CODEWITHSUNDEEP
scheme
James Anderson
James Anderson

Reputation: 203

How do I ensure the empty list isn't printed (Scheme)?

I have this code:

(define graph `(A (B (C)) (D (E)) (C (E))))

(define (prog1 graph)
    (let ([seen `()])
      (define (sub g)
          (cond 
              [(member (car g) seen) `()]
              [else 
               (set! seen (cons (car g) seen))
               (cond
                 [(null? (cdr g)) (list (car g))]
                 [else
                  (cons (car g) (map sub (cdr g)))])])) 
    (sub graph)))

It prints a connected graph where all the nodes appear once. However, if a node has already been visited I return the empty list `(). This causes a problem with the output and I don't know how to fix it:

When running (prog1 graph) The current output is: (A (B (C)) (D (E)) ())

However, I want the output to be (A (B (C)) (D (E)))

Any hint on how I can modify the code to achieve this would be great.

Upvotes: 3

Views: 114

Answers (1)

Óscar López
Óscar López

Reputation: 236004

If the empty lists only occur at the topmost level in the list, you could filter them out. Replace the last line in your procedure with this:

(filter (negate null?) (sub graph))

Or simply this:

(remove '() (sub graph))

If the empty lists occur at any nesting level, you can apply the same idea (filtering out empty lists) recursively, at each step in the traversal.

Upvotes: 1

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