Haskell mapping function to list - haskell

I am new to Haskell and I have the following problem. I have to create a list of numbers [f1, f2, f3...] where fi x = x ^ i. Then I have to create a function that applies the fi to a list of numbers. For example if I have a list lis = [4,5,6,7..] the output would be [4^1, 5^2,6^3, 7^4...]. This is what I have written so far :
powers x= [x^y |y<-[1,2,3,4]]
list = [1,2,3,4]
match :: (x -> xs) -> [x] -> [xs]
match f [] = []
match f (x:xs) = (f x) : ( match f xs )
So if I put the list = [1,2,3] the output is [1,1,1,1][2,4,8,16],[3,9,27,81] instead of [1,4,27]
Can you please tell me what is wrong and point me to the right direction?

The first issue is that powers is of type Int -> [Int]. What you really want, I think, is something of type [Int -> Int] -- a list of Int -> Int functions instead of a function that takes an Int and returns a list of Int. If you define powers like so:
powers = [(^y) | y <- [1..4]]
you can use zipWith to apply each power to its corresponding element in the list, like so:
zipWith ($) powers [1,2,3] -- returns [1,4,27]
The ($) applies its left (first) argument to its right (second) argument.
Note that using powers as defined here will limit the length of the returned list to 4. If you want to be able to use arbitrary length lists, you want to make powers an infinite list, like so:
powers = [(^y) | y <- [1..]]
Of course, as dave4420 points out, a simpler technique is to simply use
zipWith (^) [1,2,3] [1..] -- returns [1,4,27]

Your match is the standard function map by another name. You need to use zipWith instead (which you can think of as mapping over two lists side-by-side).
Is this homework?

You are currently creating a list for every input value.
What you need to do is recursively compute the appropriate
power for each input value, like this:
match f [] = []
match f (x:xs) y = (f x y) : (match f xs y+1)
Then, you can call this as match pow [1, 2, 3] 1.
This is equivalent to using zipWith and providing the desired function (pow), your input list ([1, 2, 3]) and the exponent list (a lazy one to infinity list) as arguments.

Related

Haskell Functions (map,foldr, foldl)

I am struggling to think of a way to utilize these functions for this beginner level coding class that I am taking to learn functional programming in Haskell. The functions I have to write are shown below, asum is supposed to turn a list of integers [a1,a2,..,an] into the alternating sum a1-a2+a3-a4+.… and I am not sure how to approach it with these functions. The xor function is supposed to that computes the XOR of a list of Booleans. I need some help to understand how to use these functions and it would greatly appreciated. I am also new to Haskell so any explanations would help. Thanks I have to use map foldr foldl.
asum :: (Num a) => [a] -> a
xor :: [Bool] -> Bool
I would say start by running the following, one by one, in GHCi:
:t foldr
:info foldr
:doc foldr
:t foldl
:info foldl
:doc foldl
:t map
:info map
:doc map
Or better, open hoogle.haskell.org and search each of the above mentioned functions and click on the first link.
But I agree that Haskell documentation are difficult to read, especially for beginners. I'm a beginner and I have a lot of difficulty reading and understanding them.
Here's a function that uses map and foldr to show how foldr works:
printFoldr xs = foldr (\x acc -> "(" ++ x ++ " + " ++ acc ++ " )") "0" $ map show xs
Now running watch this:
printFoldr [1..5]
-- outputs the following:
"(1 + (2 + (3 + (4 + (5 + 0 ) ) ) ) )"
This shows us how foldr is evaluated. Before going into how foldr is evaluated, let's look briefly at map.
map show [1..5]
-- outputs the following:
["1","2","3","4","5"]
This means that map takes 2 arguments. A list and a function that is applied to each element of the list. The result is a new list with the function applied to each element. Thus, applying show to each number outputs their string representation.
Back to foldr. foldr takes 3 arguments:
a function of type a -> b -> b
an initial value of type b
a list of type [a]
foldr takes each and every value of the provided list and applies this function to it. What is special is that map retains the output of the function over each iteration and passes it to the function as its second argument on the next run. Therefore it is convenient to write the function that is passed foldr as follows: (\el acc -> do something). Now on the next iteration of foldr, acc will hold the value of the previous run and el will be the current element from the list. BTW, acc stands for accumulator and el for element. This enables us to reduce elements of the provided list to something completely new.
As you can see in printFoldr, the initial value is just an empty string but it gradually adds the lists elements to it showing how it would have reduced the elements of the list to their sum.
Here's an idea:
a1-a2+a3-a4+...
=
a1-(a2-(a3-(a4-(...(an-0)...))))
This fits pretty well to the foldr pattern of recursion,
foldr f z [a1,a2,a3,a4,...,an]
=
a1`f`(a2`f`(a3`f`(a4`f`(...(an`f`z)...))))
So it can be coded by setting f = ... and z = ... and calling
asum :: (Num a) => [a] -> a
asum xs = foldr f z xs
where
f = (...)
z = (...)
You will need to complete this definition.
For the XOR of a list of Booleans, assuming it is to be True if one and only one of them is True, and False otherwise, we can imagine this sequence of transformations:
[ True, False, False, True, True, False, ...]
==>
[ t, f, f, t, t, f, ...]
where t and f are some specially chosen numbers. And then we can find the sum of this second list (not alternating sum, just a sum of a list of numbers) and check whether it is equal to ... some (other?) special number, let's call it n1:
xor :: [Bool] -> Bool
xor bools = (aNumber ... n1)
where
list1 = bools
list2 = fun1 transform list1
transform False = f
transform True = t
f = ...
t = ...
aNumber = sum list2
n1 = ...
fun1 = ...
sum listOfNums = ...
fun1 is the function which transforms each element of its argument list according to the given function, called transform above. It is one of the two functions left from the three you were given, considering we've already been using foldr.
sum is to be implemented by using the last function that's left.
FYI,
map foo [a1,a2,a3,...,an]
=
[foo a1, foo a2, foo a3, ..., foo an]
and
foldl f z [a1,a2,a3,...,an]
=
((((z`f`a1)`f`a2)`f`a3)...)`f`an

