1. TeachScheme, ReachJava Stephen Bloch, Adelphi University, Garden City, NY John Clements, Cal Poly, San Luis Obispo, CA Kathi Fisler, Worcester Polytechnic Institute, Worcester, MA Matthew Flatt, University of Utah, Salt Lake City, UT Shriram Krishnamurthi, Brown University, Providence, RI Viera K. Proulx, Northeastern University, Boston, MA Supported by NSF grant #0618543 Related work by Matthias Felleisen, Robby Findler, Kathy Gray, Eli Barzilay, et al

2. June 28 2009TeachScheme, ReachJava 20092 What is a problem? What is the meaning of life? –Not an objective question How old am I? –An objective question, which requires information you don’t have What is the smallest prime number? –An objective, fully-specified question, but once it’s answered once, it’s over

3. June 28 2009TeachScheme, ReachJava 20093 What is a problem? What is the Celsius equivalent of 45F? –Again, once it’s answered, it’s over What is the Celsius equivalent of 87F? –Ditto, but closely related What is the Celsius equivalent of ___ in Fahrenheit? –Generalizes infinitely many questions

4. June 28 2009TeachScheme, ReachJava 20094 What is a problem? Question: has a single answer Problem: large (or infinite) family of questions, sharing some features & differing in others Program: general rule for solving a problem, with input for the features that differ Computation: a program working on particular input to produce particular results

5. June 28 2009TeachScheme, ReachJava 20095 Introductions Who are you? What kind of school? students? Background in teaching CS Background in Scheme, Java, etc?

6. June 28 2009TeachScheme, ReachJava 20096 This week You’re primarily students, not teachers Get “the student experience” of this approach — only faster Time on Friday to talk pedagogy

7. June 28 2009TeachScheme, ReachJava 20097 What is “computer science”? “microscope science” — about microbes “telescope science” — about stars It’s not about the tool, it’s about what you can study using the tool! “computer science” — about information and the manipulation thereof

8. June 28 2009TeachScheme, ReachJava 20098 What is a beginning CS course about? 1)How to use Microsoft Office et al 2)Survey of the CS profession 3)A language that will get students jobs 4)Concepts, habits, & methodologies of programming Four completely different courses, each valuable for different reasons and audiences #3 useful for students about to graduate, or about to get internships and summer jobs #4 more useful as a first course. Let's call it "CS1"…

9. June 28 2009TeachScheme, ReachJava 20099 What is CS1 about?

10. June 28 2009TeachScheme, ReachJava 200910 What you teach blah algorithm blah variable blah function blah data type blah object blah re-use blah methodology blah testing blah design blah composition blah refinement blah abstraction

11. June 28 2009TeachScheme, ReachJava 200911 What they hear blah ; blah { blah ) blah ] blah return blah this.x = x; blah public static void main (String[] args) blah /* blah // blah blah if ( blah JOptionPane blah class Posn(int x,y)

12. June 28 2009TeachScheme, ReachJava 200912 Programming languages: a necessary evil Necessary: to write real programs that really run on real computers Evil: distract students from the important stuff Will be obsolete in a few years anyway

13. June 28 2009TeachScheme, ReachJava 200913 How to minimize language & IDE? Introduce features one at a time Avoid "black magic"; never ask students to write anything they can't understand now Introduce features only as necessary to teach an important concept Corollary: any feature that doesn't help teach a CS1 concept shouldn't be mentioned in CS1 Corollary: need enforced language subsets so students don't accidentally use language features they haven't seen For GUI, robotics, networking, etc. need interface between student code (in subset) and library code (in full language)

14. June 28 2009TeachScheme, ReachJava 200914 What Java do we need? When? Realistically, most of us must cover a certain amount of Java by the end of CS2 Not obvious that the best way to do this is Java from start of CS1 Instead, teach concepts & habits first, then complicated language

15. June 28 2009TeachScheme, ReachJava 200915 Alternatives to Java-first Leading alternatives: Alice, Python, Scheme All three: start w/simpler syntax while students master concepts Difference: Alice & Python usually taught imperatively; Scheme usually functionally (Functional-first Python: has potential)

16. June 28 2009TeachScheme, ReachJava 200916 Why functional? Simple, familiar semantic model –3+4*5 => 3+20 => 23 Ease of writing test suites –each test is usually one stmt, rather than "set up", "call method", "check side effects" –tests order-independent –no worry about aliasing, equals() vs. ==, etc. –If testing is easy, students may actually do it!

