#lang racket

;; CSE 413 19wi Lecture 2
;; Sample code

;; Return 2*n
(define (twice n) (* 2 n))

;; Return 2*n - version with explicit lambda
(define double (lambda (n) (* 2 n)))

;; Bind name to numeric value
(define xvii 17)

;; Functions with conditionals (if and cond)
(define (too-cold temp)
  (if (< temp 65)
      #t
      #f))

(define (weather temp)
  (if (too-cold temp)
      "brrr"
      "ahhhh"))

(define (comfort temp)
  (cond [(> temp 80) 'too-hot]
        [(> temp 60) 'nice]
        [(< temp 40) 'too-cold]
        [else 'seattle]))

;; = n!
(define (fact n)
  (if (< n 2)
      1
      (* n (fact (- n 1)))))

;; a couple of lists for examples below
(define nums '(3 5 12 -4 17))
(define pies '(apple banana cherry))

;; return the sum of the items in a list of numbers
(define (sum lst)
  (if (null? lst)
      0
      (+ (car lst) (sum (cdr lst)))))

;; return the sum of the numbers in lst ignoring other values
(define (numsum lst)
  (cond [(null? lst) 0]
        [(number? (car lst)) (+ (car lst) (numsum (cdr lst)))]
        [else (numsum (cdr lst))]))

;; return the number of items in lst
(define (len lst)
  (if (null? lst)
      0
      (+ 1 (len (cdr lst)))))

;; return a list with ys appended after xs
(define (app xs ys)
  (if (null? xs)
      ys
      (cons (car xs) (app (cdr xs) ys))))