Sarah Lim (http://sarahlim.com/eecs-111)
In most countries, the age of majority is 18. Some countries have different laws. In Scotland, for instance, the age of majority is 16.
Suppose we want to write a program to determine whether someone is a legal adult. We'll write a global version that works for most countries, and a Scotland-specific version.
Here are three DIFFERENT versions of this program.
;; Version 1
(define age-of-majority 18)
;; number -> boolean
(define (is-adult? age)
(>= age age-of-majority))
;; number -> boolean
(define (is-adult?/scotland age)
(begin (set! age-of-majority 16)
(>= age age-of-majority)));; Version 2
(define age-of-majority 18)
;; number -> boolean
(define (is-adult? age)
(>= age age-of-majority))
;; number -> boolean
(define (is-adult?/scotland age)
(local [(define age-of-majority 16)]
(is-adult? age)));; Version 3
(define age-of-majority 18)
;; number -> boolean
(define (is-adult? age)
(>= age age-of-majority))
;; number -> boolean
(define (is-adult?/scotland age)
(local [(define age-of-majority 16)
(define (helper a)
(>= a age-of-majority))]
(helper age)))Each of these programs will yield a different result for the following series of function calls:
(is-adult? 16)
(is-adult?/scotland 16)
(is-adult? 16)- What will the results of each call be, for each program? (Try to answer first without running the programs.)
- Which result (and which program) is correct?
- What is the difference between the programs? (In other words, WHY does each program yield the results it does?)
- Bonus: How would you modify the incorrect versions to be correct?
Assume you can only add new code to
is-adult?/scotland.
-
For each of the following code snippets, fill in the placeholder with ONE expression, either
maporfor-each, such that the program returns the correct results.-
(define lon (list 1 2 3 4 5)) ;; ... lon ; (list 10 20 30 40 50)
-
(define-struct person [first last]) (define sood (make-person "Sara" "Sood")) (define horswill (make-person "Ian" "Horswill")) (define zhang (make-person "Haoqi" "Zhang")) (define people (list sood horswill zhang)) (person-first sood) ; "S" (person-first horswill) ; "I" (person-first zhang) ; "H"
-
(define-struct person [first last]) (define sood (make-person "Sara" "Sood")) (define horswill (make-person "Ian" "Horswill")) (define zhang (make-person "Haoqi" "Zhang")) (define people (list sood horswill zhang)) (filter (lambda (p) (string=? (person-first p) "I")) ;; ... ) ;; (list (make-person "I" "Horswill"))
-
-
For each of the snippets above, could you produce a working solution by changing
maptofor-eachor vice versa?
Here is a recursive implementation of a function to compute the product of a list of numbers:
;; (listof number) -> number
(define (product lon)
(cond [(empty? lon) 1]
[else (* (first lon)
(product (rest lon)))]))As you've probably seen, there are lots of different ways to implement the same list operation, especially using imperative programming.
The purpose of this section is to help you develop a better intuition for WHY each of these strategies works, by implementing product in various ways.
Fill out the ... in each snippet. Each ... is either a single keyword (e.g. if, unless, begin, etc.) or a single expression.
-
Iterative Recursion
- List: argument
- Result: argument
- Helper function returns: number
(define (product lon) (local [; (listof number) number -> number (define (loop remaining result) (if (empty? remaining) ... (... (rest remaining) (* (first remaining) result))))] ...))
-
Separate result variable
- List: argument
- Result: variable
- Helper function returns: number
(define (product lon) (local [(define result ...) ;; (listof number) -> number (define (loop remaining) (if (empty? remaining) ... (begin ... (loop (rest remaining)))))] ...))
-
Separate result variable AND void helper
- List: argument
- Result: variable
- Helper function returns: void
(define (product lon) (local [(define result ...) ;; (listof number) -> void (define (loop remaining) (unless (empty? remaining) (begin ... (loop (rest remaining)))))] ...))
-
Separate result AND list variables; void helper
- List: variable
- Result: variable
- Helper function returns: void
(define (product lon) (local [(define remaining ...) (define result ...) ;; -> void (define (loop) (unless (empty? remaining) (begin ... ... (loop))))] ...))
-
Using
for-each(define (product lon) (local [...] (... (for-each (lambda (n) ...) lon) ...)))
-
BONUS: Implement the following function, using each of the constraints in the previous five steps.
;; longest : (listof string) -> number
;; returns the length of the longest string in the list- Recall this question from a previous assignment:
Write a recursive function,
depth-tree, that returns the number of levels of nesting in aListTree.;; depth-tree : ListTree -> numberFor instance,
(depth-tree 1) ;; => 0, it's not even a list (depth-tree (list 1 2)) ;; => 1 (depth-tree (list (list 1) 7 (list 1 4 2))) ;; => 2
You probably implemented this function using foldl. Rewrite it using apply.
- In general, you can use
applyandfold(l/r)for the same categories of problems (consolidating a list into a single object using some combiner function).
For instance:
(foldl + 0 (list 1 2 3 4))
(apply + (list 1 2 3 4))(foldr string-append "" (list "a" "b" "c" "d"))
(apply string-append (list "a" "b" "c" "d"))Come up with a usage example for apply that does NOT work for fold(l/r).
Hint: (require 2htdp/image)
;; Note: you'll need to
(require cs111/define-struct)
;; for this exercise.Consider the following inheritance hierarchy:
Animal
__|___
| |
Dog Cat
- Implement the following structs:
;; An Animal is an ABSTRACT base type.
;;
;; Properties:
;; - name, a string
;;
;; Methods:
;; - greet : -> string, returns "hi, I am <name>"
;; A Cat is a subtype of Animal.
;;
;; Properties:
;; - favorite-fish, one of "tuna" "mackerel" "sardine"
;;
;; Methods:
;; - greet : -> string, returns
;; "meow, I am <name> and my favorite fish is <favorite-fish>"
;; A Dog is a subtype of Animal.
;;
;; Properties:
;; - favorite-game, one of "fetch" "agility" "nap"
;;
;; Methods:
;; - greet : -> string, returns
;; "woof, I am <name> and my favorite game is <favorite-game>"-
Define the following variables:
-
A cat named "Peaches" whose favorite fish is "tuna"
(define peaches ...)
-
A cat named "Tubbs" whose favorite fish is "mackerel"
(define tubbs ...)
-
A dog named "Jake" whose favorite game is "nap"
(define jake ...)
-
-
Now change every animal's favorite (food/game), and change all of their names too.