How to map a function taking two parameters (instead of one)?

I have a Haskell problem and I came up with a solution but I just can't put pen to paper.
Basically I have a function calc that takes in an Int and returns a Float.
calc :: Int -> Int -> Float
In my function g I want to apply this function on an array
g :: [Int] -> [Float]
Since calc takes two Int parameters I want to consume two Ints (
x being the first index of the array and x+1 being second index). This will keep repeating the function on (index of array) [1][2], [2][3], [3][4] etc.
g :: [Int] -> [Float]
g [] = ""
g (x:xs) = map calc x x+1
calc :: Int -> Int -> Float
calc current last = (current - last / last) * 100
I've tried to research online on how to use map and it basically said like this? Any idea why the code doesn't want to compile and if x+1 is even a viable?
Unary map is map but binary map is zipWith. So what's to zip together? The two copies of the same list, shifted by one position one against the other:
calc :: Int -> Int -> Float
g :: [Int] -> [Float]
g xs = zipWith calc xs (drop 1 xs)
This is all. The zipping automatically stops when the shorter sequence runs out.
zipWith foo applies foo to each pair of arguments, each argument coming from the corresponding list, one element after the other. It could be implemented using zip as
zipWith foo xs ys = map (\ (x,y) -> foo x y) $ zip xs ys
= [ foo x y | (x,y) <- zip xs ys ]
but it already does this by itself.
As an illustration, drop 1 [1,2,3] == [2,3] and this means that
zipWith calc [1, 2, 3] (drop 1 [1, 2, 3]) ==
zipWith calc [1, 2, 3]
[2, 3 ] ==
[calc 1
2,
calc 2
3]
You wanted to use indices. Indices are practically never used in Haskell with lists. We instead achieve the same effect by structural jiggling, and then when we advance along the two lists one element at a time, each operation is O(1) because the next element is already at the top of the advanced list. Whereas with the indices we'd have to traverse the same list from the top anew each time, leading to O(k) time per each one (kth) element, i.e. quadratic behavior overall.
update: taking the zipping approach leads to a possibility of code fusion,
g2 xs = zipWith calc xs (drop 1 xs)
= [calc x y | (x,y) <- zip xs (drop 1 xs)]
= [calc x y | (x:y:_) <- tails xs] -- tails ~= iterate (drop 1)
because why build a new structure to house the two consecutive elements when they already appear in a structure ... the original list structure.
Interestingly, Common Lisp has this special kind of map, a map on tails, as part of the language. While the regular map is known there as mapcar, the map on tails is known there as maplist (it's a map on non-empty tails, to be precise).
I think the best a solution is
-- Returns the empty list on empty list due to laziness of zip
g xs = uncurry calc `map` zip xs (tail xs)
-- | |- this is the list of pairs with indeces: [(1,2), (2,3), (3,4) ...]
-- |- this converts a two arguments functions on a function with single 2-tuple argument
EDIT: #Will Ness solution is actually better.

