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TeachScheme, ReachJava

Learn about the new symbol and char data types in programming, their spelling rules, built-in functions, and how to use them in code. Explore examples and exercises to deepen your understanding.

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TeachScheme, ReachJava

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  1. TeachScheme, ReachJava Adelphi University Tuesday morning June 23, 2009

  2. A new type: symbol "foo-bar" is a string 'foo-bar is a symbol(yes, no closing apostrophe) Spelling rule: basically the same as a variable or function name, but preceded with apostrophe Built-in function ; symbol=? : symbol symbol -> boolean

  3. Mouse handlers • Contract : model num(x) num(y) symbol -> model • Events: 'button-down, 'button-up, 'drag, 'move, 'wheel-up, 'wheel-down, 'enter, 'leave … • Typically, one or two "interesting" events and an "anything else" category. • Code structure: cond with one or two (symbol=? ... …) questions and an else

  4. Example mouse handler Exercise 18.2.1 in book: ; add-dot-on-mouse-down : image(old) num(x) num(y) symbol(event) -> image (check-expect (add-dot-on-mouse-down BACKGROUND 35 10 'button-down)(place-image DOT 35 10 BACKGROUND)) (check-expect (add-dot-on-mouse-down BACKGROUND 35 10 'button-up)BACKGROUND)

  5. Example mouse handler (define (add-dot-on-mouse-down old x y event); old image; x, y numbers; event symbol(cond [(symbol=? event 'button-down) (place-image DOT x y old)] [else old] ; ignore this event))

  6. A new type: char "x" is a string #\x is a char 7 is a number "7" is a string #\7 is a char Built-in functions ; char=? : char char -> boolean ; string-ref : string number -> char ; make-string : char number -> string ; others (see Help Desk or Chap. 19 of book)

  7. A function using char ; starts-with-M? : string -> boolean (check-expect (starts-with-M? "a") false) (check-expect (starts-with-M? "M") true) (check-expect (starts-with-M? "abcMdef") false) (check-expect (starts-with-M? "") false) (check-expect (starts-with-M? "Morgan") true)

  8. A function using char (define (starts-with-M? str); str string; (string-ref str 0) char(char=? (string-ref str 0) #\M)) Mostly works, but crashes on empty string!

  9. A function using char (define (starts-with-M? str); str string; (string-ref str 0) char, if str non-empty(cond [(= (string-length str) 0) false] [else (char=? (string-ref str 0) #\M)])) Passes all test cases… but do we really need cond?

  10. A function using char (define (starts-with-M? str); str string; (string-ref str 0) char, if str non-empty(and (> (string-length str) 0) (char=? (string-ref str 0) #\M))) Slightly shorter and simpler

  11. A new type: key A key is either a symbol or a char(Specifically, 'left, 'right, 'down, 'up, 'escape, 'f1, 'home, 'release, …) Built-in function ; key=? : key key -> boolean

  12. Example Write an animation of a calendar that starts at the middle of the screen and moves left or right in response to left and right arrow keys on the keyboard Ex. 19.2.1 in book

  13. Animation: model & handlers Let's use a number as the model, representing the x coordinate (the y coordinate will be fixed at, say, 50) We need a redraw handler ; calendar-at-x: number -> image and a key handler ; handle-key: number char-or-symbol -> number

  14. Animation: redraw handler ; calendar-at-x: number -> image (define (calendar-at-x x); x number(place-image calendar x 50 BACKGROUND)) (check-expect (calendar-at-x 27) (place-image calendar 27 50 BACKGROUND)) (check-expect (calendar-at-x 193) (place-image calendar 193 50 BACKGROUND))

  15. Animation: key handler Model is a number representing x coordinate Want to respond to left, right arrow keys ; handle-key : number(x) key -> number (check-expect (handle-key 10 #\D) 10) (check-expect (handle-key 10 'left) 9) (check-expect (handle-key 10 'right) 11) (check-expect (handle-key 10 'up) 10)

  16. Animation: key handler (define (handle-key x key); x number; key key(cond [(key=? key 'left) …] [(key=? key 'right) …] [else …]))

