I“d write it like this:
(struct tree (val left right) #:transparent)
;; Tree = '() | (tree X Tree Tree)
(define (printer t0)
(local (;; from-t0 : the path from t0 to t
(define (printer/acc t from-t0)
(cond
[(empty? t)
(map display (reverse (cons " end" from-t0)))
(newline)]
[else
(define from-t (list* " --> " (tree-val t) from-t0))
(printer/acc (tree-left t) from-t)
(unless (empty? (tree-right t))
(printer/acc (tree-right t) from-t))])))
(printer/acc t0 '())))
(printer tree1)
> On Aug 5, 2018, at 2:09 PM, Robert Girault <rfrancoisgira...@gmail.com> wrote:
>
> I have this tree[1]:
>
> (tree
> '(game A B)
> (tree '(game A C) '() (tree '(game C B) '() (tree '(game B A) '() '())))
> (tree '(game B C) '() (tree '(game C A) '() (tree '(game A B) '() '()))))
>
> This is how I'd like to print it out:
>
> ;; (game A B) --> (game A C) --> end
> ;; (game A B) --> (game A C) --> (game C B) --> end
> ;; (game A B) --> (game A C) --> (game C B) --> (game B A) --> end
> ;; (game A B) --> (game B C) --> end
> ;; (game A B) --> (game B C) --> (game C A) --> end
> ;; (game A B) --> (game B C) --> (game C A) --> (game A B) --> end
>
> You see, some branches have no left or right nodes. In such cases,
> I'd like to avoid going down the tree, otherwise I end up displaying
> two different paths as the same path.
>
> To do that, I had to put a second base case in the function below.
> (So I'm calling displayln in two different places.) It looks ugly and
> I wonder how you would do it. Would you do it differently?
>
> (define (print-tree acc-str tree)
> (cond [(empty? tree) (displayln (string-append acc-str " end"))]
> ;; avoid going down the tree if both branches are empty
> [(and (empty? (tree-left tree))
> (empty? (tree-right tree)))
> (displayln (string-append acc-str (format "~a " (tree-root
> tree)) "--> end"))]
> [else
> (print-tree
> (string-append acc-str (format "~a " (tree-root tree)) "--> ")
> (tree-left tree))
> (print-tree
> (string-append acc-str (format "~a " (tree-root tree)) "--> ")
> (tree-right tree))]))
>
> Thank you!
>
> (*) Footnotes
>
> [1] This tree represents the following tourney. A matches B. If A
> wins, it matches C. If B wins, B matches C. In general, a winning
> player matches the third player (who is not playing). If a player
> wins two games in a row, the tourney is over. Also, if 4 matches in a
> row take place, the tourney is over.
>
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