SICP – cap. 2 – Esempio: sets come liste ordinate – 81 – esercizio

Continuo da qui, copio qui.

Exercise 2.61: Give an implementation of adjoin-set using the ordered representation. By analogy with element-of-set? show how to take advantage of the ordering to produce a procedure that requires on the average about half as many steps as with the unordered representation.

Al solito io chiamo Bill the Lizard 👽

The original implementation of adjoin-set made use of element-of-set? to check and see if the new element was already a member of the set. We no longer need to do this since we need to find the exact position in the set to insert the new element. As we scan through the set looking for the right position, we can simply return the set if we encounter the element we’re trying to place.

(define (adjoin-set x set)
  (cond ((null? set) (cons x '()))
        ((= x (car set)) set)
        ((< x (car set)) (cons x set)) 
        ((> x (car set)) (cons (car set)
                               (adjoin-set x (cdr set))))))

There are several different conditions, and we need to cover them all. First, since we’re no longer using element-of-set, we need to check to see if the set itself is null or empty. Second, if the we encounter the element we’re trying to add, we can just return the set. Next, if the element we’re adding is smaller than the first element of the set, we can simply cons the new element to the beginning of the set. Last, if the new element is greater than the first element of the set, we join the first element followed by the adjoin-set of the new element and the rest of the set.

Like element-of-set?, this implementation should only scan through half the set on average before finding the correct insertion point for the new element.

Belle anche le soluzioni di sicp-ex e Drewiki 😊

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