# Difference between revisions of "99 questions/Solutions/82"

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< 99 questions | Solutions

(Solution to Problem 82 using list comprehension added) |
(list as monad) |
||

(One intermediate revision by one other user not shown) | |||

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Brute-force search from the source, using list comprehension: | Brute-force search from the source, using list comprehension: | ||

+ | |||

+ | <haskell> | ||

import Data.List (partition) | import Data.List (partition) | ||

cycle' :: Int -> [(Int, Int)] -> [ [Int] ] | cycle' :: Int -> [(Int, Int)] -> [ [Int] ] | ||

Line 10: | Line 12: | ||

arrive = fst split | arrive = fst split | ||

go ls = [ x ++ [snd y] | x <- ls, y <- g, last x == fst y, not (snd y `elem` tail x)] | go ls = [ x ++ [snd y] | x <- ls, y <- g, last x == fst y, not (snd y `elem` tail x)] | ||

+ | |||

+ | </haskell> | ||

+ | |||

+ | ---- | ||

+ | another approach using list as monad | ||

+ | |||

+ | <haskell> | ||

+ | cycles :: (Eq a) => a -> [Arc a] -> [[a]] | ||

+ | cycles _ [] = [] | ||

+ | cycles start arcs = | ||

+ | map (map fst) $ aux start [] | ||

+ | where | ||

+ | aux current pathSoFar = | ||

+ | let nextEdges = filter ((== current) . fst) arcs | ||

+ | notCyclic = not . (\(_,t) -> (elem t $ map snd pathSoFar)) | ||

+ | noCycles = filter notCyclic nextEdges | ||

+ | in noCycles >>= \(f,t) -> do | ||

+ | if (t == start) then return $ pathSoFar ++ (f,t):[(t,t)] | ||

+ | else aux t (pathSoFar ++ [(f,t)]) | ||

+ | </haskell> | ||

+ | |||

+ | [[Category:Programming exercise spoilers]] |

## Latest revision as of 06:29, 31 July 2021

Brute-force search from the source, using list comprehension:

```
import Data.List (partition)
cycle' :: Int -> [(Int, Int)] -> [ [Int] ]
cycle' n g = search [[n]] []
where search [] result = result
search cur result = search (go active) (arrive ++ result)
where split = partition end cur
end s = (last s == n) && (length s /= 1)
active = snd split
arrive = fst split
go ls = [ x ++ [snd y] | x <- ls, y <- g, last x == fst y, not (snd y `elem` tail x)]
```

another approach using list as monad

```
cycles :: (Eq a) => a -> [Arc a] -> [[a]]
cycles _ [] = []
cycles start arcs =
map (map fst) $ aux start []
where
aux current pathSoFar =
let nextEdges = filter ((== current) . fst) arcs
notCyclic = not . (\(_,t) -> (elem t $ map snd pathSoFar))
noCycles = filter notCyclic nextEdges
in noCycles >>= \(f,t) -> do
if (t == start) then return $ pathSoFar ++ (f,t):[(t,t)]
else aux t (pathSoFar ++ [(f,t)])
```