CODEWITHSUNDEEP
haskellfunctional-programming
David
David

Reputation: 63

Return a concatenated string in Haskell recursive function

I am trying to write a function in haskell that would take an integer and return a concatenated (number of times the input) string

For Instance,

Input: 3

Output: hi1\nhi2\nhi3

main = do 

    let str = func 2 ""
    putStrLn str

func :: Int -> String -> String
func i str = do
        if i>(-1)
            then do
                str ++ "hi" ++ (show i)
                func (i-1) str

        else str

Thanking you!

Upvotes: 1

Views: 1847

Answers (3)

bipll
bipll

Reputation: 11940

I wonder if 'logarithmic' solution is faster:

main = putStrLn $mul 7 "Hi"

mul :: Int -> String -> String
mul 0 _ = ""
mul 1 s = s
mul _ "" = ""
mul n s = let
            (q, r) = n `quotRem` 2
            s' = mul q s
    in (if r == 1 then s else "") ++ s' ++ s'

Upvotes: 2

Mulan
Mulan

Reputation: 135257

This is a much more idiomatic solution than using if-else

a function that would take an integer and return a concatenated (number of times the input) string

func :: Int -> String -> String
func 0 s = ""
func n s = s ++ func (n - 1) s

main = putStrLn (func 3 "hi")

Output

hihihi

Upvotes: 3

Kapol
Kapol

Reputation: 6463

The easiest way to make your code "work" (I'll explain the double quotes later) is to call func with the concatenated string as a parameter directly, without intermediate steps:

func :: Int -> String -> String
func i str = do
        if i > (-1)
            then func (i-1) (str ++ "hi" ++ (show i) ++ "\n")       
            else str

I also added the newline character to the output, which means that the last character of the result will be a new line. Therefore it is better to write

let str = func 2 ""
putStr str

That way you'll avoid an extra new line at the end.

I wrote "works" in double quotes in the first sentence, because my code prints

hi2
hi1
hi0

You need to modify func so that the lines are printed in reverse order. Hint: you can store the lines in a list and reverse the list at the end.

P.S. I'm not sure whether zero should be a valid suffix. If not, then you have to change the condition in your if statement.

Upvotes: 1

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