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listhaskell
Babra Cunningham
Babra Cunningham

Reputation: 2967

Efficient list operations?

I have the following function that searches through [Char] and returns [Char] based on their index number:

myList = "abcdefghijk"

searchText = foldl (\acc el -> if elemIndex el myList > Just 11 then el : acc else acc) [] myList

Clearly this is somewhat inefficent as elemIndex returns the index number of each element then applies the condition.

Is there a more efficient way of performing this operation?

Upvotes: 0

Views: 88

Answers (2)

Chad Gilbert
Chad Gilbert

Reputation: 36375

Your function returns a string that skips the first twelve characters and reverses the output, dropping any characters that are also in those first twelve.

For a more efficient version of this, you could use Data.Set to store those first twelve characters for fast lookup, the filter them out and reverse the remainder of the string:

import qualified Data.Set as Set

searchText =
    let hash = (Set.fromList . take 12) myList
    in (reverse . filter (flip Set.notMember hash) . drop 12) myList

Upvotes: 1

chi
chi

Reputation: 116139

The usual approach is to pair each character with its index before doing the real processing

process $ zip [0..] myList

Now process can perform the actual computation, which can use the indexes as well as the characters.

In some contexts, this approach is known as the Schwartzian transform.

Upvotes: 2

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