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molinet
molinet

Reputation: 13

Haskell - Get the index of the first occurrence in a list of strings

I have a list of strings like the following:

[ "aaa", "aaa", "aba", "aaa"]

I want to make a function that returns the first position of the list where there is an item containing the character 'b'. How can I do it in Haskell?

Upvotes: 0

Views: 7855

Answers (3)

Chad Gilbert
Chad Gilbert

Reputation: 36375

You ask for the first position so I'm not clear whether you want an index value or the actual value. Here's a function that gives you a tuple containing both the index and the value at that index.

import Data.Maybe

firstOccurrenceOf :: Char -> [String] -> Maybe (Int, String)
firstOccurrenceOf c list =
  listToMaybe $ filter (elem c . snd) $ zip [0..] list

listToMaybe provides a safe alternative to head which will return Nothing instead of crashing on an empty list.

Upvotes: 1

Benjamin Barenblat
Benjamin Barenblat

Reputation: 1311

The List Utilities chapter of the Haskell Report describes a function findIndex which finds the index of the first element of a list that satisfies an arbitrary predicate. It can be implemented as

findIndex p xs =
  case [ i | (x, i) <- zip xs [0..], p x ] of
    [] -> Nothing
    e:_ -> Just e

With that, you can find the first element of the list that contains 'b' with

findIndex ('b'`elem`) ["aaa", "aaa", "aba", "aaa"]

Upvotes: 2

jamshidh
jamshidh

Reputation: 12070

Here is a function that will do what you want:

getIndexWithb = findIndex ('b' `elem`)

You will need to import Data.List to make this work. Also note that this function returns type Maybe Int for the case that 'b' never appears in any of the strings.

Usage:

> getIndexWithb [ "aaa", "aaa", "aba", "aaa"]
Just 2

Upvotes: 2

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