Lecture 3 solutions

src/Lecture1.hs

@@ -31,14 +31,11 @@ module Lecture1
     , lowerAndGreater
     ) where
 
--- VVV If you need to import libraries, do it after this line ... VVV
-
--- ^^^ and before this line. Otherwise the test suite might fail  ^^^
-
 {- | Specify the type signature of the following function. Think about
 its behaviour, possible types for the function arguments and write the
 type signature explicitly.
 -}
+makeSnippet :: Int -> [Char] -> [Char]
 makeSnippet limit text = take limit ("Description: " ++ text) ++ "..."
 
 {- | Implement a function that takes two numbers and finds sum of
@@ -53,8 +50,8 @@ their squares.
 Explanation: @sumOfSquares 3 4@ should be equal to @9 + 16@ and this
 is 25.
 -}
--- DON'T FORGET TO SPECIFY THE TYPE IN HERE
-sumOfSquares x y = error "TODO!"
+sumOfSquares :: Int -> Int -> Int
+sumOfSquares x y = x * x + y * y
 
 {- | Implement a function that returns the last digit of a given number.
 
@@ -66,8 +63,8 @@ sumOfSquares x y = error "TODO!"
 🕯 HINT: use the @mod@ function
 
 -}
--- DON'T FORGET TO SPECIFY THE TYPE IN HERE
-lastDigit n = error "lastDigit: Not implemented!"
+lastDigit :: Int -> Int
+lastDigit n = mod (abs n) 10
 
 {- | Write a function that takes three numbers and returns the
 difference between the biggest number and the smallest one.
@@ -81,7 +78,11 @@ and 1 is the smallest, and 7 - 1 = 6.
 Try to use local variables (either let-in or where) to implement this
 function.
 -}
-minmax x y z = error "TODO"
+minmax :: Int -> Int -> Int -> Int
+minmax x y z =
+    let maxVal = max x (max y z)
+        minVal = min x (min y z)
+    in maxVal - minVal
 
 {- | Implement a function that takes a string, start and end positions
 and returns a substring of a given string from the start position to
@@ -98,7 +99,16 @@ start position can be considered as zero (e.g. substring from the
 first character) and negative end position should result in an empty
 string.
 -}
-subString start end str = error "TODO"
+subString :: Int -> Int -> [Char] -> [Char]
+subString start end str
+    | end < 0   = ""
+    | start < 0 = take (end + 1) str
+    | otherwise = take (end - start + 1) (drop start str)
+
+{- Alternative one-line implementation:
+
+subString start end str = take (max (-1) end - max 0 start + 1) (drop start str)
+-}
 
 {- | Write a function that takes a String — space separated numbers,
 and finds a sum of the numbers inside this string.
@@ -108,7 +118,8 @@ and finds a sum of the numbers inside this string.
 
 The string contains only spaces and/or numbers.
 -}
-strSum str = error "TODO"
+strSum :: [Char] -> Int
+strSum str = sum (map read (words str))
 
 {- | Write a function that takes a number and a list of numbers and
 returns a string, saying how many elements of the list are strictly
@@ -123,4 +134,22 @@ and lower than 6 elements (4, 5, 6, 7, 8 and 9).
 
 🕯 HINT: Use recursion to implement this function.
 -}
-lowerAndGreater n list = error "TODO"
+lowerAndGreater :: Int -> [Int] -> [Char]
+lowerAndGreater n list = go 0 0 list
+  where
+    go :: Int -> Int -> [Int] -> [Char]
+    go lower greater l
+        | null l     = display lower greater
+        | head l > n = go (lower + 1) greater (tail l)
+        | head l < n = go lower (greater + 1) (tail l)
+        | otherwise  = go lower greater (tail l)
+
+    display :: Int -> Int -> [Char]
+    display lower greater =
+        show n
+        ++ " is greater than "
+        ++ show greater
+        ++ " elements and lower than "
+        ++ show lower
+        ++ " elements"
+

src/Lecture2.hs

@@ -40,10 +40,6 @@ module Lecture2
     , constantFolding
     ) where
 
--- VVV If you need to import libraries, do it after this line ... VVV
-
--- ^^^ and before this line. Otherwise the test suite might fail  ^^^
-
 {- | Implement a function that finds a product of all the numbers in
 the list. But implement a lazier version of this function: if you see
 zero, you can stop calculating product and return 0 immediately.
@@ -167,12 +163,12 @@ data Knight = Knight
 dragonFight = error "TODO"
 
 ----------------------------------------------------------------------------
--- Extra Challenges
+-- Challenges
 ----------------------------------------------------------------------------
 
-{- The following exercises are considered optional. Some of them might be more
-challenging. However, you still may find some of them easier than some of the
-previous ones. Difficulty is a relative concept.
+{- The following exercises are considered more challenging. However,
+you still may find some of them easier than some of the previous
+ones. Difficulty is a relative concept.
 -}
 
 {- | Write a function that takes a list of numbers and returns 'True'
@@ -293,3 +289,4 @@ Folding" optimization on the given expression.
 -}
 constantFolding :: Expr -> Expr
 constantFolding = error "TODO"
+

src/Lecture3.hs

@@ -34,9 +34,6 @@ module Lecture3
     , apply
     ) where
 
--- VVV If you need to import libraries, do it after this line ... VVV
-
--- ^^^ and before this line. Otherwise the test suite might fail  ^^^
 
 -- $setup
 -- >>> import Data.Semigroup
@@ -250,3 +247,4 @@ Just [8,9,10]
 
 -}
 apply = error "TODO"
+