Reverse Scheme List
Write a function in Scheme that reverses a list. The output should be a new list that is the reverse of the original list.
(define (reverse lst) ; Your Code Here )
And here’s a test to help you out:
(define (test) (assert-equal '(1 2 3) (reverse '(3 2 1))) (assert-equal '(3) (reverse '(3))) (assert-equal '() (reverse '())))
(define (reverse lst) (if (null? lst) lst (append (reverse (cdr lst)) (list (car lst)))))
In this problem, we first check to make sure that the input
lst is not empty. If it is, we can just return the empty list. Otherwise, we want to take off the first item, reverse the rest of the list, and then attach the original first item to the end of the reversed list.
We note that we have to use
append in this case because of the way that
cons works by creating a pair out of two elements. If we want to use
cons to form a list, we’d have to have a list as the second argument. In this case, all we have is an element, the first item of the list. By using
append, we can reverse the rest of the list (getting a list back) and then create a one-item list out of the first element and attach it to the end of the reversed list.
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