# Haskell Quiz/DayRange/Solution Jethr0

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< Haskell Quiz | DayRange(Difference between revisions)

m (using a datatype, added type declarations) |
m (using a modified version of groupBy) |
||

Line 4: | Line 4: | ||

-- > dayRange [1,2,3,6,7] | -- > dayRange [1,2,3,6,7] | ||

-- "Mon-Wed, Sat, Sun" | -- "Mon-Wed, Sat, Sun" | ||

− | + | module DayRange where | |

+ | import Data.List (intersperse,sort) | ||

+ | -- > dayRange [1,2,3,6,7] | ||

+ | -- "Mon-Wed, Sat, Sun" | ||

+ | data Weekday = Mon | Tue | Wed | Thu | Fri | Sat | Sun deriving (Show,Enum) | ||

+ | |||

dayRange :: [Int] -> String | dayRange :: [Int] -> String | ||

− | dayRange = sepComma . map range . map (map toWeekday) . | + | dayRange = sepComma . map range . map (map toWeekday) . groupBy' (\a b -> a+1 == b) . sort |

where sepComma = concat . intersperse ", " | where sepComma = concat . intersperse ", " | ||

toWeekday x = show $ (toEnum (x-1) :: Weekday) | toWeekday x = show $ (toEnum (x-1) :: Weekday) | ||

Line 13: | Line 18: | ||

| otherwise = head xs ++ "-" ++ last xs | | otherwise = head xs ++ "-" ++ last xs | ||

− | -- | + | -- groupBy compares any element to the first one of the group |

− | + | -- groupBy' instead compares an element to the last added group element | |

− | + | groupBy' :: (a -> a -> Bool) -> [a] -> [[a]] | |

− | where | + | groupBy' f (x:xs) = gb f xs [[x]] |

− | else ( | + | where gb f (x:xs) ((a:as):bs) = gb f xs $ if f a x then ((x:a:as):bs) |

− | + | else ([x]:(a:as):bs) | |

+ | gb _ [] as = reverse . map reverse $ as | ||

</haskell> | </haskell> |

## Revision as of 17:41, 10 January 2007

-- > dayRange [1,2,3,6,7] -- "Mon-Wed, Sat, Sun" module DayRange where import Data.List (intersperse,sort) -- > dayRange [1,2,3,6,7] -- "Mon-Wed, Sat, Sun" data Weekday = Mon | Tue | Wed | Thu | Fri | Sat | Sun deriving (Show,Enum) dayRange :: [Int] -> String dayRange = sepComma . map range . map (map toWeekday) . groupBy' (\a b -> a+1 == b) . sort where sepComma = concat . intersperse ", " toWeekday x = show $ (toEnum (x-1) :: Weekday) range xs | length xs < 3 = sepComma xs | otherwise = head xs ++ "-" ++ last xs -- groupBy compares any element to the first one of the group -- groupBy' instead compares an element to the last added group element groupBy' :: (a -> a -> Bool) -> [a] -> [[a]] groupBy' f (x:xs) = gb f xs [[x]] where gb f (x:xs) ((a:as):bs) = gb f xs $ if f a x then ((x:a:as):bs) else ([x]:(a:as):bs) gb _ [] as = reverse . map reverse $ as