steps:
Shuffle the list, xs = shuffle [1..n];
Random mutate a number x in xs to y, where 1 <= y <= n;
After these 2 steps, the new list is denoted as ys.
I wrote a program. It takes ys, and outputs (x, y), the function prototype is like this:
solve :: [a] -> (a, a)
I want to use Test.QuickCheck to test my program. How to generate such test data? I see there is a function in QuickCheck
shuffle :: [a] -> Gen [a]
But I have no idea about how to use it.
The Gen monad in QuickCheck is basically a state monad with a random number generator as its state. So when you see
shuffle :: [a] -> Gen [a]
That means that this argument takes a list and returns a shuffled list "in" the Gen monad.
Because its a monad you can put it inside a do clause. Its not quite clear what you are asking for, but I think its something like this:
myTest :: Integer -> Gen [Integer]
myTest 0 = return []
myTest n = do
ns <- shuffle [1..n]
x <- choose (0,n-1)
y <- choose (1,n)
let (ns1,ns2) = splitAt x ns
return $ ns1 ++ [y] ++ drop 1 ns2`
You can run an action in the Gen monad using generate, which returns a value in IO, or you can set up a new type for your test data and make it an instance of Arbitrary, which contains the function
arbitrary :: Gen a
Edit: Or as Zeta pointed out in the comments you can use forAll:
quickCheck $ forAll (myTest 10) $ \x -> ....

Building up a list in Haskell

What I am wanting to do is create a list of random integers, with no duplicates. As a first step, I have a function which makes a list of n random samples. How does one write this in a more Haskell idiomatic way, where an empty list does not need to be passed in to start the list off? I am sure I am missing something basic and fundamental.
-- make a list of random integers.
-- takes a size, and an empty list.
-- returns a list of that length of random numbers.
f :: Int -> [Int] -> IO [Int]
f l xs | length xs >= l = return (xs)
f l xs = do
r <- randomRIO (1, 40) :: IO Int
f l $ r : x
Usage:
*Main> f 6 []
[10,27,33,35,31,28]
Ultimately this function will have filtering to check for duplicate insertions, but that is a separate question. Although this may look like homework, it is not, but part of my own attempt to come to grips with the State monad as used for random number generation, and finding I am stuck at a much earlier spot.
Well, you can operate on the output of the recursive call:
f :: Int -> IO [Int]
f 0 = return []
f n = do
r <- randomRIO (1, 40)
xs <- f (n-1)
return $ r : xs
Note however that it's important the the operation you perform on the result is fast. In this case r : xs is constant time. However if you replace the last line with (say):
return $ xs ++ [r]
this would change the complexity of the function from linear to quadratic because every ++ call will have to scan all the sequence of previously generated numbers before appending the new one.
However you could simply do:
f n = sequence $ replicate n (randomRIO (1, 40))
replicate creates a [IO Int] list of length n made of randomRIO actions and sequence takes an [IO a] and turns it into an IO [a] by executing all the actions in order and collecting the results.
Even simpler, you could use replicateM which is already the function you want:
import Control.Monad(replicateM)
f n = replicateM n (randomRIO (1, 40))
or in point-free style:
f :: Int -> IO [Int]
f = flip replicateM $ randomRIO (1, 40)
This uses a Set to keep track of numbers already generated:
import System.Random
import qualified Data.Set as Set
generateUniqueRandoms :: (Int, Int) -> Int -> IO [Int]
generateUniqueRandoms range#(low, high) n =
let maxN = min (high - low) n
in
go maxN Set.empty
where
go 0 _ = return []
go n s = do
r <- getUniqueRandom s
xs <- go (n-1) (Set.insert r s)
return $ r : xs
getUniqueRandom s = do
r <- randomRIO range
if (Set.member r s) then getUniqueRandom s
else return r
Here is some sample output:
Main> generateUniqueRandoms (1, 40) 23
[29,22,2,17,5,8,24,27,10,16,6,3,14,37,25,34,30,28,7,31,15,20,36]
Main> generateUniqueRandoms (1, 40) 1000
[33,35,24,16,13,1,26,7,14,11,15,2,4,30,28,6,32,25,38,22,17,12,20,5,18,40,36,39,27,9,37,31,21,29,8,34,10,23,3]
Main> generateUniqueRandoms (1, 40) 0
[]
However, it is worth noting that if n is close to the width of the range, it'd be much more efficient to shuffle a list of all numbers in the range and take the first n of that.

How to generate an infinite list of random numbers using randomRIO?

