# Euler problems/181 to 190

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## Revision as of 19:53, 24 March 2008

## Contents |

## 1 Problem 181

Investigating in how many ways objects of two different colours can be grouped.

## 2 Problem 182

RSA encryption.

Solution:

fun a1 b1 = sum [ e | e <- [2..a*b-1], gcd e (a*b) == 1, gcd (e-1) a == 2, gcd (e-1) b == 2 ] where a = a1-1 b = b1-1 problem_182 = fun 1009 3643

## 3 Problem 183

Maximum product of parts.

Solution:

-- Does the decimal expansion of p/q terminate? terminating p q = 1 == reduce [2,5] (q `div` gcd p q) where reduce [] n = n reduce (x:xs) n | n `mod` x == 0 = reduce (x:xs) (n `div` x) | otherwise = reduce xs n -- The expression (round $ fromIntegral n / e) computes the integer k -- for which (n/k)^k is at a maximum. Also note that, given a rational number -- r and a natural number k, the decimal expansion of r^k terminates if -- and only if the decimal expansion of r does. answer = sum [if terminating n (round $ fromIntegral n / e) then -n else n | n <- [5 .. 10^4]] where e = exp 1 main = print answer

## 4 Problem 184

Triangles containing the origin.

Solution:

problem_184 = undefined

## 5 Problem 185

Number Mind

Solution:

This approach does **NOT** solve the problem in under a minute,
unless of course you are extremely lucky. The best time I've
seen so far has been about 76 seconds. Before I came up with
this code, I tried to search for the solution by generating a
list of all possible solutions based on the information given in
the guesses. This was feasible with the 5 digit problem, but
completely intractable with the 16 digit problem. The approach
here, simple yet effective, is to make a random guess, and then
vary each digit in the guess from [0..9], generating a score of
how well the guess matched the given numbermind clues. You then
*improve* the guess by selecting those digits that had a unique
low score. It turns out this approach converges rather quickly,
but can often be stuck in cycles, so we test for this and try a
differenct random first guess if a cycle is detected. Once you
run the program, you might have time for a cup of coffee, or
maybe even a dinner. HenryLaxen 2008-03-12

import Data.List import Control.Monad import Data.Char import System.Random type Mind = [([Char],Int)] values :: [Char] values = "0123456789" score :: [Char] -> [Char] -> Int score guess answer = foldr (\(a,b) y -> if a == b then y+1 else y) 0 (zip guess answer) scores :: Mind -> [Char] -> [Int] scores m g = map (\x -> abs ((snd x) - score (fst x) g)) m scoreMind :: Mind -> [Char] -> Int scoreMind m g = sum $ scores m g ex1 :: Mind ex1 = [("90342",2), ("39458",2), ("51545",2), ("34109",1), ("12531",1), ("70794",0)] ex2 :: Mind ex2 = [ ("5616185650518293",2), ("3847439647293047",1), ("5855462940810587",3), ("9742855507068353",3), ("4296849643607543",3), ("3174248439465858",1), ("4513559094146117",2), ("7890971548908067",3), ("8157356344118483",1), ("2615250744386899",2), ("8690095851526254",3), ("6375711915077050",1), ("6913859173121360",1), ("6442889055042768",2), ("2321386104303845",0), ("2326509471271448",2), ("5251583379644322",2), ("1748270476758276",3), ("4895722652190306",1), ("3041631117224635",3), ("1841236454324589",3), ("2659862637316867",2)] guesses :: [Char] -> Int -> [[Char]] guesses str pos = [ left ++ n:(tail right) | n<-values] where (left,right) = splitAt pos str bestGuess :: Mind -> [[Char]] -> [Int] bestGuess mind guesses = let scores = map (scoreMind mind) guesses bestScore = minimum scores bestGuesses = findIndices (==bestScore) scores in bestGuesses iterateGuesses :: Mind -> [Char] -> [Char] iterateGuesses mind value = let allguesses = map (guesses value) [0..(length value)-1] mins = map (bestGuess mind) allguesses in nextguess value mins nextguess :: [Char] -> [[Int]] -> [Char] nextguess prev mins = let choose x = if length (snd x) == 1 then intToDigit ((snd x)!!0) else fst x both = zip prev mins in foldr (\x y -> (choose x) : y) "" both iterateMind :: Mind -> [Char] -> [([Char], Int)] iterateMind mind n = let a = drop 2 $ inits $ iterate (iterateGuesses mind) n b = last $ takeWhile (\x -> (last x) `notElem` (init x)) a c = map (scoreMind mind) b in zip b c randomStart :: (Num a, Enum a) => a -> IO [Char] randomStart n = mapM (\_ -> getStdRandom (randomR ('0','9'))) [1..n] main :: IO () main = do let ex = ex1 x <- randomStart (length (fst (head ex))) let y = iterateMind ex x let done = (snd (last y) == 0) when done (putStrLn $ (fst.last) y) unless done main

Here's another solution, and this one squeaks by in just under a minute on my machine. The basic idea is to just do a back-tracking search, but with some semi-smart pruning and guess ordering. The code is in pretty much the order I wrote it, so most prefixes of this code should also compile. This also means you should be able to figure out what each function does one at a time.

