CODEWITHSUNDEEP
haskell
benharris
benharris

Reputation: 597

In Haskell how can you multiply a string?

I'm trying to write a function that takes a String and an Int and returns that string "int" times. That is:

duplicate :: String -> Int -> String

If I were to write duplicate "Hello" 3 the output should be "HelloHelloHello".

Upvotes: 20

Views: 15795

Answers (5)

Asokan Pichai
Asokan Pichai

Reputation: 11

Again a beginners attempt, using recursion

duplicate s n = if n <= 1 then s else duplicate (n-1) s ++ s

though it is a little unclear what the function should do if n is negative or zero. So I chose to return the string itself.

Upvotes: 1

Chris Taylor
Chris Taylor

Reputation: 47392

A String is just a synonym for a list of Char, and the list type is a Monad. Therefore

duplicate :: Int -> String -> String
duplicate n str = [1..n] >>= const str

Or, if you wanted to get all point-free

duplicate = (. const) . (>>=) . enumFromTo 1

Edit

As suggested in the comments

duplicate n str = [1..n] >> str

or

duplicate = (>>) . enumFromTo 1

Upvotes: 9

Aadit M Shah
Aadit M Shah

Reputation: 74204

You can use replicate and concat as follows:

duplicate :: [a] -> Int -> [a]
duplicate = flip $ (concat .) . replicate

-- or as larsmans suggested:

duplicate :: [a] -> Int -> [a]
duplicate = (concat .) . flip replicate

Then use it as duplicate "Hello" 3.

Upvotes: 5

Shanthakumar
Shanthakumar

Reputation: 768

You can use pattern matching.

duplicate _ 0 = []
duplicate xs n = xs ++ duplicate xs (n-1)

or

duplicate xs n  | n==0 = []
                | otherwise = xs ++ duplicate xs (n-1)

Upvotes: 4

Nikita Volkov
Nikita Volkov

Reputation: 43310

Easily:

duplicate :: String -> Int -> String
duplicate string n = concat $ replicate n string

The $ is a function of type (a -> b) -> a -> b. The language allows the functions with non-alpha-numeric names to be used in infix form (as operators). I.e., the body of the function above is absolutely identical to the following expression:

($) concat (replicate n string)

What $ does is just allows you to get rid of braces. Meaning that the above expressions are just an alternative to the following expression:

concat (replicate n string)

Upvotes: 23

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