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Chapter 1 Tasks #557

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Chapter 1 Tasks
naprof007 Mar 23, 2023
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Chapter 1 Solutions fixes
naprof007 Mar 24, 2023
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Chapter 2 Solution
naprof007 Mar 24, 2023
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Chapter 4 Solutions
naprof007 Mar 25, 2023
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70 changes: 42 additions & 28 deletions src/Chapter1.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -209,31 +209,31 @@ 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
<INSERT THE RESULT INSTEAD OF THE TEXT>
True :: Bool
>>> :t 'a'
<INSERT THE RESULT INSTEAD OF THE TEXT>
'a' :: Char
>>> :t 42
<INSERT THE RESULT INSTEAD OF THE TEXT>
42 :: Num a => a

A pair of boolean and char:
>>> :t (True, 'x')
<INSERT THE RESULT INSTEAD OF THE TEXT>
(True, 'x') :: (Bool, Char)

Boolean negation:
>>> :t not
<INSERT THE RESULT INSTEAD OF THE TEXT>
not :: Bool -> Bool

Boolean 'and' operator:
>>> :t (&&)
<INSERT THE RESULT INSTEAD OF THE TEXT>
(&&) :: Bool -> Bool -> Bool

Addition of two numbers:
>>> :t (+)
<INSERT THE RESULT INSTEAD OF THE TEXT>
(+) :: Num a => a -> a -> a

Maximum of two values:
>>> :t max
<INSERT THE RESULT INSTEAD OF THE TEXT>
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.
Expand Down Expand Up @@ -301,43 +301,43 @@ expressions in GHCi
functions and operators first. Remember this from the previous task? ;)

>>> 1 + 2
<INSERT THE RESULT INSTEAD OF THE TEXT>
3

>>> 10 - 15
<INSERT THE RESULT INSTEAD OF THE TEXT>
-5

>>> 10 - (-5) -- negative constants require ()
<INSERT THE RESULT INSTEAD OF THE TEXT>
15

>>> (3 + 5) < 10
<INSERT THE RESULT INSTEAD OF THE TEXT>
True

>>> True && False
<INSERT THE RESULT INSTEAD OF THE TEXT>
False

>>> 10 < 20 || 20 < 5
<INSERT THE RESULT INSTEAD OF THE TEXT>
True

>>> 2 ^ 10 -- power
<INSERT THE RESULT INSTEAD OF THE TEXT>
1024

>>> not False
<INSERT THE RESULT INSTEAD OF THE TEXT>
True

>>> div 20 3 -- integral division
<INSERT THE RESULT INSTEAD OF THE TEXT>
6

>>> mod 20 3 -- integral division remainder
<INSERT THE RESULT INSTEAD OF THE TEXT>
2

>>> max 4 10
<INSERT THE RESULT INSTEAD OF THE TEXT>
10

>>> min 5 (max 1 2)
<INSERT THE RESULT INSTEAD OF THE TEXT>
2

>>> max (min 1 10) (min 5 7)
<INSERT THE RESULT INSTEAD OF THE TEXT>
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
Expand Down Expand Up @@ -429,6 +429,7 @@ task is to specify the type of this function.
49
-}

squareSum :: Num a => a -> a -> a
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We explain Num in the following chapters, so you are one step ahead 😸

In here Int -> Int -> Int would also work, as a more specific type 🙂

squareSum x y = (x + y) * (x + y)


Expand All @@ -449,7 +450,7 @@ 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
Expand Down Expand Up @@ -490,7 +491,8 @@ 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 :: Integral a => a -> a
lastDigit n = abs n `mod` 10


{- |
Expand Down Expand Up @@ -520,7 +522,7 @@ 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 abs x < abs y then x else y


{- |
Expand Down Expand Up @@ -554,7 +556,11 @@ 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 :: Ord a => a -> a -> a -> a
mid x y z
| x <= y && x > z || x >= y && x < z = x
| y <= x && y > z || y >= x && y < z = y
| otherwise = z

{- |
=⚔️= Task 8
Expand All @@ -568,7 +574,10 @@ True
>>> isVowel 'x'
False
-}
isVowel c = error "isVowel: not implemented!"
isVowel :: Char -> Bool
isVowel c
| c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' = True
| otherwise = False
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Yes, this is right!

As || operator already returns the Bool, you can also skip the cases and use just it:

Suggested change
| c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' = True
| otherwise = False
= c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u'

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Yes, this is right!

