/
2.66.scm
50 lines (45 loc) · 1.89 KB
/
2.66.scm
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(define (make-tree entry left right) (list entry left right))
(define (entry tree) (car tree))
(define (left-branch tree) (cadr tree))
(define (right-branch tree) (caddr tree))
(define (make-record key data) (cons key data))
(define (key record) (car record))
(define (data record) (cdr record))
(define (tree->list tree)
(define (copy-to-list tree result-list)
(if (null? tree)
result-list
(copy-to-list (left-branch tree)
(cons (entry tree)
(copy-to-list (right-branch tree)
result-list)))))
(copy-to-list tree '()))
(define (list->tree elts)
(define (partial-tree elts n)
(if (= n 0)
(cons '() elts)
(let* ((left-size (quotient (- n 1) 2))
(left-result (partial-tree elts left-size))
(left-tree (car left-result))
(non-left-elts (cdr left-result))
(this-entry (car non-left-elts))
(right-size (- n (+ left-size 1)))
(right-result (partial-tree (cdr non-left-elts) right-size))
(right-tree (car right-result))
(remaining-elts (cdr right-result)))
(cons (make-tree this-entry
left-tree
right-tree)
remaining-elts))))
(car (partial-tree elts (length elts))))
(define (lookup k records)
(let* ((this-record (entry records))
(this-key (key this-record)))
(cond ((= k this-key) this-record)
((< k this-key) (lookup k (left-branch records)))
((> k this-key) (lookup k (right-branch records))))))
(lookup 4 (list->tree (list (make-record 1 "apple")
(make-record 2 "banana")
(make-record 3 "color")
(make-record 4 "dog")
(make-record 5 "egg"))))