Rules of Processing Lists Announcements HW3 is tougher than was - PowerPoint PPT Presentation

Rules of Processing Lists

Announcements • HW3 is tougher than was HW2; plan to spend a little more time on it. • Monday was the last day that I’ll be sharing the lecture- capture. • It’ll be back near end of semester, for review, etc. • End of class feedback: on a scrap of paper, feel free to write down anything that left you confused/unhappy/whatever, and hand it to the TA at the door. (If class seemed OK, no need to do anything!)

Evaluation examples i. (last class) (define (add1 x) (+ x 1)) (+ (add1 3) (add1 4)) • Why? • T o understand subtleties in material later today • T o make it easy to write the Rackette project

From the Rackette handout • “Practice evaluating Rackette expressions by hand. Specifjcally, write out your evaluations for the ``Practice on Paper'' section of this assignment and bring them to your Design Check.” • “For practice, process the following programs in either example's style, starting from the usual initial top-level environment:” i. (let ((x 19)) (+ x 32)) ii. ((lambda (x) (/ x 10)) 100)

Evaluation examples ii. Today (quickly!) (define (add1 x) (+ x 1))

Name login QUIZ <your answers here > • [Quiz contents deleted]

Quick additions: and and or <and-expr> := ( and <expr> <expr> <expr>* ) • Idea: ( and x y) is true only if x and y are both true • Formalized a little ( and x y) evaluates to true only if x and y both evaluate to true • Rule (the "and" rule) to evaluate (and b1 b2 b3 … bn) we • Evaluate b1, whose value must be a boolean (or it's an error) • If b1's value is false , then the and-expression's value is false and we are done. • Otherwise, we evaluate b2; if b2's value is false , then the and- expression's value is false and we are done. • Otherwise, we continue in this manner until bn; if all the expressions have evaluated to true , then the and-expression's value is true.

Quick additions: and and or <or-expr> := ( or <expr> <expr> <expr>* ) • Idea: ( or x y) is true if either x or y or both are true • Formalized a little ( or x y) evaluates to true only if at least one of x or y evaluates to true • Rule (the "or" rule) to evaluate ( or b1 b2 b3 … bn) we • Evaluate b1, whose value must be a boolean (or it's an error) • If b1's value is true , then the or-expression's value is true and we are done. • Otherwise, we evaluate b2; if b2's value is true, then the or-expression's value is true and we are done. • Otherwise, we continue in this manner until bn; if all the expressions have evaluated to false , then the or-expression's value is false.

Notes • and and or are keywords • "and" and "or" are not procedures • The procedure-application-expression rule says that you "evaluate all the arguments", but the and- and or-rules say "evaluate them one at a time, and if you get a certain result, then don't evaluate any more of them!" • Called short-circuiting • You'll see later on why it's built in •

Notes • One more boolean thing: the "not" procedure: • (not true ) => false • (not false ) => true • Activity: write “not

Design-recipe steps already done for you! ;; Data definition: ;; bool: true, false ;; ;; not : bool -> bool ;; input: ;; a boolean, b ;; output ;; a boolean; if b is true, output is false, and vice versa ;; (define (not b) … ) (check-expect (not true) false)

Solution ;; Data definition: ;; bool: true, false ;; ;; not : bool -> bool ;; input: ;; a boolean, b ;; output ;; a boolean; if b is true, output is false, and vice versa ;; (define (not b) (if b false true)) (check-expect (not true) false)

Last syntax piece for a while: if <if-expr> := ( if <expr> <expr> <expr> ) • Idea: ( if condition x y) is x if condition is true, y if it's false. • Rule (the "if" rule) to evaluate ( if condition yes-result no-result) we • Evaluate condition , whose value must be a boolean (or it's an error) • If the value is true , we evaluate yes-result, whose value is the value of the if-expression • If the value is false , we evaluate no-result, whose value is the value of the if-expression • NB for those familiar with other languages: An if- expression is an expression , not a "statement"; it has a

Activity Write an expression involving x and y which evaluates to 37 if x is larger than 5, or y is less than 10, and evaluates to -1 otherwise.

Activity Write an expression involving x and y which evaluates to 37 if x is larger than 5, or y is less than 10, and evaluates to -1 otherwise. (if (or (> x 5) (< y 10)) 37 -1) (if (or (> x 5) (< y 10)) 37 -1)

Lists • Racket is derived from Scheme which is derived from LISP • LISP stands for “LISt Processing language” • Abstraction of lists: • A list can be empty (no items) or • There’s a fjrst item, and the rest of the list • You can add a new fjrst item (lengthening the list) •

T erminology • atomic data: num, bool, string • compound data: lists!

Lists • There are two ways to build a list, indeed, two difgerent kinds of lists. • First “constructor”: empty • Builds a list that corresponds to the notion of a list with no items in it. • Along with this comes a predicate, empty? • No surprises: (empty? empty) => true • Note: empty is a keyword ; you cannot defjne “empty”! • Also can’t use it as the name of an argument to a function… or any kind of name!

Keywords so far • define • and • or • if • cond • else • true • false • empty

The other way to create a list: cons • “cons” is short for “construct” cons: item * list -> list • How could we use cons right now? (cons 13 …) • Need a list to fjll in for “…” • We only have one choice: empty! (cons 13 empty)

T ype-predicate? • Yes, it’s called “ cons?” as expected (cons? empty) => false (cons? (cons 13 empty)) => true

First and rest – deconstructing a list • (first empty) … error • (rest empty) … error • (first (cons 3 (cons 4 empty)) ) => 3 • (rest (cons 3 (cons 4 empty))) => (cons 4 empty) • General rule: • (first (cons a b)) => a • (rest (cons a b)) => b

Activities • Construct a list containing 2 and 3 in that order (so that “fjrst” of the list is 2). • What is (first (cons 4 (cons 3 empty))) ? • What is (rest (cons 2 empty)) ? • Build a list z containing (cons 2 empty) and (cons 3 empty) , in that order • What is (first (rest z)) ? • What is (first (first z)) ?