DrRacket: How to remove elements in a list that are greater/lower than a certain number-CodePudding

Im trying to write a small program that will take a list as an input and the first function is supposed to remove values that are greater than 4 and the other one is supposed to remove values that are lower than 4 in the check-expects. I figure this is a simple solution but I cant seem to figure it out as Im still new to lists and data structures. Any help would be appreciated, here is the code:

(define (removehigher lon n)
  (cond [(> n lon)(remove (lon))]
        [(< n lon) true]))

(define (removelower lon n)
  (cond [(> n lon) true]
        [(< n lon) (remove(lon))]))

(check-expect(removehigher(list 0 1 2 3 4 5 6)4)(list 0 1 2 3))
(check-expect(removelower(list 0 1 2 5 6 7) 3)(list 5 6 7))

CodePudding user response：

There is a function called filter which would do this.

(define (my-filter pred lst (acc '()))
  (cond [(null? lst) (reverse acc)]
        [(pred (car lst)) (my-filter pred (cdr lst) (cons (car lst) acc))]
        [else (my-filter pred (cdr lst) acc)]))

It is actually an in-built function filter. I use my- as prefix to not to overwrite this in-built function.

Once you have the function filter (or my-filter), you can write your desired functions easily - it is then about to define the predicate function (pred) correctly for each of the cases:

(define (remove-higher lst n)
  (filter (lambda (x) (<= x n)) lst))

(define (remove-lower lst n)
  (filter (lambda (x) (<= n x)) lst))

Alternatively, one could also use append-map:

(define (remove-higher lst n)
  (append-map (lambda (x) (if (<= x n) (list x) '())) lst))

(define (remove-lower lst n)
  (append-map (lambda (x) (if (<= n x) (list x) '())) lst))

Or define filter using append-map:

(define (my-filter pred lst)
  (append-map (lambda (x) (if (pred x) (list x) '())) lst))

append-map in turn can be defined:

(define (my-append-map func lst)
  (apply append (map func lst)))