Question

Please use Haskell, thank you!

For this question, you will implement (under specific constraints) a recursive function in Haskell that takes a list of characters as an argument and returns a list that contains only every third element from the argument list in the same order. As a clarifying example, this function, when passed the argument list “ABCDEFGHIJKLMNO”, should return the list “ CFILO”. It should also be noted that your function must work (i.e., not terminate with an error) even if the number of elements in the argument list is less than three. You may NOT use the !! operator to complete this question, and you may NOT use any function that you haven’t written here (i.e., you may NOT use any built-in functions, but you may use other functions if you write them yourself).

Write this function (including a type declaration) without selector functions (even if you wrote them yourself), using only pattern matching.

Answer 1

--function definition which accepts a list and returns a list
listThird :: [a] -> [a]
--for no errors return empty list for 0, 1 or 2 elements
listThird [] = []
listThird [x] = []
listThird [_, x] = []
--call the function recursively over the rest of the list which is parted as 1,2, 3rd element and the rest of the list
--undersore represents a single element
listThird (_:_:x:xs) = x:listThird xs

main = do
putStrLn "The input is:" --display this line
-- .. operator automatically creates whole list from A to O
let p = ['A'..'O'] --variable to assign the string
--Any random string can be assigned as below.
--let p = "ABCDEFG"
print(p)
--display the output by calling the function with p as argument
putStrLn "The output is:"
print(listThird p ) --calling a function

Above is the code in the hs file. Second method of defining a random string is shown in the comments above, uncomment and use if needed. remove the two hyphens to uncomment

Sample output