How to find the strings of a string list that start with a given prefix?-CodePudding

I want to write a helper function that could return a string list

helper ['Excuse','Me', 'Exit', 'And'] 'Ex'

and this should return

['Excuse','Exit']

I search up and find in Data.List have the isInfixOf function could compare two string, but how could I recursive call to find the correct String list

helper :: [String] -> String -> [String]
helper [] _ = []  
helper (x:xs) std 
    | isInfixOf x std == True : helper

CodePudding user response：

You're almost entirely there.

To call helper on a list ["Excuse", "Me", "Exit", "And"] named, oh let's say myList, you'd write:

helper myList "Ex"

Now, when you pattern match that list with x:xs, xs is the tail of the list, which is just another list. To call helper recursively on this, you'd just write helper xs std.

You do not need tto explicitly compare the return of isInfixOf to True. You also need to handle the case where isInfixOf x std is not true.

helper :: [String] -> String -> [String]
helper [] _ = []  
helper (x:xs) std 
    | isInfixOf x std = x : helper xs std
    | otherwise = helper xs std

CodePudding user response：

There is not really a reason to do that with a recursive function. You want to keep elements of a list that fulfill a certain predicate ("Ex" being an infix of the element). The filter function does exactly that.

import Data.List (isInfixOf)

helper :: Eq a => [[a]] -> [a] -> [[a]]
helper = flip (filter . isInfixOf)

For example:

helper ["Excuse", "Me", "Exit", "And"] "Ex"
-- ["Excuse","Exit"]