How does listx2 = [x * 2 | x<- numberList] work?

So I m watching a very basic Tutorial, and I m at list comprehension where this comes up:
listx2 = [x * 2 | x<- numberList]
with numberList being a list of numbers
So this takes every number in the list and duplicates it, so numberList = [1,2] results in [2,4].
But HOW does the whole Syntax come together?
I know that x * 2 is the doubleing, but the rest just doesn't make sense to me.
| is the "or" Symbol as far as I know,and what does it do there?
x <- numberList gives x a number from the list, but why does it take just a number? and why so nicely one after the other? There is no recursion or anything that tells it to do one element at a time...
I learn stuff by understanding it, so is that even possible here or do I just have to accept this as "thats how it goes" and memorize the pattern?
List comprehensions use their own special syntax, which is
[ e | q1, q2, ..., qn ]
The | is not an "or", it's part of the syntax, just as [ and ].
Each qi can be of the following forms.
x <- list chooses x from the list
condition is a boolean expression, which discards the xs chosen before if the condition is false
let y = expression defines variable y accordingly
Finally, e is an expression which can involve all the variables defined in the qi, and which forms the elements in the resulting list.
What you see is syntactical sugar. So Haskell does not interpret the pipe (|) as a guard, etc. It sees the list comprehension as a whole.
This however does not mean that the <- are picked at random. Actually list comprehension maps nicely on the list monad. What you see is syntactical sugar for:
listx2 = do
x <- numberList
return x*2
Now a list type [] is actually a monad. It means that we have written:
listx2 = numberList >>= \x -> return (x*2)
Or even shorter:
listx2 = numberList >>= return . (*2)
Now the list monad is defined as:
instance Monad [] where
return x = [x]
xs >>= k = concat $ fmap k xs
So this means that it is equivalent to:
listx2 = numberList >>= return . (*2)
listx2 = concat (fmap (return . (*2)) numberList)
listx2 = concat (fmap (\x -> [2*x]) numberList)
Now for a list fmap is equal to map, so:
listx2 = concat $ map (\x -> [2*x]) numberList
listx2 = concatMap (\x -> [2*x]) numberList
so that means that for every element x in the numberList we will generate a singleton list [2*x] and concatenate all these singleton lists into the result.

Haskell: min distance between neighbor numbers on a list

I'm trying to define a functino that finds the minimum distance between to neighbor numbers on a list
something like this:
minNeighborsDistance [2,3,6,2,0,1,9,8] => 1
My code looks like this:
minNeighborsDistance [] = []
minNeighborsDistance (x:xs) = minimum[minNeighborsDistance xs ++ [subtract x (head xs)]]
Although this seems to run, once I enter a list I receive an Exception error.
I'm new to Haskell I would appreciate any help in this matter.
If you pass a singleton list to minNeighborsDistance then
It'll fail to match [] in the first line, then
it'll successfully match (x:xs) assigning the single value to x and the empty like to xs, then
it'll throw an error when you try to access the head of an empty list.
Further, since you call minNeighborsDistance recursively then you'll always eventually call it on a singleton list excepting when you pass it an empty list.
Here's what I came up with:
minDistance l = minimum . map abs . zipWith (-) l $ tail l
Try this:
minDistance list = minimum (distance list)
where
distance list = map abs $ zipWith (-) list (tail list)
distance calculates the absolute value of the list being subtracted with itself shifted by 1 position:
[2,3,6,2,0,1,9,8] -- the 8 is skipped but it does not make a difference
- [3,6,2,0,1,9,8]
= [1,3,4,2,1,8,1]
minDistance now just gets the smallest element of the resulting list.
Your question is a bit unclear (a type signature would really help here), but if you're wanting to calculate the difference between adjacent elements of the list, then find the minimum of those numbers, I would say the most clear way is to use some extra pattern matching:
-- Is this type you want the function to have?
minNeighborsDistance :: [Int] -> Int
minNeighborsDistance list = minimum $ go list
where
go (x:y:rest) = (x - y) : go (y:rest)
go anythingElse = [] -- Or just go _ = []
However, this won't quite give you the answer you want, because the actual minimum for your example list would be -4 when you go from 6 to 2. But this is an easy fix, just apply abs:
minNeighborsDistance :: [Int] -> Int
minNeighborsDistance list = minimum $ go list
where
go (x:y:rest) = abs (x - y) : go (y:rest)
go anythingElse = []
I've used a helper function to calculate the differences from element to element, then the top-level definition calls minimum on that result to get the final answer.
There is an easier way, though, if you exploit a few functions in Prelude, namely zipWith, map, and drop:
minNeighborsDistance :: [Int] -> Int
minNeighborsDistance list
= minimum -- Calculates the minimum of all the distances
$ (maxBound:) -- Ensures we have at least 1 number to pass to
-- minimum by consing the maximum possible Int
$ map abs -- Ensure all differences are non-negative
-- Compute the difference between each element. I use "drop 1"
-- instead of tail because it won't error on an empty list
$ zipWith (-) list (drop 1 list)
So combined into one line without comments:
minNeighborsDistance list = minimum $ (maxBound:) $ map abs $ zipWith (-) list $ drop 1 list

