Implement 2.91 division
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@ -609,13 +609,17 @@
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(error "Polys not in same var -- ADD-POLY"
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(list p1 p2))))
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(define (sub-poly p1 p2)
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(define (negate-term term)
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(make-term (order term) (negate (coeff term))))
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(define (negate-terms terms)
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(map negate-term terms))
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(define (sub-poly p1 p2)
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(if (same-variable? (variable p1) (variable p2))
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(make-poly (variable p1)
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(add-terms (term-list p1)
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(map negate-term (term-list p2))))
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(negate-terms (term-list p2))))
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(error "Polys not in same var -- ADD-POLY"
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(list p1 p2))))
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@ -778,7 +782,8 @@
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(attach-tag term-type (cdr terms))))
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(define (empty-termlist? term-list) (null? (contents term-list)))
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(define (make-term order coeff) (list order coeff))
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(define (make-term order coeff)
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(list order coeff))
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(define (order term) (car term))
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(define (coeff term) (cadr term))
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@ -843,53 +848,48 @@
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(define (div-poly p1 p2)
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(if (same-variable? (variable p1) (variable p2))
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(make-poly (variable p1)
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(div-terms (term-list p1)
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(term-list p2)))
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(let ((result (div-terms (term-list p1) (term-list p2)))
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(var (variable p1)))
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(list
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(tag (make-poly var (car result)))
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(tag (make-poly var (cadr result)))))
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(error "Polys not in same var -- MUL-POLY"
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(list p1 p2))))
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; Division can be performed via long division. That is, divide the highest-order
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; term of the dividend by the highest-order term of the divisor. The result is
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; the first term of the quotient. Next, multiply the result by the divisor,
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; subtract that from the dividend, and produce the rest of the answer by
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; recursively dividing the difference by the divisor. Stop when the order of the
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; divisor exceeds the order of the dividend and declare the dividend to be the
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; remainder. Also, if the dividend ever becomes zero, return zero as both
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; quotient and remainder.
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;
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; We can design a div-poly procedure on the model of add-poly and mul-poly. The
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; procedure checks to see if the two polys have the same variable. If so,
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; div-poly strips off the variable and passes the problem to div-terms, which
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; performs the division operation on term lists. Div-poly finally reattaches the
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; variable to the result supplied by div-terms. It is convenient to design
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; div-terms to compute both the quotient and the remainder of a division.
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; Div-terms can take two term lists as arguments and return a list of the
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; quotient term list and the remainder term list.
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;
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; dividend
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; --------
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; divisor
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;
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; Complete the following definition of div-terms by filling in the missing
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; expressions. Use this to implement div-poly, which takes two polys as arguments
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; and returns a list of the quotient and remainder polys.
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; Division can be performed via long division. That is, divide the
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; highest-order term of the dividend by the highest-order term of the
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; divisor. The result is the first term of the quotient. Next, multiply the
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; result by the divisor, subtract that from the dividend, and produce the
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; rest of the answer by recursively dividing the difference by the divisor.
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; Stop when the order of the divisor exceeds the order of the dividend and
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; declare the dividend to be the remainder. Also, if the dividend ever
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; becomes zero, return zero as both quotient and remainder.
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(define (div-terms L1 L2)
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(define (negate-term t)
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(make-term (order t) (negate (coeff t))))
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(define (negate-terms terms)
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(let ((term-type (type-tag terms))
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(term-list (contents terms)))
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(cons term-type (map negate-term term-list))))
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(if (empty-termlist? L1)
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(list L1 L2)
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(let ((t1 (first-term L1))
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(t2 (first-term L2)))
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(let ((t1 (first-term L1)) ; dividend
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(t2 (first-term L2))) ; divisor
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(if (> (order t2) (order t1))
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(list (empty-termlist L2) L1)
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(let ((new-c (div (coeff t1) (coeff t2)))
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(new-o (- (order t1) (order t2)))
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(new-term (make-term (- (order t1) (order t2))
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(div (coeff t1) (coeff t2)))))
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; XXX: continue here
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(let ((rest-of-result (div-terms (empty-termlist L1) L2)))
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(let ((new-dividend (add-terms
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L1
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(negate-terms
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(mul-terms
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(adjoin-term new-term (empty-termlist L1))
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L2)))))
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(let ((rest-of-result (div-terms new-dividend L2)))
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(list (adjoin-term new-term (car rest-of-result))
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(cadr rest-of-result))))))))
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(cadr rest-of-result)))))))))
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(define (=zero?-poly p)
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(define (=zero?-terms terms)
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@ -953,10 +953,10 @@
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(define p2 (make-poly-dense 'x '((2 1) (0 -1))))
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(assert (mul p1 p1) (make-poly-dense 'x '((10 1) (5 -2) (0 1))))
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(assert (mul p1 p2) (make-poly-dense 'x '((7 1) (5 -1) (2 -1) (0 1))))
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(display p1) (newline)
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(display p2) (newline)
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(display (div p1 p2)) (newline)
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(let ((result (div p1 p2)))
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(assert (car result) (make-poly-dense 'x '((3 1) (1 1))))
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(assert (cadr result) (make-poly-dense 'x '((1 1) (0 -1)))))
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(display "\nex-2.92\n")
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;(display "\nex-2.93\n")
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