How to compare 2 lists and return another list with equal elements? - Haskell

Question

My code is this

comparar :: [Int] -> [Int] -> [Int]
comparar [] [] = []
comparar (a:as) (b:bs) = [x | x <- as, x == b] ++ [x | x <- bs, x == a]

What’s wrong with the code above?

Situation: I want to learn how to find equal elements in 2 lists or more, to make the MDC (Maximum Common Divisor)

Example: I want to do the MDC of the list [150,200,300]

create a function that generates the list of splitters of each elements

divisors 150 = [1,2,3,5,6,10,15,25,30,50,75,150]

divisors 200 = [1,2,4,5,8,10,20,25,40,50,100,200]

divisors 300 = [1,2,3,4,5,6,10,12,15,20,25,30,50,60,75,100,150,300]

compare "splitters 150" with "splitters 200" with function compare (that is, the code up there) and the result is [1,2,5,10,25,50] and then I compare this to the new list with "divisors 300" that will give the same thing, use the function Greater to find the largest number and that will be the MDC.

Answer 1

From what I understand you need a function that removes the elements in common from two lists? Proposed function:

comparar :: [Int] -> [Int] -> [Int]
comparar [] l  = []
comparar l  [] = []
comparar (h:t) l2
    | elem h l2 = h:comparar t l2
    | otherwise = comparar t l2

using lists in comprehension becomes simpler:

comparar :: [Int] -> [Int] -> [Int]
comparar l l2 = [ x | x <- l , elem x l2]