Dr. Racket Recursion count occurrences

问题

I'm new to Racket and trying to learn it. I'm working through some problems that I'm struggling with. Here is what the problem is asking:

Write a definition for the recursive function occur that takes a data expression a and a list s and returns the number of times that the data expression a appears in the list s.

Example:

(occur '() '(1 () 2 () () 3)) =>3

(occur 1 '(1 2 1 ((3 1)) 4 1)) => 3 (note that it only looks at whole elements in the list)

(occur '((2)) '(1 ((2)) 3)) => 1

This is what I have written so far:

 (define occur
      (lambda (a s)
        (cond
          ((equal? a (first s))
          (else (occur a(rest s))))))

I'm not sure how to implement the count. The next problem is similar and I have no idea how to approach that. Here is what this problem says:

(This is similar to the function above, but it looks inside the sublists as well) Write a recursive function atom-occur?, which takes two inputs, an atom a and a list s, and outputs the Boolean true if and only if a appears somewhere within s, either as one of the data expressions in s, or as one of the data expression in one of the data expression in s, or…, and so on.

Example:

(atom-occur? 'a '((x y (p q (a b) r)) z)) => #t

(atom-occur? 'm '(x (y p (1 a (b 4)) z))) => #f

Any assistance would be appreciated. Thank you.

回答1:

In Racket, the standard way to solve this problem would be to use built-in procedures:

(define occur
  (lambda (a s)
    (count (curry equal? a) s)))

But of course, you want to implement it from scratch. Don't forget the base case (empty list), and remember to add one unit whenever a new match is found. Try this:

(define occur
  (lambda (a s)
    (cond
      ((empty? s) 0)
      ((equal? a (first s))
       (add1 (occur a (rest s))))
      (else  (occur a (rest s))))))

The second problem is similar, but it uses the standard template for traversing a list of lists, where we go down on the recursion on both the first and the rest of the input list, and only test for equality when we're in an atom:

(define atom-occur?
  (lambda (a s)
    (cond
      ((empty? s) #f)
      ((not (pair? s))
       (equal? a s))
      (else (or (atom-occur? a (first s))
                (atom-occur? a (rest s)))))))

来源：https://stackoverflow.com/questions/26936949/dr-racket-recursion-count-occurrences

标签

recursion

racket