17. June 28 2009TeachScheme, ReachJava 200917 But what about OOP? Students need to learn OOP, and an OO language, by 2nd or 3rd semester OOP is a terrific way to organize multi-KLOC programs First-term students don't write multi-KLOC programs OOP's benefits aren't apparent in first term; students see only the overhead (Challenge: write a short, OO C++/Java program that wouldn't be simpler without OO)

18. June 28 2009TeachScheme, ReachJava 200918 Main points of TSRJ Multi-language approach to CS1/CS2 –Start in Scheme, develop concepts & habits –Switch to Java after 2-4 months Step-by-step design recipe in both languages –concrete questions & products at each step –test-first methodology –strong emphasis on data types –shape of data determines shape of code & test cases DrScheme development environment –beginner-friendly –interactive testing and experimentation –enforces language subsets w/appropriate messages

19. June 28 2009TeachScheme, ReachJava 200919 Let’s try DrScheme DrScheme Interactions pane (bottom half) Literal expressions –7 –"hello world" –copy and paste picture from Web browser –true Try www.adelphi.edu/sbloch/book/pictures/

20. June 28 2009TeachScheme, ReachJava 200920 Operating on pictures (reflect-vert paste-picture-from-Web) Same with reflect-horiz, rotate-cw, rotate-ccw, rotate-180 (image-above picture1 picture2) Same with image-beside, overlay –These three actually accept two or more pictures. Syntax rule: ( operation pic1 … ) Terminology: expression, function, argument, value, literal

21. June 28 2009TeachScheme, ReachJava 200921 Composing functions > (image-beside (reflect-horiz )) Exercise: write expressions to produce

22. June 28 2009TeachScheme, ReachJava 200922 Scheme syntax All expressions fully parenthesized; no order of operations to memorize All functions (both built-in and user-defined) are prefix, inside parentheses Some functions have arity 1, some 2, some "2 or more", etc. as appropriate.

23. June 28 2009TeachScheme, ReachJava 200923 Definitions pane Editable and savable "Run" button evaluates all expressions in order, replacing old Interactions Try 7 "hello world" copied image from Web (image-above (rotate-cw picture) (rotate-ccw picture))

24. June 28 2009TeachScheme, ReachJava 200924 The Stepper Type several nested expressions into Definitions pane Click "Step" button at top See each sub-expression replaced with its value, one by one Valuable for students who never really "got" algebra

25. June 28 2009TeachScheme, ReachJava 200925 Variable definitions (define calendar ) (image-above calendar (reflect-vert calendar)) Define another variable to hold a different picture copied from the Web; write some expressions using both. Note no declared data type; data have types, variables don't

26. June 28 2009TeachScheme, ReachJava 200926 Building images from scratch (rectangle 34 21 "solid" "blue") (circle 18 "outline" "green") (ellipse 34 55 "outline" "purple") (triangle 22 "solid" "pink") (star 7 20 10 "solid" "blue") (text "Hello there" 18 "blue") Try your own variations (define blue-star (star 7 20 10 "solid" "blue")) Try modifying & combining these using image-above, overlay, reflect-vert, rotate-cw, surround, etc.

27. June 28 2009TeachScheme, ReachJava 200927 Terminology Three kinds of literals: image (pasted from Web browser), number, and string. Data types

28. June 28 2009TeachScheme, ReachJava 200928 Function contracts ; reflect-vert : image -> image ; reflect-horiz, rotate-cw, rotate-ccw, rotate-180 : similar ; image-beside : image image … -> image ; image-above, overlay : similar ; rectangle : number(width) number(height) string(solid/outline) string(color) -> image Summarizes what a function takes in and returns, in a brief, standard notation Note semicolon for comment-to-end-of-line

29. June 28 2009TeachScheme, ReachJava 200929 More image functions ; crop-bottom : image number(pixels) -> image ; crop-top, crop-left, crop-right : similar ; image-width : image -> number ; image-height : similar ; place-image : image(foreground) number(x) number(y) image(background) -> image ; text : string(text) number(size) string(color) -> image

30. June 28 2009TeachScheme, ReachJava 200930 Defining functions > (define (mirror picture) (image-beside picture (reflect-horiz picture))) (mirror calendar) (mirror (rotate-cw (triangle 10 "solid" "blue"))) Note no declared parameter or return types; again, data have types Try (mirror calendar hacker) -- wrong number of arguments! Try (mirror 7) -- wrong type of argument!

31. June 28 2009TeachScheme, ReachJava 200931 Exercise Define a function four-square that takes in an image and produces a 2x2 arrangement of it like

32. June 28 2009TeachScheme, ReachJava 200932 Worked Exercise Define a function counterchange that takes in two images and produces a 2x2 arrangement like My answer: (define (counterchange topleft topright) (image-above (image-beside topleft topright) (image-beside topright topleft)))

33. June 28 2009TeachScheme, ReachJava 200933 Testing functions With English descriptions: –(mirror calendar) "should be a calendar on the left, and a right-left-reflected calendar on the right" With check-expect: –(check-expect (mirror calendar) (image-beside calendar (reflect-horiz calendar))) Put either or both in Definitions window; hit Run & see what happens. Try with wrong function definition!

