CODEWITHSUNDEEP
haskell
Nia
Nia

Reputation: 151

Writing a prefix function Haskell

I am new to Haskell [I am loving Haskell by the way :)]. The problem is that I want to write a function myPrefix that takes as input a list, and gives as output the list consisting of all prefixes of the input-list. For Example:

*Main> prefix [1,2,3]
[[],[1],[1,2],[1,2,3]]

But I can not think of a way to do it. If some one can please give me a push in the right direction, I will be in debt.

I have tried to find related posts but I could not find any, SO I posted a question.

Thank you in advance!!!

Upvotes: 0

Views: 864

Answers (3)

MiFi
MiFi

Reputation: 759

Poin-free solution:

prefixes = foldr (\el acc -> [] : map (el:) acc) [[]]

Upvotes: 0

user6428287
user6428287

Reputation: 

suffixes, prefices :: [a] -> [[a]]

suffixes []          = [] : []
suffixes lst@(_ : s) = lst : suffixes s

prefices []  = [] : []
prefices lst = lst : prefices (init lst)

Upvotes: 0

chi
chi

Reputation: 116174

The library function inits does what you want.

Otherwise, consider this:

prefix :: [a] -> [[a]]
prefix [] = baseCase
prefix (x:xs) = doSomethingTo (prefix xs)

Can you guess what baseCase and doSomethingTo should be?

For the latter: think about how to transform

prefix [2,3] = [[], [2], [2,3]]

into

prefix [1,2,3] = [[], [1], [1,2], [1,2,3]]

A way to do that is to first transform [[], [2], [2,3]] into [[1], [1,2], [1,2,3]] and then add the additional [] in front of that.

Upvotes: 2

Related Questions