As || operator already returns the Bool, you can also skip the cases and use just it:

Oh, yeah :) I've just forgot it for a moment))



{- |
Expand Down Expand Up @@ -632,7 +641,11 @@ 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 :: Integral a => a -> a
sumLast2 n = b + a where
a = an `mod` 10
b = (an `div` 10) `mod` 10
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That is a wonderful solution! 👏🏼 You correctly noticed that it is the div and mod, cool 😎

One hint to make your solution even shorter: you can see that you use both:

mod m 10
div m 10

The standard library has the divMod function, that actually combines inside both div and mod. And this is exactly what you use!.

So you could write it this way:

(x, y) = divMod m 10

You can see how we could pattern match on the pair 🙂

an = abs n


{- |
Expand All @@ -653,7 +666,8 @@ 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 :: Integral t => t -> t
firstDigit n = let an = abs n in if an < 10 then an else firstDigit (an `div` 10)


{-
Expand Down
66 changes: 54 additions & 12 deletions src/Chapter2.hs
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Expand Up @@ -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
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


-}
Expand All @@ -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!"]



Expand Down Expand Up @@ -336,7 +359,9 @@ 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 (b - a + 1) (drop a xs)

{- |
=⚔️= Task 4
Expand All @@ -349,7 +374,8 @@ 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 (length l `div` 2) l


{- |
Expand Down Expand Up @@ -501,7 +527,9 @@ True
>>> isThird42 [42, 42, 0, 42]
False
-}
isThird42 = error "isThird42: Not implemented!"
isThird42 :: (Eq a, Num a) => [a] -> Bool
isThird42 (_ : _ : 42 : _) = True
isThird42 _ = False


{- |
Expand Down Expand Up @@ -606,7 +634,8 @@ 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


{- |
Expand All @@ -621,7 +650,10 @@ 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 [] = []
takeEven [x] = [x]
takeEven (x:_:xs) = x : takeEven xs
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That's great!

Note, that in the first two cases you return the list as it is. In this case, you could also right the function with just two cases, by switching them around:

takeEven (x:_:xs) = x : takeEven xs
takeEven xs = xs

Though, I think that your solution is more explicit, therefore, more readable! 👏🏼


{- |
=🛡= Higher-order functions
Expand Down Expand Up @@ -728,7 +760,7 @@ value of the element itself
🕯 HINT: Use combination of 'map' and 'replicate'
-}
smartReplicate :: [Int] -> [Int]
smartReplicate l = error "smartReplicate: Not implemented!"
smartReplicate l = concat (map (\x -> replicate x x) l)
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Great! 👍🏼
If you want to write this function even more elegantly, you can use the standard function concatMap which is a combination of concat and map as the name suggests 🙂
After that, you could notice, that you would be able to eta-reduce on the last argument :)


{- |
=⚔️= Task 9
Expand All @@ -741,7 +773,8 @@ 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 :: (Foldable t, Eq a) => a -> [t a] -> [t a]
contains n = filter (n `elem`)


{- |
Expand Down Expand Up @@ -781,13 +814,15 @@ 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 :: Ord a => a -> [a] -> [a]
listElementsLessThan x = filter (< x)

-- Can you eta-reduce this one???
pairMul xs ys = zipWith (*) xs ys
pairMul :: Num c => [c] -> [c] -> [c]
pairMul = zipWith (*)
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Eta-reduction award 🏆


{- |
=🛡= Lazy evaluation
Expand Down Expand Up @@ -842,7 +877,10 @@ list.

🕯 HINT: Use the 'cycle' function
-}
rotate = error "rotate: Not implemented!"
rotate :: Int -> [a] -> [a]
rotate n l
| n < 0 = []
| otherwise = take (length l) (drop n (cycle l))
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Awesome!

You can optimise it a little bit by using mod n (length l) as in this case you won't overgo if n is too huge 🙂


{- |
=💣= Task 12*
Expand All @@ -858,7 +896,11 @@ 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 [] = []
rewind l = rewind' [] l where
rewind' reversed [] = reversed
rewind' reversed (x:xs) = rewind' (x : reversed) xs
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Fantastic solution 🎉

Nothe, that there is no need to pattern match on the empty list in the rewind itself as you check this case in rewind' anyway, so you can get rid of this case 🙂



{-
Expand Down
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