list-index-ops.hs

-- Index based list operations using folds
-- 
-- Author: Umair Saeed
-- Date: 05/17/2015
-- 

-- insert at an index, implemented via foldr
insertAt :: a -> [a] -> Int -> [a]
insertAt x ys n = foldr insertHelper [] $ zip [0..] ys
	where 
		insertHelper (i,y) acc = if i == n
			then x : y : acc
			else y : acc


-- delete at an index, implemented via foldr
deleteAt :: [a] -> Int -> [a]
deleteAt ys n = foldr deleteHelper [] $ zip [0..] ys
	where
		deleteHelper (i,y) acc = if i == n then acc else y : acc


-- update at an index, implemented via foldr
updateAt :: a -> [a] -> Int -> [a]
updateAt x ys n = foldr updateHelper [] $ zip [0..] ys
	where
		updateHelper (i,y) acc = if i == n then x : acc else y : acc


-- insertAt, implemented via foldl
insertAtL :: a -> [a] -> Int -> [a]
insertAtL x ys n = foldl insertLHelper [] $ zip [0..] ys
    where 
        insertLHelper acc (i,y) = if i == n
            then acc ++ [x] ++ [y]
            else acc ++ [y]

-- deleteAt, implemented via foldl
deleteAtL :: [a] -> Int -> [a]
deleteAtL ys n = foldl deleteLHelper [] $ zip [0..] ys
    where
        deleteLHelper acc (i,y) = if i == n then acc else acc ++ [y]

-- updateAt, implemented via foldl
updateAtL :: a -> [a] -> Int -> [a]
updateAtL x ys n = foldl updateLHelper [] $ zip [0..] ys
    where
        updateLHelper acc (i,y) = if i == n then acc ++ [x] else acc ++ [y]





-- Wrapper to help with printing the results
insertAtTester :: (Show a) => a -> [a] -> Int -> String
insertAtTester x ys n = "Insert " ++ (show x) ++ " at " ++ (show n) ++ ": " ++ show (insertAt x ys n)

deleteAtTester :: (Show a) => [a] -> Int -> String
deleteAtTester ys n = "Delete at " ++ (show n) ++ ": " ++ show (deleteAt ys n)

updateAtTester :: (Show a) => a -> [a] -> Int -> String
updateAtTester x ys n = "Update " ++ (show x) ++ " at " ++ (show n) ++ ": " ++ show (updateAt x ys n)


-- Main function, with some examples to test the functions
main = do
    let testStr = "abcdefghi"
    putStrLn $ "Test string is: " ++ testStr
    putStrLn "---"
    putStrLn $ insertAtTester 'X' testStr 0
    putStrLn $ insertAtTester 'X' testStr 2
    putStrLn $ insertAtTester 'X' testStr 8
    putStrLn $ insertAtTester 'X' testStr (-2)
    putStrLn $ insertAtTester 'X' testStr 10
    putStrLn "---"
    putStrLn $ deleteAtTester testStr 0
    putStrLn $ deleteAtTester testStr 2
    putStrLn $ deleteAtTester testStr 8
    putStrLn $ deleteAtTester testStr (-2)
    putStrLn $ deleteAtTester testStr 10
    putStrLn "---"
    putStrLn $ updateAtTester 'X' testStr 0
    putStrLn $ updateAtTester 'X' testStr 2
    putStrLn $ updateAtTester 'X' testStr 8
    putStrLn $ updateAtTester 'X' testStr (-2)
    putStrLn $ updateAtTester 'X' testStr 10