;; 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-intermediate-lambda-reader.ss" "lang")((modname week6_w_full) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))
; count-odds : list-of-numbers -> number
; returns number of odd numbers in list
(define (count-odds lon)
  (foldr add-if-odd 0 lon)
  )
;  (cond [(empty? lon) 0]
 ;       [else (cond [(odd? (first lon)) (+ 1 (count-odds (rest lon)))]
  ;                  [else (count-odds (rest lon))])]))

(define (add-if-odd x y)
(cond [(odd? x) (+ 1 y)]
                  [else y]))

(check-expect (add-if-odd 1 7) 8)

(check-expect (count-odds (list 1 2 3 4)) 2)
(check-expect (count-odds empty) 0)

               
               

(foldr + 0 (list 1 2 3 4))


(define (add3toeach lon)
  (map (lambda (x) (+ x 3)) lon))

(check-expect (add3toeach (list 1 2 3 4)) (list 4 5 6 7))

;((lambda (x y z) (* x (+ y 3) (sqrt z))) 2 3 4)

; adder : number -> (number -> number)
(define adder (lambda (x) (lambda (y) (+ x y))))

(define xx (adder 3))

(define delta 0.000001)

; deriv : (number -> number) -> (number -> number)
; returns the derivative of a function
(define (deriv f)
  (lambda (x)
    (/ (- (f (+ x delta))
          (f (- x delta)))
       (* 2 delta)
    )))

;(define (sqr n) (* n n))

(define 2x (deriv sqr))

(define (cube n) (* n n n))

(check-expect (2x 10) 20)