Filter

From Distributed Interactive Functional Fiction Programming

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filter - use 'property' function and a list and return a new list constructed from elements of the old list that have a given property

Haskell:

filter':: (a -> Bool) -> [a] -> [a]
filter' _ [] = []
filter' p (x:xs) 
          | p x = x : filter' p xs
          | otherwise = filter' p xs

-- Example of property function
isEven :: Int -> Bool
isEven n = (n `mod` 2 == 0)

-- Example of using filter':
testFilter = filter' isEven [1,2,3,4]
-- returns [2,4]

Scheme:

(define (filter p ls)
  (cond ((null? ls) '())
        (else (let ((x (car ls)) (xs (cdr ls)))
                (if (p x) (cons x (filter p xs))
                    (filter p xs))))
        ))
                    
(define (isEven n)
  (zero? (modulo n 2)))

(define testFilter (filter isEven '(1 2 3 4)))