CODEWITHSUNDEEP
haskell
James Powell
James Powell

Reputation: 207

Haskell - Creating rectangle of 1s with height and width of user input

I am trying to create a function that will take two integer values that correspond to the width and height of a rectangle of 1s that is outputted like so:

 Main> rectangle 3 4
 1111
 1111
 1111

I am a beginner at Haskell and have no experience, so any pointers would be appreciated, and therefore only basic Haskell operations should be used. Thanks.

Upvotes: 0

Views: 944

Answers (1)

Tobi Nary
Tobi Nary

Reputation: 4596

rectangle :: Int -> Int -> String
rectangle h w = unlines (replicate h (replicate w '1'))

Although this is a pure function, it does show the relevant part. You can just putStrLn (rectangle 3 4) to have it printed out as expected in ghci, rather than wrapped in a show.

Giving it a second thought, here's a short walkthrough.

replicate :: Int -> a -> [a]
unlines :: [String] -> String

As you can see, replicate w '1'creates a list of w times the charakter 1. Because String = [Char], the result is a String of ones, as many as w says.

Now, this String is replicated again, h times, giving a list of h times that string.

unlines now concatenates those strings, inserting a new line character between the strings.

The result is what you'd expect, only that ghci (which you appear to be using) is wrapping each expression's result in a show call. So, to do exactly what you want to achieve, a call to putStr in needed as so:

impureRectangle :: Int -> Int -> IO ()
impureRectangle x y = putStr (rectangle x y)

Note that monads (or IO, as the first monad, people use to get to know as such) are not the easiest things to get your head around. I'd suggest staying pure until you feel safe.

Upvotes: 5

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