SICP 习题2.2 线段的中点

线段的中点(x1+x22,y1+y22)

(define make-segment cons)
(define start-segment car)
(define end-segment cdr)

(define make-point cons)
(define x-point car)
(define y-point cdr)

(define average
(lambda (x y)
(/ (+ x y) 2)))

(define mid-point
(lambda (seg)
(make-point (average (x-point (start-segment seg)) (x-point (end-segment seg))) (average (y-point (start-segment seg)) (y-point (end-segment seg))))))

(define a (make-point 1 2))
(define b (make-point 3 4))
(define seg (make-segment a b))

(define print-point
(lambda (p)
(newline)
(display "(")
(display (x-point p))
(display ".")
(display (y-point p))
(display ")")))

(newline)
(print-point (mid-point seg))