Learning Haskell: String manipulation question

Question

How can I write a function in Haskell, that takes an input string in the format a1a2a3 and expands into a1a2a2a3a3a3. For example input string "code" would be expanded into "coodddeeee"

Answer 1

So you want the nth character repeated n times.

f :: String -> String
f x = concatMap g  (zip x [1..])
   where
       g (x,y) = replicate y x

I'm sure there's an easier way to do this.

Explanation: First we get the string and pair it with it's place in the list (starting at 1). This is what zip does:

Prelude> zip "code" [1..]
[('c',1),('o',2),('d',3),('e',4)]

Now the function g (x,y) uses the replicate function which replicates whatever you want y times. So we replicate x, y times.

Prelude> g ('z',4)
"zzzz"

If we map this function over the list produced you get the result:

Prelude> map g $ zip "code" [1..]
["c","oo","ddd","eeee"]

If you have a list of strings, you can concatenate them together using concat. concatMap applies the function g to each pair of letter and number and then concatenates the string into the final result.

Prelude> concat $ map g $ zip "code" [1..]
"coodddeeee"

Basically: concat $ map g -> concatMap g

EDIT: now it works, it can also be done in one line thusly:

f x = concatMap (\(a,b)->replicate b a ) $ zip x [1..]

Output:

Prelude> f "lambda"
"laammmbbbbdddddaaaaaa"