TeachScheme, ReachJava Adelphi University Wednesday morning June 24, 2008

Review definition by choices definition by parts char, symbol, key & mouse handlers posns defining other structs definition by choices, revisited (unions) lists (2 choices, one of which has 2 parts, one of which is the orig. type) structural recursion: apply already-learned coding patterns to self-referential type

Lists A list is either empty, or (cons element-type list) (define (function-on-list L) (cond [(empty? L) … ] [(cons? L) ; (first L) element-type ; (rest L) list ; (function-on-list (rest L)) whatever

Counting ; count-dolls : list-of-strings -> number "Examples of count-dolls:" (check-expect (count-dolls empty) 0) (check-expect (count-dolls (cons "ball" empty)) 0) (check-expect (count-dolls (cons "doll" empty)) 1) (check-expect (count-dolls (cons "ball" (cons "doll" (cons "doll" (cons "bat" (cons "doll" empty)))))) 3) You try this one. Hint: you'll need a "cond" inside the "cons?" case.

My answer ; count-dolls : list-of-strings -> number (define (count-dolls toys) (cond [(empty? toys) ; toys an empty list 0 ; from one of the test cases ] [(cons? toys) ; toys a non-empty list of strings ; (first toys) a string ; (rest toys) a list of strings ; (count-dolls (rest toys)) a number ; (string=? (first toys) "doll") a boolean (cond [(string=? (first toys) "doll") (+ 1 (count-dolls (rest toys)))] [else (count-dolls (rest toys))]) ]))

A slightly different approach ; count-dolls : list-of-strings -> number (define (count-dolls toys) (cond [(empty? toys) ; toys an empty list 0 ; from one of the test cases ] [(cons? toys) ; toys a non-empty list of strings ; (first toys) a string ; (rest toys) a list of strings ; (count-dolls (rest toys)) a number (+ (1-if-doll (first toys)) (count-dolls (rest toys))) ])) ; "helper" function 1-if-doll : string -> number ; returns 1 if the string is "doll", 0 if it's anything else.

A simpler syntax ; list : object … -> list (list "ball" "doll" "doll" "bat" "doll") is equivalent to (cons "ball" (cons "doll" (cons "doll" (cons "bat" (cons "doll" empty))))) Set Language -> Beginning Student with List Abbreviations Now output will be in "list" syntax rather than "cons" syntax No internal difference; any function that worked before will still work

List exercises Write a function add-positives which takes in a list of numbers and computes the sum of the positive ones, ignoring any zeroes or negatives. Write a function largest which takes in a non-empty list of numbers and returns the largest of them. (Hint: need to define new data type "non-empty list of numbers". Either one element, i.e. (cons # empty), or more than one, i.e. (cons # nelon).)

Functions producing lists Write a function add-1-to-each which takes in a list of numbers and produces a list of numbers, each 1 more than the corresponding number in the input. Write a function convert-grades which takes in a list of numeric grades and produces a list of corresponding letter grades ("A", "B", etc.) in the same order. Write a function substitute which takes in two strings (old and new) and a list of strings, and produces the same list with all occurrences of old replaced with new.

Functions producing lists Write a function keep-positives which takes in a list of numbers and returns a list of only the positive numbers in the list, in the same order, omitting negatives and zeroes. Write a function sort which takes in a list of numbers and returns a list of the same numbers in increasing order. (Hint: don't worry about "what sorting algorithm"; just follow the recipe. You'll need an auxiliary function; again, just follow the recipe.)

Extra credit: lists of lists Write a function permutations which takes in a list (of anything) and returns a list of lists, comprising all the possible orderings of the original list. For example, (check-expect (permutations (list 1 2 3)) (list (list 1 2 3) (list 1 3 2) (list 2 1 3) (list 2 3 1) (list 3 1 2) (list 3 2 1))) Write a function subsets which takes in a list (of anything) and returns a list of lists, comprising all the subsets of the original list (don't worry about order). For example, (check-expect (subsets (list 1 2 3)) (list (list) (list 3) (list 2) (list 2 3) (list 1) (list 1 3) (list 1 2) (list 1 2 3)))

Lists of structs An employee-list is either empty, or (cons employee employee-list) An employee is… See employees.scm as starting point Since an emp-list has a part which is an emp, function-on-emp-list calls function-on-emp on that part

Lists of structs Write total-salary which takes in a list of employees & adds up their salaries Write any-over-100k? which takes in a list of employees & tells whether any of them earn over $100,000/year Write count-over-100k which takes in a list of employees & tells how many of them earn over $100,000/year Write give-10%-raises which takes in a list of employees & returns a similar list in which everybody's salary has increased by 10% Write fire-over-100k which takes in a list of employees & returns a list of the ones who don't earn over $100,000/year. Write highest-paid, which takes in a non-empty list of employees & returns the highest-paid one. (If there's a tie, return the first of the highest-paid employees.)

Lists of unions An animal-list is either empty, or (cons animal animal-list) An animal is either a dog, or a fish A dog is… A fish is… See animals.scm as a starting point Since a list of animals has a part which is an animal, function-on- animal-list calls function-on-animal (which in turn calls function-on-dog, function-on-fish, etc.)

Lists of unions Write count-tigers, which takes in a list of animals and returns how many of them are tigers Write extract-fish, which takes in a list of animals and returns a list of only the fish, in the same order they were in before Write lookup-age, which takes in a list of animals and a string, and if there is a dog with that name in the list, returns the dog's age. (If there's more than one, use the first one you find.) If not, return -1.