Office Hours Code

01-28-oh-1.rkt

#lang racket

;; Define a function that evaluates to #t if
;; the list l contains the number 7
;;
;; - what do we do when l is '()
;; - what do we do when l is (cons hd tl)
(define (contains-7 l)
  (if (empty? l)
      #f
      ;; we know it's false
      (if (equal? (first l) 7)
          #t
          (contains-7 (rest l)))))

;; (sum-list l) returns the list of l for *any* list l 
(define (sum-list l)
  (if (empty? l)
      0 ;; (sum-list '()) => 0 
      (+ (first l) (sum-list (rest l))))) ;; 

#;(define (sum-list l)
  (if (empty? l)
      0 ;; (sum-list '()) => 0 
      (let ([a0 (first l)])
        (let ([a1 (rest l)])
          (let ([a2 (sum-list a1)])
            (+ a0 a2))))))

;; a list is either '() or (cons x y)
;; struct Cons {
;;   usize car;
;;   usize cdr;
;; }

;; takes a list l, adds 1 to every element of l
;; (listof number?) -> (listof number?)
;;
;; (add1-to-all '()) => '()
;; (add1-to-all '(3)) => '(4)
;; (add1-to-all '(1 2 3)) => '(2 3 4)
(define (add1-to-all l)
  (if (empty? l)
      '()
      (cons (+ 1 (first l)) (add1-to-all (rest l)))))

;; (add1-to-all '(0 1 2 3 ... 999)) => '(1 2 ... 1000)