尾递归 Map function RACKET

Question

The map function in Racket takes a function and a list and applies the function to each of the items of the list recursively. I'm trying to convert the map into a tail-recursive function, how can this be done?

Answer 1

Where are you getting stuck? You can define your own map procedure with an inner loop procedure.

(define (map f in)
  (define (loop out lst)
    (if #|exit condition here|#
        #|return output|#
        (loop (cons #|add to output|# out)
              #|sub problem|#)))
  (loop null in)) ; run the loop

(map (lambda (x) (* x x)) '(1 2 3 4 5))

'(1 4 9 16 25)

This pattern is so common in racket that there is a named let syntax that makes it easier for us to use.

(define (map f in)
  (let loop ((out null) (lst in))
    (if #|exit condition here|#
        #|return output|#
        (loop (cons #|add to output|# out)
              #|sub problem|#))))

Another option is to use continuations.

(define (map f in)
  (let (loop (return identity) (lst in))
    (if #|exit condition|#
        (return null)
        (loop (lambda (out) (return (cons #|add to output|# out)))
              #|sub problem|#))))