I'm trying to generate an infinite list of random numbers using randomRIO.
import System.Random
g :: IO [Integer]
g = do
n <- randomRIO (1,6) :: IO Integer
f <- g
return (n : f)
It compiles but hangs when run.
The problem with your code is that it never teminates. Whenever you execute g, it initially calcuates the first random number and stores it in n. Then it again recursively calls back to the same function g which goes on and on. To get an infinite list you can use the randomRs function:
g2 :: IO [Integer]
g2 = do
g <- newStdGen
return $ randomRs (1,6) g
And then you can produce any number of random numbers using it:
λ> g2 >>= \x -> return $ take 5 $ x
[5,4,6,5,6]
Well, you can't, for much the same reason why you can't lazily mapM over State (and then use the result state afterwards). randomRIO produces a single value and only hands on the monadic program flow after this value has been produced.
The way to produce infinite streams of random values is of course randomRs. You can easily couple it with getStdRandom to produce such streams right in IO:
import Control.Arrow
randomRsIO :: Random a => (a,a) -> IO [a]
randomRsIO range = getStdRandom $ split >>> first (randomRs range)
With that,
g :: IO [Integer]
g = randomRsIO (1,6)
You wanted an answer using randomRIO. I know its been a long time since the question was asked.
This is using liftM where you can generate a random number in range low l to high h and keep appending to a list num times. So we have a list of length num of random numbers.
import Control.Monad
import System.Random
gx :: Int -> Int -> Int -> IO [Int]
gx l h num = do
x <- randomRIO(l,h)
if num <=1 then return [x] else liftM (x:) (gx l h (num-1))

Random numbers without duplicates

Simulating a lottery of 6 numbers chosen from 40, I want to create a list of numbers in Haskell using the system random generator but eliminate duplicates, which often arise.
If I have the following:
import System.Random
main :: IO ()
main = do
rs <- forM [1..6] $ \_x -> randomRIO (1, 40) :: (IO Int)
print rs
this is halfway. But how do I filter out duplicates? It seems to me I need a while loop of some sort to construct a list filtering elements that are already in the list until the list is the required size. If I can generate an infinite list of random numbers and filter it inside the IO monad I am sure that would work, but I do not know how to approach this. It seems while loops are generally deprecated in Haskell, so I am uncertain of the true Haskeller's way here. Is this a legitimate use case for a while loop, and if so, how does one do that?
The function you are looking for is nub from Data.List, to filter dublicates.
import Data.List
import System.Random
main = do
g <- newStdGen
print . take 6 . nub $ (randomRs (1,40) g :: [Int])
If you don't mind using a library, then install the random-shuffle package and use it like this:
import System.Random.Shuffle
import Control.Monad.Random
main1 = do
perm <- evalRandIO $ shuffleM [1..10]
print perm
If you want to see how to implement a naive Fischer-Yates shuffle using lists in Haskell, have a look at this code:
shuffle2 xs g = go [] g (length xs) xs
where
go perm g n avail
| n == 0 = (perm,g)
| otherwise = let (i, g') = randomR (0,n-1) g
a = avail !! i
-- can also use splitAt to define avail':
avail' = take i avail ++ drop (i+1) avail
in go (a:perm) g' (n-1) avail'
main = do
perm <- evalRandIO $ liftRand $ shuffle2 [1..10]
print perm
The parameters to the go helper function are:
perm - the constructed permutation so far
g - the current generator value
n - the length of the available items
avail - the available items - i.e. items not yet selected to be part of the permutation
go simply adds a random element from avail to the permutation being constructed and recursively calls itself with the new avail list and new generator.
To only draw k random elements from xs, just start go at k instead of length xs:
shuffle2 xs k g = go [] g k xs
...
You could also use a temporary array (in the ST or IO monad) to implement a Fischer-Yates type algorithm. The shuffleM function in random-shuffle uses a yet completely different approach which you might find interesting.
Update: Here is an example of using an ST-array in a F-Y style algorithm:
import Control.Monad.Random
import Data.Array.ST
import Control.Monad
import Control.Monad.ST (runST, ST)
shuffle3 :: RandomGen g => Int -> g -> ([Int], g)
shuffle3 n g0 = runST $ do
arr <- newListArray (1,n) [1..n] :: ST s (STUArray s Int Int)
let step g i = do let (j,g') = randomR (1,n) g
-- swap i and j
a <- readArray arr i
b <- readArray arr j
writeArray arr j a
writeArray arr i b
return g'
g' <- foldM step g0 [1..n]
perm <- getElems arr
return (perm, g')
main = do
perm <- evalRandIO $ liftRand $ shuffle3 20
print perm
I've used the Fisher Yates Shuffle in C++ with a decent random number generator to great success. This approach is very efficient if you are willing to allocate an array for holding numbers 1 to 40.
Going the strict IO way requires to break down nub, bringing the condition into the tail recursion.
import System.Random
randsf :: (Eq a, Num a, Random a) => [a] -> IO [a]
randsf rs
| length rs > 6 = return rs
| otherwise = do
r <- randomRIO (1,40)
if elem r rs
then randsf rs
else randsf (r:rs)
main = do
rs <- randsf [] :: IO [Int]
print rs
If you know what you do unsafeInterleaveIO from System.IO.Unsafe can be handy, allowing you to generate lazy lists from IO. Functions like getContents work this way.
import Control.Monad
import System.Random
import System.IO.Unsafe
import Data.List
rands :: (Eq a, Num a, Random a) => IO [a]
rands = do
r <- randomRIO (1,40)
unsafeInterleaveIO $ liftM (r:) rands
main = do
rs <- rands :: IO [Int]
print . take 6 $ nub rs
You commented:
The goal is to learn how to build a list monadically using filtering. It's a raw newbie question
Maybe you should change the question title then! Anyways, this is quite a common task. I usually define a combinator with a general monadic type that does what I want, give it a descriptive name (which I didn't quite succeed in here :-) and then use it, like below
import Control.Monad
import System.Random
-- | 'myUntil': accumulate a list with unique action results until the list
-- satisfies a test
myUntil :: (Monad m, Eq a) => ([a] -> Bool) -> m a -> m [a]
myUntil test action = myUntil' test [] action where
myUntil' test resultSoFar action = do
if test resultSoFar then
return resultSoFar
else do
x <- action
let newResults = if x `elem` resultSoFar then resultSoFar
else resultSoFar ++ [x] -- x:resultSoFar
myUntil' test newResults action
main :: IO ()
main = do
let enough xs = length xs == 6
drawNumber = randomRIO (0, 40::Int)
numbers <- myUntil enough drawNumber
print numbers
NB: this is not the optimal way to get your 6 distinct numbers, but meant as an example how to, in general, build a list monadically using a filter that works on the entire list
It is in essence the same as Vektorweg's longest answer, but uses a combinator with a much more general type (which is the way I like to do it, which may be more useful for you, given your comment at the top of this answer)

