;; The first three lines of this file were inserted by DrScheme. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-beginner-reader.ss" "lang")((modname lect02a) (read-case-sensitive #t) (teachpacks ((lib "universe.ss" "teachpack" "2htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "universe.ss" "teachpack" "2htdp")))))
; blend3 : string, string, string -> string
; return w1,w2 blended, then a space, then w2,w3 blended.
; (See lect01c-examples.ss, for def'n of 'blend'.)
;
(define (blend3 w1 w2 w3)
  ;; N.B. Requires `blend` from lect01c-examples.ss.
  (string-append (blend w1 w2) " " (blend w2 w3)))


; To write: `blend3`:
(check-expect (blend3 "Ian" "Thomas" "Barland")
              "Imas Tholand")
(check-expect (blend3 "a" "b" "c") "b c")
(check-expect (blend3 "hey" "there" "") "here th")
(check-expect (blend3 "hey" "" "there") "h ere")
(check-expect (blend3 "hey" "there" "everybody")
              "here thybody")
; N.B. This last test-case is unnecessary: it doesn't
; really test anything that the first example didn't.





; area-of-disc : real -> real
;
(define (area-of-disc r)
  (* pi r r))

; area-of-ring : real, real -> real
; COmpute the surface area of an annulus whose
; outer radius is `outer` and inner radius is `inner`;
; must have (<= 0 inner outer).
;
(define (area-of-ring outer inner)
  (- (area-of-disc outer) (area-of-disc inner)))

(check-expect (area-of-disc 0)
              0)
(check-within (area-of-disc 1) pi 0.0000001)
(check-within (area-of-disc 10)
              314.1592653589793238462643383279
              0.000001)

(check-within (area-of-ring 0 0)
              0
              0)
(check-within (area-of-ring 1 0)
              pi
              0.0001)
(check-within (area-of-ring 17 17)
              0
              0.0001)
(check-within (area-of-ring 10 5)
              (- 314.159265 (area-of-disc 5))
              0.0001)