diff --git a/src/Chapter1.hs b/src/Chapter1.hs index 406deeaca..d5afb3e95 100644 --- a/src/Chapter1.hs +++ b/src/Chapter1.hs @@ -72,6 +72,9 @@ the `.hs` extension. -} module Chapter1 where +import Data.List (sort) + + {- | In Haskell, we have __expressions__. Expressions can be represented by some primitive values (numbers: 1, 100; characters: 'a', 'z'; booleans: True, False; @@ -209,31 +212,31 @@ So, the output in this example means that 'False' has type 'Bool'. > Try to guess first and then compare your expectations with GHCi output >>> :t True - +True :: Bool >>> :t 'a' - +'a' :: Char >>> :t 42 - +42 :: Num a => a A pair of boolean and char: >>> :t (True, 'x') - +(True, 'x') :: (Bool, Char) Boolean negation: >>> :t not - +not :: Bool -> Bool Boolean 'and' operator: >>> :t (&&) - +(&&) :: Bool -> Bool -> Bool Addition of two numbers: >>> :t (+) - +(+) :: Num a => a -> a -> a Maximum of two values: >>> :t max - +max :: Ord a => a -> a -> a You might not understand each type at this moment, but don't worry! You've only started your Haskell journey. Types will become your friends soon. @@ -301,43 +304,43 @@ expressions in GHCi functions and operators first. Remember this from the previous task? ;) >>> 1 + 2 - +3 >>> 10 - 15 - +-5 >>> 10 - (-5) -- negative constants require () - +15 >>> (3 + 5) < 10 - +True >>> True && False - +False >>> 10 < 20 || 20 < 5 - +True >>> 2 ^ 10 -- power - +1024 >>> not False - +True >>> div 20 3 -- integral division - +6 >>> mod 20 3 -- integral division remainder - +2 >>> max 4 10 - +10 >>> min 5 (max 1 2) - +2 >>> max (min 1 10) (min 5 7) - +5 Because Haskell is a __statically-typed__ language, you see an error each time you try to mix values of different types in situations where you are not @@ -429,6 +432,7 @@ task is to specify the type of this function. 49 -} +squareSum :: Int -> Int -> Int squareSum x y = (x + y) * (x + y) @@ -449,7 +453,7 @@ Implement the function that takes an integer value and returns the next 'Int'. function body with the proper implementation. -} next :: Int -> Int -next x = error "next: not implemented!" +next x = x + 1 {- | After you've implemented the function (or even during the implementation), you @@ -490,7 +494,8 @@ Implement a function that returns the last digit of a given number. whether it works for you! -} -- DON'T FORGET TO SPECIFY THE TYPE IN HERE -lastDigit n = error "lastDigit: Not implemented!" +lastDigit :: Int -> Int +lastDigit n = mod (abs n) 10 {- | @@ -520,7 +525,10 @@ branches because it is an expression and it must always return some value. satisfying the check will be returned and, therefore, evaluated. -} closestToZero :: Int -> Int -> Int -closestToZero x y = error "closestToZero: not implemented!" +closestToZero x y = if a < b then x else y + where + a = abs x + b = abs y {- | @@ -553,8 +561,9 @@ value after "=" where the condition is true. Casual reminder about adding top-level type signatures for all functions :) -} - -mid x y z = error "mid: not implemented!" +mid :: Int -> Int -> Int -> Int +mid x y z = sort xs !! 1 + where xs = [x, y, z] {- | =⚔️= Task 8 @@ -568,7 +577,10 @@ True >>> isVowel 'x' False -} -isVowel c = error "isVowel: not implemented!" +isVowel :: Char -> Bool +isVowel c + | c `elem` ['a', 'e', 'i', 'o', 'u'] = True + | otherwise = False {- | @@ -632,7 +644,12 @@ Try to introduce variables in this task (either with let-in or where) to avoid specifying complex expressions. -} -sumLast2 n = error "sumLast2: Not implemented!" +sumLast2 :: Int -> Int +sumLast2 x = (+) (m n) (m a) + where + n = abs x + a = div n 10 + m o = mod o 10 {- | @@ -653,7 +670,10 @@ You need to use recursion in this task. Feel free to return to it later, if you aren't ready for this boss yet! -} -firstDigit n = error "firstDigit: Not implemented!" +firstDigit :: Int -> Int +firstDigit i = go $ abs i + where go n = if x == 0 then n else go x + where x = div n 10 {- diff --git a/src/Chapter2.hs b/src/Chapter2.hs index b98ceaf7d..d76b9d6d7 100644 --- a/src/Chapter2.hs +++ b/src/Chapter2.hs @@ -136,42 +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" :: String 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 +>5>> :t reverse +reverse :: [a] -> [a] Take first N elements of a list: >>> :t take +take :: Int -> [a] -> [a] Create a 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 -} @@ -186,30 +196,43 @@ 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!"] @@ -336,7 +359,10 @@ from it! ghci> :l src/Chapter2.hs -} subList :: Int -> Int -> [a] -> [a] -subList = error "subList: Not implemented!" +subList a b xs + | a < 0 || b < 0 = [] + | otherwise = take n (drop a xs) + where n = b - a + 1 {- | =⚔️= Task 4 @@ -349,7 +375,9 @@ Implement a function that returns only the first half of a given list. "b" -} -- PUT THE FUNCTION TYPE IN HERE -firstHalf l = error "firstHalf: Not implemented!" +firstHalf :: [a] -> [a] +firstHalf l = take n l + where n = length l `div` 2 {- | @@ -501,7 +529,9 @@ True >>> isThird42 [42, 42, 0, 42] False -} -isThird42 = error "isThird42: Not implemented!" +isThird42 :: (Num a, Eq a) => [a] -> Bool +isThird42 (_ : _ : 42 : _) = True +isThird42 _ = False {- | @@ -606,7 +636,8 @@ 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 {- | @@ -621,7 +652,10 @@ Write a function that takes elements of a list only in even positions. >>> takeEven [2, 1, 3, 5, 4] [2,3,4] -} -takeEven = error "takeEven: Not implemented!" +takeEven :: [a] -> [a] +takeEven (x : _ : xs) = x : takeEven xs +takeEven (x : xs) = x : takeEven xs +takeEven [] = [] {- | =🛡= Higher-order functions @@ -728,7 +762,8 @@ value of the element itself 🕯 HINT: Use combination of 'map' and 'replicate' -} smartReplicate :: [Int] -> [Int] -smartReplicate l = error "smartReplicate: Not implemented!" +smartReplicate [] = [] +smartReplicate (x:xs) = replicate x x ++ smartReplicate xs {- | =⚔️= Task 9 @@ -741,7 +776,8 @@ 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 n = filter (elem n) {- | @@ -781,13 +817,15 @@ 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 :: Num a => [a] -> [a] -> [a] +pairMul = zipWith (*) {- | =🛡= Lazy evaluation @@ -842,7 +880,11 @@ list. 🕯 HINT: Use the 'cycle' function -} -rotate = error "rotate: Not implemented!" +rotate :: Int -> [a] -> [a] +rotate r xs + | r < 0 = [] + | otherwise = take n $ drop r $ cycle xs + where n = length xs {- | =💣= Task 12* @@ -858,7 +900,11 @@ 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 lst = go lst [] + where + go [] y = y + go (x:xs) y = go xs (x:y) {-