Write a scheme that returns a list of even numbers

Question

Write a Scheme function that returns the list of even numbers in a given list of integer numbers.The list may not be simple,nested lists can occur and you need to find the even numbers inside those.

(DEFINE (evenlist numberlist) using this as a body and getting the result sth like this (DEFINE (evenlist numberlist) Result:(2 4 6 4)

(define (evenlist numberlist)
  (cond
    ((null? numberlist) '())
    (else(not (= 0 (modulo (numberlist) 2))(evenlist(car numberlist))))
    ))

this is what i have done , i am new to lisp so dont blame me :(

Answer 1

You have to do a bit more of work if the input list is an arbitrarily nested list of lists. This is the standard template to process such lists (cases: empty list, non-list or a list of lists), plus additional logic to flatten the result (that's why append is used here):

(define (evenlist numberlist)
  (cond ((null? numberlist) '())
        ((not (pair? numberlist))
         (if (even? numberlist) (list numberlist) '()))
        (else
         (append (evenlist (car numberlist))
                 (evenlist (cdr numberlist))))))