#lang racket

;; cse 413 12au lecture 3 sample functions
;; note: some of these functions like len and app are reimplementations
;; of ones in the standard library to demonstrate how these are implemented.
;; use the library versions in your own code.

;; = larger value of a or b
(define (biggest a b) (if (> a b) a b))

;; same as above using cond instead of if
(define (bigger a b) 
  (cond ((> a b) a)
        (else b)))

;; = description of the weather depending on the temperature
;; (uses cond for multi-way decision)
(define (weather temp)
  (cond ((> temp 80) 'warm)
        ((< temp 60) 'cold)
        (else 'seattle)))

;; = length of list lst
(define (len lst)
  (if (null? lst)
      0
      (+ 1 (len (cdr lst)))))

;; = sum of items in list lst, which must be numbers
(define (lstsum lst)
  (if (null? lst)
      0
      (+ (car lst) (lstsum (cdr lst)))))

;; = sum of numbers in list lst.  non-numeric list elements are ignored.
(define (numsum lst)
  (cond ((null? lst) 0)
        ((number? (car lst)) (+ (car lst) (numsum (cdr lst))))
        (else (numsum (cdr lst)))))

;; = list with contents of lst1 followed by contents of lst2
(define (app lst1 lst2)
  (if (null? lst1)
      lst2
      (cons (car lst1) (app (cdr lst1) lst2))))