Chapter 2, Building Abstractions with Data
Section - 2.5 - Systems with Generic Operations
Exercise 2.89
I got something interesting to learn here. The =zero? procedure I implemented for polynomial in solution 2.87 was not working correctly!. Because I was using internal representation of the term-list in =zero? implementation(iterating it using car and cadr - inside list-and). The lesson learned is the code/procedures which are external to term-list api shoud not use the internal representation. Well, it seems obvious to not do such bad things :)
Here are the required changes:
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(define (adjoin-term term term-list)
(define (iter count terms)
(if (= count 0)
terms
(iter (- count 1) (cons 0 terms))
)
)
(let ((cof (coeff term))
(count (- (order term) (length term-list)))
)
(cond
((=zero? cof) term-list)
((< count 0) (error "Can not add term - order of passed term is already present in the list"))
(else (cons cof (iter count term-list)))
)
)
)
(define (first-term term-list) (make-term (- (length term-list) 1) (car term-list)))
;all other procedures of term-list will remain same
And here goes the new =zero? implementation:
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(define (=is-zero? poly)
(define (iter terms)
(if (empty-termlist? terms)
#t
(and (=zero? (coeff (first-term terms))) (iter (rest-terms terms)))
)
)
(iter (term-list poly))
)
Test:
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> (display (add (make-polynomial 'x (list 2 3)) (make-polynomial 'x (list 4 5))))
(polynomial x (int . 6) (int . 8))
> (display (add (make-polynomial 'x (list 2 3)) (make-polynomial 'x (list 4 5 6))))
(polynomial x 4 (int . 7) (int . 9))
>