Thank You developers!! #514

abhideepd · 2022-02-26T15:45:09Z

First of all, thank you for the hard work and thoughtfulness given to create this course, it has really helped me get my heads around Haskell.

It would make my day if you could suggest me some areas of improvements or some suggestions to hone skills in Haskell and Functional Programming

Regards

Solutions for Chapter {1}

cc @vrom911 @chshersh

vrom911

Fantastic work 💯
I have added some comment, hope it would be helpful 🙂

vrom911 · 2022-03-07T09:44:43Z

src/Chapter1.hs

@@ -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


Great!
In Haskell, all operators are separated by spaces from its arguments 👾
So it would be:

Suggested change

next x = x+1

next x = x + 1

vrom911 · 2022-03-07T09:49:35Z

src/Chapter1.hs

@@ -490,7 +491,7 @@ 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 n = mod (abs n) 10


Nice work!

We don't explain this in our course, but you can also use the infix form of calling functions! (Meaning, the function can be written in between of its arguments):slightly_smiling_face:

lastDigit n = abs n `mod` 10

(Note that you can remove (), they are redundant in that case )

You can read a bit more on the infix here: https://kowainik.github.io/posts/fixity#notation

vrom911 · 2022-03-07T09:50:47Z

src/Chapter1.hs

@@ -520,7 +521,7 @@ 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


No need to have brackets here. You can skip it in if-then-else constructions:

Suggested change

closestToZero x y = if (abs x < abs y) then x else y

closestToZero x y = if abs x < abs y then x else y

vrom911 · 2022-03-07T09:53:28Z

src/Chapter1.hs

+    | y<=(max x z)&&y>=(min x z)=y
+    | x<=(max z y)&&x>=(min z y)=x
+    | otherwise =z


vrom911 · 2022-03-07T09:58:34Z

src/Chapter1.hs

@@ -632,8 +642,7 @@ 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 n = if (abs n)<10 then (abs n) else (if (abs n)<100 then (div (abs n) 10)+(mod (abs n) 10) else sumLast2 (mod (abs n) 100))


Wow, one-liner 😮 Nice recursive solution!

It is a bit difficult to read though 🙂 I would suggest to align it at different lines first:

Suggested change

sumLast2 n = if (abs n)<10 then (abs n) else (if (abs n)<100 then (div (abs n) 10)+(mod (abs n) 10) else sumLast2 (mod (abs n) 100))

sumLast2 n =

if abs n < 10

then abs n

else

if abs n < 100

then div (abs n) 10 + mod (abs n) 10

else sumLast2 (mod (abs n) 100)

Then I see that 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 🙂

vrom911 · 2022-03-07T10:12:58Z

src/Chapter2.hs

+    | null x=[]
+    | otherwise=(head x):(head x):duplicate (tail x)


Here it is also better to use pattern matching instead of using unsafe head function. (It is also more efficient 🙂 )

vrom911 · 2022-03-07T10:13:11Z

src/Chapter2.hs

+takeEven x
+    | ((length x) == 0) = []
+    | ((length x) == 1) = head x:[]
+    | otherwise=(head x):takeEven (tail (tail x))


Same here 🙂

vrom911 · 2022-03-07T10:14:03Z

src/Chapter2.hs

-contains = error "contains: Not implemented!"
-
+--contains = error "contains: Not implemented!"
+contains a x= filter (elem a) x


Now, after you've mastered eta-reduction, you can apply such a technique to this function as well 🙂
But your implementation is already great 💯

vrom911 · 2022-03-07T10:14:20Z

src/Chapter2.hs


 -- Can you eta-reduce this one???
-pairMul xs ys = zipWith (*) xs ys
+pairMul  = zipWith (*) 


Eta-reduction award 🏆

vrom911 · 2022-03-07T10:14:58Z

src/Chapter2.hs

+--rewind = error "rewind: Not Implemented!"
+rewind x
+    | null x=[]
+    | otherwise=(rewind (tail x))++((head x):[])


Your solution is correct! However, it is slow. In lists, it is quite slow to add anything at the end of the list. That is why it is always better to rewrite it with the : cons. Remember the explanation with trains? 🚂 🚋 🚋

That is why a more efficient solution is with the accumulator and the recursive function that will do the addition at the start of the list which is instant!

You can read a bit more about the go pattern in here: https://kowainik.github.io/posts/haskell-mini-patterns#recursive-go

abhideepd added 2 commits February 23, 2022 13:48

Update Chapter1.hs

55e57a3

final

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abhideepd requested review from chshersh and vrom911 as code owners February 26, 2022 15:45

abhideepd added 3 commits February 26, 2022 21:55

changes made for advanced test

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solutions

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chapter 2 final updates for advanced test

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vrom911 approved these changes Mar 7, 2022

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Thank You developers!! #514

Thank You developers!! #514

abhideepd commented Feb 26, 2022

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	closestToZero x y = if (abs x < abs y) then x else y
	closestToZero x y = if abs x < abs y then x else y

-sumLast2 n = if (abs n)<10 then (abs n) else (if (abs n)<100 then (div (abs n) 10)+(mod (abs n) 10) else sumLast2 (mod (abs n) 100))
+sumLast2 n =
+    if  abs n < 10
+    then abs n
+    else
+        if abs n < 100
+        then div (abs n) 10 + mod (abs n) 10
+        else sumLast2 (mod (abs n) 100)

Thank You developers!! #514

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Thank You developers!! #514

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abhideepd commented Feb 26, 2022

Solutions for Chapter {1}

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