import Control.Monad import Data.List import qualified Data.Set as S ensure p x = guard (p x) >> return x selectDistinct 0 _ = [[]] selectDistinct n [] = [] selectDistinct n (x:xs) = map (x:) (selectDistinct (n - 1) xs) ++ selectDistinct n xs data Environment a = Environment { guesses :: [(Int, [a])] , restrictions :: [S.Set a] , assignmentsLeft :: Int } deriving (Eq, Ord, Show) reorder e = e { guesses = sort . guesses $ e } domain = S.fromList "0123456789" initial = Environment gs (replicate a S.empty) a where a = length . snd . head $ gs gs = [(2, "5616185650518293"), (1, "3847439647293047"), (3, "5855462940810587"), (3, "9742855507068353"), (3, "4296849643607543"), (1, "3174248439465858"), (2, "4513559094146117"), (3, "7890971548908067"), (1, "8157356344118483"), (2, "2615250744386899"), (3, "8690095851526254"), (1, "6375711915077050"), (1, "6913859173121360"), (2, "6442889055042768"), (0, "2321386104303845"), (2, "2326509471271448"), (2, "5251583379644322"), (3, "1748270476758276"), (1, "4895722652190306"), (3, "3041631117224635"), (3, "1841236454324589"), (2, "2659862637316867")] acceptableCounts e = small >= 0 && big <= assignmentsLeft e where ns = (0:) . map fst . guesses $ e small = minimum ns big = maximum ns positions s = map fst . filter (not . snd) . zip [0..] . zipWith S.member s acceptableRestriction r (n, s) = length (positions s r) >= n acceptableRestrictions e = all (acceptableRestriction (restrictions e)) (guesses e) firstGuess = head . guesses sanityCheck e = acceptableRestrictions e && acceptableCounts e solve e@(Environment _ _ 0) = [[]] solve e@(Environment [] r _) = sequence $ map (S.toList . (domain S.\\)) r solve e' = do is <- m newE <- f is rest <- solve newE return $ interleaveAscIndices is (l is) rest where f = ensure sanityCheck . update e m = selectDistinct n (positions g (restrictions e)) e = reorder e' l = fst . flip splitAscIndices g (n, g) = firstGuess e splitAscIndices = indices 0 where indices _ [] xs = ([], xs) indices n (i:is) (x:xs) | i == n = let (b, e) = indices (n + 1) is xs in (x:b, e) | True = let (b, e) = indices (n + 1) (i:is) xs in (b, x:e) interleaveAscIndices = indices 0 where indices _ [] [] ys = ys indices n (i:is) (x:xs) ys | i == n = x : indices (n + 1) is xs ys | True = head ys : indices (n + 1) (i:is) (x:xs) (tail ys) update (Environment ((_, a):gs) r l) is = Environment newGs restriction (l - length is) where (assignment, newRestriction) = splitAscIndices is a (_, oldRestriction) = splitAscIndices is r restriction = zipWith S.insert newRestriction oldRestriction newGs = map updateEntry gs updateEntry (n', a') = (newN, newA) where (dropped, newA) = splitAscIndices is a' newN = n' - length (filter id $ zipWith (==) assignment dropped) problem_185 = head . solve $ initial

## 6 Problem 187

Semiprimes

Solution:

The solution to this problem isn't terribly difficult, once you know that the numbers the problem is referring to are called semiprimes. In fact there is an excellent write-up at: http://mathworld.wolfram.com/Semiprime.html which provides an explicit formula for the number of semiprimes less than a given number.

The problem with this formula is the use of pi(n) where pi is the number of primes less than n. For Mathematica users this is no problem since the function is built in, but for us poor Haskeller's we have to build it ourselves. This is what took the most time for me, to come up with a pi(n) function that can compute pi(50000000) in less than the lifetime of the universe. Thus I embarked on a long and circuitous journey that eventually led me to the PI module below, which does little more than read a 26MB file of prime numbers into memory and computes a map from which we can calculate pi(n). I am sure there must be a better way of doing this, and I look forward to this entry being amended (or replaced) with a more reasonable solution. HenryLaxen Mar 24, 2008

module PI (prime,primes,pI) where import Control.Monad.ST import System.IO.Unsafe import qualified Data.Map as M import Data.ByteString.Char8 (readFile,lines,readInt) import Data.Maybe import Data.Array.ST import Data.Array.IArray r = runSTUArray (do a <- newArray (0,3001134) 0 :: ST s (STUArray s Int Int) writeArray a 0 1 mapM_ (\(x,y) -> writeArray a x y) (zip [0..] primes) return a) {-# NOINLINE s #-} s = M.fromDistinctAscList $ map (\(x,y) -> (y,x)) (assocs r) prime n = r!n pI :: Int -> Int pI n = case M.splitLookup n s of (_,Just x,_) -> (x+1) (_,_,b) -> snd (head (M.assocs b)) {-# NOINLINE primes #-} primes :: [Int] primes = unsafePerformIO $ do l <- Data.ByteString.Char8.readFile "primes_to_5M.txt" let p = Data.ByteString.Char8.lines l r = map (fst . fromJust . readInt) p::[Int] return r import PI import Data.List s n k = ( n `div` (prime (k-1))) - k + 1 semiPrimeCount n = let last = pI $ floor $ sqrt (fromIntegral n) s k = pI ( n `div` (prime (k-1))) - k + 1 pI2 = foldl' (\x y -> s y + x) 0 [1..last] in pI2 main = print (semiPrimeCount 100000000)