SICP Exercise 2.12

sicp

Question

Define a constructor make-centre-percent that takes a centre and a percentage tolerance and produces the desired interval. You must also define a selector percent that produces the percentage tolerance for a given interval. The centre selector is the same as the one shown above.

Answer

(define (make-centre-percent c p)
  (let ((w (* c (/ p 100))))
    (make-centre-width c w)))

(define (percent i)
  (let ((w (width i)))
    (* (/ w (centre i)) 100)))

Let’s test it:

(define i (make-centre-percent 50 20))
(percent i)

Results:

20