I'm trying to write a function in racket (delete-all xx elt) which returns a new list with all occurrences of elt removed

Question

This is what i have but it only works if the elt appear at the start of the list

(define (delete-all xx elt)
  (cond ((null? xx) null)
        ((equal? elt (car xx)) (delete (cdr xx) elt))))

Answer 1

You're missing an extra case: what happens if the current element is not the one you want to delete? Here's the general idea of what needs to be done, I'm not giving you a straight answer because this looks like homework (you should use the homework tag in your question). Better fill-in the blanks yourself:

(define (delete-all xx elt)
  (cond ((null? xx)            ; base case: empty list
         null)                 ; return the empty list
        ((equal? elt (car xx)) ; current element needs to be removed
         <???>)                ; ignore current element and make recursive call
        (else                  ; current element needs to be added to the list
         (<???> (car xx) <???>)))) ; add current element and make recursive call

Also, don't call delete in your answer, given that this is a recursive solution, you need to call delete-all instead, but with appropriate arguments to keep the recursion going until the base case is reached. Hint : what about cons and cdr ?

Answer 2

You could also use filter , that is if you're allowed to use higher order functions :

(define (delete-all xx elt)
  (filter (lambda (y) (not(eq? xx y))) elt))

Answer 3

(define (delete-all xx elt)
  remove* xx elt)