sicp-ex-2.1

  
 ;; ex 2.1 
  
 (define (numer x) (car x)) 
  
 (define (denom x) (cdr x)) 
  
 (define (print-rat x) 
   (newline) 
   (display (numer x)) 
   (display "/") 
   (display (denom x))) 
  
 (define (make-rat n d) 
   (let ((g ((if (< d 0) - +) (gcd n d)))) 
     (cons (/ n g) (/ d g)))) 
  
 ;; Testing 
 (print-rat (make-rat 6 9)) ; 2/3 
 (print-rat (make-rat -6 9)) ; -2/3 
 (print-rat (make-rat 6 -9)) ; -2/3 
 (print-rat (make-rat -6 -9)) ; 2/3 
  

This forces denominator to be positive by making g (which is (gcd n d)) same sign as d

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