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The filtering function
For practical purposes it is important to have a memory efficient filter function
(define (filter pred lst)
  (reverse (filter-help pred lst '())))

(define (filter-help pred lst res)
  (cond ((null? lst) res)
        ((pred (car lst)) 
           (filter-help pred (cdr lst)  (cons (car lst) res)))
           (filter-help pred (cdr lst)  res))))
Go to exercise
A straightforward filter function