;; simple expressions
'(1 2 3)
(= 4 5)
(- 54 3)
(+ 4 5 23)
(eq? #t #t)
(eq? #t #f)
(car '(1 2 3))
(first '(1 2 3))
(cdr '(1 2 3))
(rest '(1 2 3))
(cdar '((1 2) 3 5))
(cadr '((1 2) 3 5))

;; variable pi
(define pi 3.14159)
pi

;; lambdas
(lambda (x y)
  (* 2 (- x y)))

((lambda (x y)
   (* 2 (- x y)))
 5 1)

(define double-diff
  (lambda (x y)
    (* 2 (- x y))))

(double-diff (- 2 3) (/ 2 3))

(define (double-diff2 x y)
  (* 2 (- x y)))

(double-diff2 (- 2 3) (/ 2 3))

;; area of a circle
(define (circle-area r)
  (* pi r r))

;; function that takes an atom and attempts to respond to it appropriately
(define (response s)
  (cond ((eqv? s 'hello) "Hi there.")
        ((eqv? s 'goodbye) "Until next time.")
        ((eqv? s 'time) "The time is 10:00AM.")
        (else "I Don't Understand.")))

;; recursive functions

;; function that sums elements in a list
(define (sum l)
  (if (null? l)
      0
      (+ (car l) (sum (cdr l)))))

;; function that sums elements in a list
(define (sum2 l)
  (if (null? l)
      0
      (+ (first l) (sum (rest l)))))

;; returns the nth fibonacci number
(define (fib n)
  (if (eqv? n 1)
      1
      (if (eqv? n 2)
          1
          (+ (fib (- n 1)) (fib (- n 2))))))

;; returns the nth fibonacci number
(define (fib2 n)
  (cond ((= n 1) 1)
        ((= n 2) 1)
        (else (+ (fib2 (- n 1)) (fib2 (- n 2))))))