Haskell - counting the number of occurrences of a value in a list

user_10605246 0
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So still working through Haskell tutorial...

One problem posed, is to write a function using:

count :: Eq a => [a] -> a -> Int

That can take a list of numbers & a value, and tell you how many times the value you specify occurs in the list.

It says to see if you can write it using List Comprehension, and again using Explicit Recursion...

AND, to use it to not just count occurrences of numbers -- but of letters, for instance, how many times does 's' occur in 'she sells sea shells'.

So I got:

countListComp :: Eq a => [a] -> a -> Int 
countListComp [] find = 0
countListComp ys find = length xs
    where xs = [xs | xs <- ys, xs == find]

and:

countRecursion :: Eq a => [a] -> a -> Int
countRecursion [] find = 0
countRecursion (x:xs) find 
    | find == x = 1 + (countRecursion xs find)
    | otherwise = countRecursion xs find

So it's counting the occurrences of numbers in a list just fine, like so:

ghci > countListComp [1,3,2,3,4,3] 3
3

ghci > countRecursion [6,9,7,9,8,9] 9
3

but when i look for a specific letter, it does this:

ghci > countListComp ["she sells sea shells"] "s"
0

ghci > countRecursion ["she sells sea shells"] "s"
0

it also said to try to count something else 'countable', like how many lists are there... so I tried:

ghci > countListComp [[1,2,3],[3,2,1],[4,5,6]] []
0

is there something wrong with my code, or am I not specifying what to look for correctly? I'm thinking it's the latter... because the following works:

For example, looking for how many times 's' occurs in 'she sells sea shells'... do I really have to put each individual letter in quotes with a comma between?? Like:

ghci > countRecursion ['s','h','e',' ','s','e','l','l','s',' ','s','e','a',' ','s','h','e','l','l','s'] 's'
6

And do I have to look for a specific list? Or is there a way to look for just a list with anything in it?

haskell
3 Answers

Problem with countListComp ["she sells sea shells"] "s" is you have list of string.

You probably mean countListComp "she sells sea shells" 's'

Sting is just alias to list of character.

With countListComp [[1,2,3],[3,2,1],[4,5,6]] [] is different problem. It doesn't count how many list you have. It count how many list equals to [] you have.

If you try countListComp [[1,2,3],[],[4,5,6]] [] or countListComp [[1,2,3],[3,2,1],[4,5,6]] [3,2,1] you get 1.

user_36569040

Try seeing what the first item in "she sells sea shells" is:

ghci> head "she sells sea shells"
=> 's'

's' is a Char, while "s" is a single-item [Char].

user_6254030

In my opinion, you have two mistakes here.

First, when you pass ["she sells sea shells"] to your function you actually pas a list of list of chars to your function. So function call should be as the following.

countListComp "she sells sea shells" <second_parameter>

Second problem in the function call is, String is a list of chars, and list in Haskell consists of a head and tail list. So when you pass "s" as string, instead of char, you actually pass ['s',[]]. So the right function call should be:

countListComp "she sells sea shells" 's'
user_64876070
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