  17. Animation: key handler (define (handle-key x key); x number; key key(cond [(key=? key 'left) (- x 1)] [(key=? key 'right) (+ x 1)] [else x]))

  18. Running the animation (define WIDTH 300) (define HEIGHT 100) (define BACKGROUND(rectangle WIDTH HEIGHT "solid" "white")) (run-animation WIDTH HEIGHT(/ WIDTH 2) 1(on-redraw calendar-at-x)(on-key handle-key))

  19. Defining a type "by choices" • Type = "this string or that string or …" • Type = "this range of numbers or that range of numbers or …" • Type = "this type or that type or …" • Test cases: at least one for each category of input or output

  20. Code for definition by choices • A cond w/ N branches, where N is the number of categories in the input type • For strings, usually use string=? • For numbers, <, >, <=, >=, =, … • For symbols, symbol=? • For chars, char=? • For types, …?

  21. Discriminator functions (built-in) • number? : anything -> boolean • string? : anything -> boolean • image? : anything -> boolean • symbol? : anything -> boolean • char? : anything -> boolean • etc. Tell whether something is of the specified type

  22. Example Write a function big? that takes in a number or a string or an image. Numbers > 1000 are "big"; strings longer than 10 characters are "big"; images with height* width > 10000 are "big".

  23. Contract & data analysis ; big? : number, string, or image -> boolean ; Data analysis: input is either number, string, or image ; In each case, it's broken down into two sub-categories

  24. Test cases (check-expect (big? 347) false) (check-expect (big? 1000) false) (check-expect (big? 1200) true) (check-expect (big? "alison") false) (check-expect (big? "passphrase") false) (check-expect (big? "brontosaurus") true) (check-expect (big? calendar) false) (check-expect (big? (rectangle 200 50 "solid" "purple")) false) (check-expect (big? (ellipse 200 60 "outline" "chartreuse")) true)

  25. Skeleton & inventory (define (big? thing); thing a string, number, or image)

  26. Body, first draft (define (big? thing); thing string, number, or image(cond [ q a ] [ q a ] [ q a ]))

  27. Body, second draft (define (big? thing); thing string, number, or image(cond [(string? thing) a ] [(number? thing) a ] [(image? thing) a ]))

  28. Body, third draft (define (big? thing); thing string, number, or image(cond [(string? thing) (> (string-length thing) 10)] [(number? thing)(> thing 1000)] [(image? thing)(> (* (image-width thing) (image-height thing)) 10000)]))

  29. Example: animation using type choice • Key handler that treats all chars one way, all symbols another way • Exercises 20.4.1, 20.4.2 in book

  30. Recall left/right arrow animation • Could obviously have picture move up or down, rather than left or right • Could have circle of radius that increases or decreases • etc. • How about a picture that moves up, down, left, or right?Problem: need to "remember" both x and y coordinates

  31. Definition by parts Sometimes several related data need to be passed around together, e.g. • name, SSN, and salary of employee • x and y coordinates of 2-D point • name, length, and favorite food of boa-constrictor Scheme allows you to define a new data type with several "parts"

  32. A predefined struct There's a built-in type named posn, which has two parts x and y. Built-in functions ; make-posn : number(x) number(y) -> posn ; posn-x : posn -> number ; posn-y : posn -> number ; posn? : anything -> boolean

  33. For those who know Java… • make-posnis a constructor. • posn-xandposn-y are getter methods. • There are also setter methods, but we'll get to them later.

  34. Using posns "Examples of the posn type:" (make-posn 3 5) (make-posn 12 -7) (define here (make-posn 14 9)) (check-expect (posn-x here) 14) (check-expect (posn-y here) 9)

  35. Writing functions on posns Almost any function that takes in a posn looks like (define (function-on-posn where) ) (the "skeleton" step of the recipe)

  36. Writing functions on posns Almost any function that takes in a posn looks like (define (function-on-posn where); where posn ) (the "inventory" step of the recipe)

  37. Writing functions on posns Almost any function that takes in a posn looks like (define (function-on-posn where); where posn; (posn-x where) number; (posn-y where) number ) (the "inventory" step of the recipe, continued)