How do I split a list into sublists at certain points?

How do I manually split [1,2,4,5,6,7] into [[1],[2],[3],[4],[5],[6],[7]]? Manually means without using break.
Then, how do I split a list into sublists according to a predicate? Like so
f even [[1],[2],[3],[4],[5],[6],[7]] == [[1],[2,3],[4,5],[6,7]]
PS: this is not homework, and I've tried for hours to figure it out on my own.
To answer your first question, this is rather an element-wise transformation than a split. The appropriate function to do this is
map :: (a -> b) -> [a] -> [b]
Now, you need a function (a -> b) where b is [a], as you want to transform an element into a singleton list containing the same type. Here it is:
mkList :: a -> [a]
mkList a = [a]
so
map mkList [1,2,3,4,5,6,7] == [[1],[2],...]
As for your second question: If you are not allowed (homework?) to use break, are you then allowed to use takeWhile and dropWhile which form both halves of the result of break.
Anyway, for a solution without them ("manually"), just use simple recursion with an accumulator:
f p [] = []
f p (x:xs) = go [x] xs
where go acc [] = [acc]
go acc (y:ys) | p y = acc : go [y] ys
| otherwise = go (acc++[y]) ys
This will traverse your entire list tail recursively, always remembering what the current sublist is, and when you reach an element where p applies, outputting the current sublist and starting a new one.
Note that go first receives [x] instead of [] to provide for the case where the first element already satisfies p x and we don't want an empty first sublist to be output.
Also, this operates on the original list ([1..7]) instead of [[1],[2]...]. But you can use it on the transformed one as well:
> map concat $ f (odd . head) [[1],[2],[3],[4],[5],[6],[7]]
[[1,2],[3,4],[5,6],[7]]
For the first, you can use a list comprehension:
>>> [[x] | x <- [1,2,3,4,5,6]]
[[1], [2], [3], [4], [5], [6]]
For the second problem, you can use the Data.List.Split module provided by the split package:
import Data.List.Split
f :: (a -> Bool) -> [[a]] -> [[a]]
f predicate = split (keepDelimsL $ whenElt predicate) . concat
This first concats the list, because the functions from split work on lists and not list of lists. The resulting single list is the split again using functions from the split package.
First:
map (: [])
Second:
f p xs =
let rs = foldr (\[x] ~(a:r) -> if (p x) then ([]:(x:a):r) else ((x:a):r))
[[]] xs
in case rs of ([]:r) -> r ; _ -> rs
foldr's operation is easy enough to visualize:
foldr g z [a,b,c, ...,x] = g a (g b (g c (.... (g x z) ....)))
So when writing the combining function, it is expecting two arguments, 1st of which is "current element" of a list, and 2nd is "result of processing the rest". Here,
g [x] ~(a:r) | p x = ([]:(x:a):r)
| otherwise = ((x:a):r)
So visualizing it working from the right, it just adds into the most recent sublist, and opens up a new sublist if it must. But since lists are actually accessed from the left, we keep it lazy with the lazy pattern, ~(a:r). Now it works even on infinite lists:
Prelude> take 9 $ f odd $ map (:[]) [1..]
[[1,2],[3,4],[5,6],[7,8],[9,10],[11,12],[13,14],[15,16],[17,18]]
The pattern for the 1st argument reflects the peculiar structure of your expected input lists.

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