;; The first three lines of this file were inserted by DrRacket. 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 pizza) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f () #f)))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; A hello-world program:
"Hello World"


; Racket is an industrial-strength scheme
; Scheme is an elegant, minimal version of Lisp
; Lisp is the 2nd-oldest high-level language still in use (after Fortran)
;   (Lisp ~ 1965)

; When calling 'substring', we pass it 3 inputs (string, natural-number, natural-number),
; and we get a string back.
;   In Ada, python:      substring("Mississippi", 2, 5)

; When calling '+', we pass it 2 [or more] inputs (number, number)
; and we get a number back
;; In Ada, python:      2 + 3
;; "operators" are just functions that are called with a different syntax.


; IN racket:  "(" means 'call a function'.
; Syntax:    (  <function>   <expression>... )



;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; pizza-area : non-negative real -> non-negative real
; The area (in sq.in.) of a pizza, whose diameter is `diam` (in inches)
; 
(define (pizza-area diam)
  (* pi (sqr (/ diam 2))))



(check-expect (pizza-area 0) 0)
(check-within (pizza-area 2) pi 0.0000001)
(check-within (pizza-area 20) (* 100 pi)  0.0000001)
(check-within (pizza-area 2.5) 4.91  0.03)