Haskell - How do I get Random Points (Int,Int)

I'm trying to get a set of random points (x,y) for drawing graph nodes to a screen. I need one randomly generated point for each node name passed in.
I found this code on a SO page, and modified it slightly to work for me, but it doesn't really do what I need.
I need a list of random (as random as possible) (Int,Int).
Anyway, here is what I have so far, and of course, it gives the same values every time, so it isn't particularly random :)
rndPoints :: [String] -> [Point]
rndPoints [] = []
rndPoints xs = zip x y where
size = length xs
x = take size (tail (map fst $ scanl (\(r, gen) _ -> randomR (25::Int,1000::Int) gen) (random (mkStdGen 1)) $ repeat ()))
y = take size (tail (map fst $ scanl (\(r, gen) _ -> randomR (25::Int,775::Int) gen) (random (mkStdGen 1)) $ repeat ()))
Any help would be much appreciated.
First, let's clean up your code a bit. There is a plural version of randomR that delivers an infinite list of random values: randomRs. This simplifies things a bit:
rndPoints1 :: [String] -> [Point]
rndPoints1 [] = []
rndPoints1 xs = zip x y
where
size = length xs
x = take size $ randomRs (25, 1000) (mkStdGen 1)
y = take size $ randomRs (25, 775) (mkStdGen 1)
We can simplify that further, by using zip's property that it stops after the shorter list is exhausted:
rndPoints2 :: [a] -> [Point]
rndPoints2 xs = map snd $ zip xs $ zip x y
where
x = randomRs (25, 1000) (mkStdGen 1)
y = randomRs (25, 775) (mkStdGen 1)
Notice I've also generalized the type of incoming list to just [a]. Since the values are never used, they needn't be Strings!
Now, it gives the same value every time because it uses mkStdGen to create a pseudo-random generator from the same seed (1) each time. If you want it to be different each time, then you need to create a generator in IO which can be based on the radom state of the computer. Rather than put the whole computation in IO, it is cleaner to pass in a StdGen:
rndPoints3 :: StdGen -> [Point]
rndPoints3 sg = zip x y
where
(sg1, sg2) = split sg
x = randomRs (25, 1000) sg1
y = randomRs (25, 775) sg2
pointsForLabels :: [a] -> StdGen -> [(a, Point)]
pointsForLabels xs sg = zip xs $ rndPoints3 sg
example3 :: [a] -> IO [(a, Point)]
example3 xs = newStdGen >>= return . pointsForLabels xs
Here, newStdGen creates a new pseudo-random generator each time, but it is in IO. That is passed eventually to a pure (non-IO) function rndPoints3 that takes the generator, and returns an infinite list of random Points. Within that function, split is used to create two generators from it, and each is used to derive the random list of coordinates.
pointsForLables now separates out the logic of matching up a new random point for each label. I also changed it to return the more likely useful pairs of labels and Points.
Finally, example3 lives in IO, and creates the generator and passes it all into the otherwise pure code.
I ended up using MonadRandom for this. I think the code was a little clearer and easier for me to understand. You could adapt the following code to address the original question.
import Control.Applicative
import Control.Monad.Random
type Point = (Float, Float)
type Poly = [Point]
randomScalar :: (RandomGen g) => Rand g Float
randomScalar = getRandomR (-500, 500)
randomPoint :: (RandomGen g) => Rand g Point
randomPoint = (,) <$> randomScalar <*> randomScalar
randomPoly :: (RandomGen g) => Int -> Rand g Poly
randomPoly n = sequence (replicate n randomPoint)

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