  38. Using posns Write a function right-of-100? which takes in a posn and tells whether its x coordinate is greater than 100 Work this out together

  39. Creating posns Write a function diagonal-posn which takes in a number and produces a posn whose x and y coordinates are both that number Work this out together

  40. Exercises on posns Write a function above-main-diagonal? which takes in a posn and tells whether it is above the diagonal line "x=y" (note that positive x is to the right, and positive y is down, as usual in computer graphics) Write a function posn=? which takes in two posns and tells whether they have the same x coordinate and the same y coordinate Write a function swap-x-y which takes in a posn and returns a posn whose x coordinate is the y coordinate of the given posn, and vice versa.

  41. Optional exercises on posns Write a function distance which takes in two posns and returns their Euclidean distance, which is given by the formula sqrt((x1-x2)^2 + (y1-y2)^2) Write a function scale-posn which takes in a posn and a number and produces a posn by multiplying each coordinate of the given posn by the number Write a function add-posns which takes in two posns and produces a posn whose x coordinate is the sum of their x coordinates, and likewise for the y coordinates

  42. Animations with posn models Write an animation of a picture that moves up, down, left, and right in response to the corresponding arrow keys

  43. Model & handlers • Model is a posn representing center of picture • We'll need a redraw handler; calendar-at-posn : posn -> imageand a key handler; handle-key : posn key -> posn

  44. Redraw handler ; calendar-at-posn : posn -> image (check-expect (calendar-at-posn (make-posn 27 104)) (place-image calendar 27 104 BACKGROUND)) (check-expect (calendar-at-posn (make-posn 215 6)) (place-image calendar 215 6 BACKGROUND)) (define (calendar-at-posn where); where posn; (posn-x where) number; (posn-y where) number(place-image calendar (posn-x where) (posn-y where) BACKGROUND))

  45. Key handler: contract & examples ; handle-key : posn key -> posn (check-expect (handle-key (make-posn 12 47) #\v)(make-posn 12 47)) (check-expect (handle-key (make-posn 12 47) 'up)(make-posn 12 46)) (check-expect (handle-key (make-posn 12 47) 'down)(make-posn 12 48)) (check-expect (handle-key (make-posn 12 47) 'left)(make-posn 11 47)) (check-expect (handle-key (make-posn 12 47) 'right)(make-posn 13 47)) (check-expect (handle-key (make-posn 12 47) ‘home)(make-posn 12 47))

  46. Key handler: skeleton & inventory ; handle-key : posn key -> posn (define (handle-key old-place key) (cond [(key=? key 'left) …] [(key=? key 'right) …] [(key=? key 'up) …] [(key=? key 'down) …] [else old-place]))

  47. Key handler: skeleton & inventory ; handle-key : posn key -> posn (define (handle-key old-place key) (cond [(key=? key 'left) (add-posns old-place (make-posn -1 0))] [(key=? key 'right) (add-posns old-place (make-posn 1 0))] [(key=? key 'up) (add-posns old-place (make-posn 0 -1))] [(key=? key 'down) (add-posns old-place (make-posn 0 1))] [else old-place]))

  48. Or more briefly… ; handle-key : posn char-or-symbol -> posn (define (handle-key old-place key)(add-posns old-place (cond [(symbol=? key 'left) (make-posn -1 0))] [(symbol=? key 'right) (make-posn 1 0)) [(symbol=? key 'up) (make-posn 0 -1)) [(symbol=? key 'down) (make-posn 0 1)) [else (make-posn 0 0))))

  49. Running the animation (define WIDTH 300) (define HEIGHT 300) (define BACKGROUND (rectangle WIDTH HEIGHT "solid" "white")) (run-animation WIDTH HEIGHT(make-posn (/ WIDTH 2) (/ HEIGHT 2))1(on-redraw calendar-at-posn)(on-key handle-key))

  50. Exercises • Write an animation of a picture that, every second, teleports to a random location within the window, but ignores key & mouse events • Write an animation of a picture that moves randomly up, down, left, or right by a pixel each 0.1 second

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