diff --git a/src/Chapter2.hs b/src/Chapter2.hs index 812311d2e..fb16cc487 100644 --- a/src/Chapter2.hs +++ b/src/Chapter2.hs @@ -136,43 +136,52 @@ functions in GHCi and insert the corresponding resulting output below: List of booleans: >>> :t [True, False] +[True, False] :: [Bool] String is a list of characters: >>> :t "some string" +"some string" :: [Char] Empty list: >>> :t [] +[] :: [a] Append two lists: >>> :t (++) +(++) :: [a] -> [a] -> [a] Prepend an element at the beginning of a list: >>> :t (:) +(:) :: a -> [a] -> [a] Reverse a list: >>> :t reverse +reverse :: [a] -> [a] Take first N elements of a list: >>> :t take +take :: Int -> [a] -> [a] Create list from N same elements: >>> :t replicate +replicate :: Int -> a -> [a] Split a string by line breaks: >>> :t lines +lines :: String -> [String] Join a list of strings with line breaks: >>> :t unlines - +unlines :: [String] -> String -} @@ -186,31 +195,31 @@ Evaluate the following expressions in GHCi and insert the answers. Try to guess first, what you will see. >>> [10, 2] ++ [3, 1, 5] - +[10,2,3,1,5] >>> [] ++ [1, 4] -- [] is an empty list - +[1,4] >>> 3 : [1, 2] - +[3,1,2] >>> 4 : 2 : [5, 10] -- prepend multiple elements - +[4,2,5,10] >>> [1 .. 10] -- list ranges - +[1,2,3,4,5,6,7,8,9,10] >>> [10 .. 1] - +[] >>> [10, 9 .. 1] -- backwards list with explicit step - +[10,9,8,7,6,5,4,3,2,1] >>> length [4, 10, 5] -- list length - +3 >>> replicate 5 True - +[True,True,True,True,True] >>> take 5 "Hello, World!" - +"Hello" >>> drop 5 "Hello, World!" - +", World!" >>> zip "abc" [1, 2, 3] -- convert two lists to a single list of pairs - +[('a',1),('b',2),('c',3)] >>> words "Hello Haskell World!" -- split the string into the list of words - +["Hello","Haskell","World!"] 👩‍🔬 Haskell has a lot of syntax sugar. In the case with lists, any @@ -336,7 +345,9 @@ from it! ghci> :l src/Chapter2.hs -} subList :: Int -> Int -> [a] -> [a] -subList = error "subList: Not implemented!" +subList from to l + | from < 0 || to < 0 || to < from = [] + | otherwise = take (to - from + 1) (drop from l) {- | =⚔️= Task 4 @@ -348,8 +359,10 @@ Implement a function that returns only the first half of a given list. >>> firstHalf "bca" "b" -} --- PUT THE FUNCTION TYPE IN HERE -firstHalf l = error "firstHalf: Not implemented!" +firstHalf :: [a] -> [a] +firstHalf l = + let halfLen = div (length l) 2 + in take halfLen l {- | @@ -500,7 +513,9 @@ True >>> isThird42 [42, 42, 0, 42] False -} -isThird42 = error "isThird42: Not implemented!" +isThird42 :: [Int] -> Bool +isThird42 (_ : _ : 42 : _) = True +isThird42 _ = False {- | @@ -605,7 +620,8 @@ Implement a function that duplicates each element of the list -} duplicate :: [a] -> [a] -duplicate = error "duplicate: Not implemented!" +duplicate [] = [] +duplicate (x:xs) = x : x : duplicate xs {- | @@ -620,7 +636,10 @@ Write a function that takes elements of a list only on even positions. >>> takeEven [2, 1, 3, 5, 4] [2,3,4] -} -takeEven = error "takeEven: Not implemented!" +takeEven :: [a] -> [a] +takeEven [] = [] +takeEven [x] = [x] +takeEven (x : _ : xs) = x : takeEven xs {- | =🛡= Higher-order functions @@ -727,7 +746,7 @@ value of the element itself 🕯 HINT: Use combination of 'map' and 'replicate' -} smartReplicate :: [Int] -> [Int] -smartReplicate l = error "smartReplicate: Not implemented!" +smartReplicate = concatMap (\x -> replicate x x) {- | =⚔️= Task 9 @@ -740,7 +759,8 @@ the list with only those lists that contain a passed element. 🕯 HINT: Use the 'elem' function to check whether an element belongs to a list -} -contains = error "contains: Not implemented!" +contains :: Int -> [[Int]] -> [[Int]] +contains x = filter (elem x) {- | @@ -780,13 +800,15 @@ Let's now try to eta-reduce some of the functions and ensure that we mastered the skill of eta-reducing. -} divideTenBy :: Int -> Int -divideTenBy x = div 10 x +divideTenBy = div 10 -- TODO: type ;) -listElementsLessThan x l = filter (< x) l +listElementsLessThan :: Int -> [Int] -> [Int] +listElementsLessThan x = filter (< x) -- Can you eta-reduce this one??? -pairMul xs ys = zipWith (*) xs ys +pairMul :: [Int] -> [Int] -> [Int] +pairMul = zipWith (*) {- | =🛡= Lazy evaluation @@ -841,7 +863,13 @@ list. 🕯 HINT: Use the 'cycle' function -} -rotate = error "rotate: Not implemented!" +rotate :: Int -> [a] -> [a] +rotate n l + | n < 0 = [] + | null l = [] + | otherwise = + let len = length l + in take len (drop (mod n len) (cycle l)) {- | =💣= Task 12* @@ -857,7 +885,12 @@ and reverses it. function, but in this task, you need to implement it manually. No cheating! -} -rewind = error "rewind: Not Implemented!" +rewind :: [a] -> [a] +rewind = go [] + where + go :: [a] -> [a] -> [a] + go res [] = res + go res (x:xs) = go (x : res) xs {-