CODEWITHSUNDEEP
stringhaskell
MKB 06
MKB 06

Reputation: 63

Cut first x characters from a String in Haskell

I'm trying to create a function that concatenates parts of strings in a list to another. I already know how to select the strings I want but now I need the first part of them (and it's not always the same value).

So I want to cut the first part of a string

-- here is a function that should take a string and the numbers
-- of characters we want to take from the start
cutString :: Int -> String -> String
cutString x str = cut x str -- here's a way to take the first x characters

And the way to use it:

print (cutString 3 "Hello World")
Output --> "Hel"

Is there any simple ways to do that?

Thanks for any help or advices.

Upvotes: 1

Views: 7343

Answers (2)

Adam Smith
Adam Smith

Reputation: 54203

You can implement take yourself with explicit recursion:

-- note the signature here is less general than it need be:
-- cutString :: Int -> [a] -> [a]
cutString :: Int -> String -> String
cutString 0 _      = []
cutString n (x:xs)
  | n > 0     = x : cutString (n-1) xs
  | otherwise = []
cutString _ _      = []

The first alternation is your usual base case -- taking zero elements of anything is the empty string: [].

The second alternation is your recursive case. If n is zero, execution hits the first alternation, so we only have to worry about two cases here: either n is greater than zero in which case we give back one character and recurse with n-1, or n is less than zero in which case we catch the edge case and give back something sensible. Taking less than zero elements from a string isn't a sensible thing to do, so you could choose your response here -- error might be appropriate.

The third alternation is the fallback and is another base case. If n is still not zero, but we've run out of items in the string, stop recursing and give back []. Without this case, the code crashes on cutString 100 "Hello" (anything where n > length str)

Upvotes: 2

Chad Gilbert
Chad Gilbert

Reputation: 36375

See take:

take :: Int -> [a] -> [a]

take n, applied to a list xs, returns the prefix of xs of length n, or xs itself if n > length xs

Using it in Prelude shows:

Prelude> take 3 "Hello"
"Hel"

Upvotes: 6

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