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Solutions for Chapter 1 #547

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Solutions for Chapter 1 #547

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ee10bbc
Part 1 for Chapter 1
Oct 13, 2022
51b36ed
Finished Chapter1
Oct 14, 2022
d2df71a
Kleinigkeiten...
Oct 14, 2022
d12851b
Handle negative numbers correctly
Oct 14, 2022
f632b62
Omit redundant brackets
guntbert Oct 27, 2022
4640b1d
Fix firstDigit for negative numbers
Oct 27, 2022
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73 changes: 43 additions & 30 deletions src/Chapter1.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -72,6 +72,7 @@ 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;
Expand Down Expand Up @@ -209,31 +210,31 @@ 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 p => p

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 +302,43 @@ 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 +430,7 @@ task is to specify the type of this function.
49
-}

squareSum :: Int -> Int -> Int
squareSum x y = (x + y) * (x + y)


Expand All @@ -448,8 +450,9 @@ Implement the function that takes an integer value and returns the next 'Int'.
every type ｑ.β˜†.*ｑ. No need to worry much about "error" here, just replace the
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 +493,8 @@ 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


{- |
Expand Down Expand Up @@ -520,7 +524,7 @@ 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 @@ -553,8 +557,8 @@ 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 = head (tail(sort[x,y,z]))
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{- |
=βš”οΈ= Task 8
Expand All @@ -568,7 +572,10 @@ True
>>> isVowel 'x'
False
-}
isVowel c = error "isVowel: not implemented!"
isVowel :: Char -> Bool
isVowel c
| elem c ['a', 'e', 'i', 'o', 'u'] = True
| otherwise = False
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{- |
Expand Down Expand Up @@ -631,8 +638,11 @@ Implement a function that returns the sum of the last two digits of a number.
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 n =
let digit1 = mod (abs n) 10
digit2 = mod (div (abs n) 10) 10
Comment on lines +643 to +644
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That is a wonderful solution! πŸ‘πŸΌ 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 πŸ™‚

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Hmm...
I fail to see how this would be simpler - I need to apply the modulo-division by 10 to the result of the first division whereas divMod only applies it to the original value albeit as div and as mod at the same time.

in digit1 + digit2


{- |
Expand All @@ -653,7 +663,10 @@ 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 n
| abs n < 10 = abs n
| otherwise = firstDigit (div (abs n) 10)


{-
Expand Down