20 hours ago

my attempt to do the exercises in sicp.

## Sunday, July 6, 2008

### sicp exercise 2.2

; Exercise 2.2. Consider the problem of representing line segments in a plane. Each segment is

; represented as a pair of points: a starting point and an ending point. Define a constructor

; make-segment and selectors start-segment and end-segment that define the representation of

; segments in terms of points. Furthermore, a point can be represented as a pair of numbers:

; the x coordinate and the y coordinate. Accordingly, specify a constructor make-point and

; selectors x-point and y-point that define this representation. Finally, using your selectors

; and constructors, define a procedure midpoint-segment that takes a line segment as argument

; and returns its midpoint (the point whose coordinates are the average of the coordinates of

; the endpoints). To try your procedures, you'll need a way to print points:

(begin

(define (print-segment s)

(display "(")

(print-point (start-point s))

(display ",")

(print-point (end-point s))

(display ")")

)

(define (print-point p)

(display "(")

(display (x-point p))

(display ",")

(display (y-point p))

(display ")")

)

(define make-segment (lambda (start end )(cons start end)))

(define start-point (lambda (segment) (car segment)))

(define end-point (lambda (segment) (cdr segment)))

(define make-point (lambda (x y) ( cons x y)))

(define x-point (lambda (p) ( car p )))

(define y-point (lambda (p) ( cdr p )))

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

(define midpoint-segment (lambda (segment)

(make-point

(middle (x-point (start-point segment)) (x-point (end-point segment)))

(middle (y-point (start-point segment)) (y-point (end-point segment))))))

(print-segment (make-segment (make-point 2 3) (make-point 5 6)))

(newline)

(print-point (midpoint-segment ( make-segment (make-point 2 3) (make-point 5 6))))

(newline)

)

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