34. June 28 2009TeachScheme, ReachJava 200934 Design recipes We could write counterchange in an ad-hoc way, but in general we use… Step-by-step recipes to get from English-language description to working, tested code One recipe for functions, one for data types, one for abstractions, one for event-driven animations…

35. June 28 2009TeachScheme, ReachJava 200935 How to accomplish anything 1)Figure out what you want to do 2)Do it 3)Check that you did it right

36. June 28 2009TeachScheme, ReachJava 200936 How to write a function/method Figure out what you want to do Contract: Specify name, inputs, outputs Data definition: Identify any new data types in the problem Examples: Write test cases with expected answers Do it Skeleton: Write boilerplate code for function definition Inventory: Write available expressions, their types, and (later) their values for a non-trivial test case Body: Fill in details based on problem to solve Check that you did it right Testing: Run test cases

37. June 28 2009TeachScheme, ReachJava 200937 The design recipe in practice contract: ; counterchange : image (topleft) image(topright) -> image data definition: problem only involves images, which are predefined test cases: (check-expect (counterchange calendar schemelogo) (image-above (image-beside calendar schemelogo) (image-beside schemelogo calendar))) (check-expect (counterchange book (ellipse 30 10 "outline" "pink")) (image-above (image-beside book (ellipse 30 10 "outline" "pink")) (image-beside (ellipse 30 10 "outline" "pink") book))

38. June 28 2009TeachScheme, ReachJava 200938 The design recipe in practice skeleton: (inserted before examples) (define (counterchange topleft topright) ) inventory: (define (counterchange topleft topright) ; topleft an image ; topright an image ) body: (fill in something real, using expressions from inventory) (define (counterchange topleft topright) ; topleft an image ; topright an image (image-above (image-beside topleft topright) (image-beside topright topleft))) testing: hit "Run" and see whether answers match

39. June 28 2009TeachScheme, ReachJava 200939 The result ; counterchange : image (topleft) image(topright) -> image (check-expect (counterchange calendar schemelogo) (image-above (image-beside calendar schemelogo) (image-beside schemelogo calendar))) (check-expect (counterchange book (ellipse 30 10 "outline" "pink")) (image-above (image-beside book (ellipse 30 10 "outline" "pink")) (image-beside (ellipse 30 10 "outline" "pink") book)) (define (counterchange topleft topright) ; topleft an image ; topright an image (image-above (image-beside topleft topright) (image-beside topright topleft)))

40. June 28 2009TeachScheme, ReachJava 200940 Testing again Change the definition so that it's wrong; run the tests again and see what happens.

41. June 28 2009TeachScheme, ReachJava 200941 Design recipes as pedagogy Note "test-first" methodology (a la XP); identify special cases before writing code Use as grading rubric: partial credit for each step First steps are in comments; non-threatening, avoids "blank page syndrome" Each step has concrete questions and concrete products I don't help with step N until I see step N-1 Same steps apply in Java, C++, etc. (but more complicated) Know-it-all students usually try to skip the recipe… until Chapter 12 of HtDP…

42. June 28 2009TeachScheme, ReachJava 200942 Exercises Write contracts and test cases, but no definitions, for 1)a function named copies-beside that takes in a number and an image, and produces that many copies of the image side by side 2)a function named pinwheel that takes in an image & produces a 2x2 rotated thing like 3)a function named checkerboard2 that takes in two color names & produces a 2x2 checkerboard in those colors

43. June 28 2009TeachScheme, ReachJava 200943 Exercises Write skeletons & definitions for pinwheel and checkerboard2 (you don't know how to do copies-beside yet) Do all the steps to define lollipop, which takes in two numbers and a color name, and creates a picture of a lollipop w/specified radius, stick length, and color

44. June 28 2009TeachScheme, ReachJava 200944 Discussion break How is this different from what you've done in the past? How much explaining would it take for your students? I have a lot of mathophobic students, so I start with images rather than numbers and algebra. The prefix notation doesn't throw them, because they don't already "know" the "right" notation for operating on images.

45. June 28 2009TeachScheme, ReachJava 200945 Animation (run-animation 50 50 calendar 0.5 (on-tick rotate-cw)) ; run-animation : number(width) number(height) image number(tick-interval) handler … -> boolean Every tick-interval, applies handler to old image to get new image Try some variations: different widths, heights, images, tick- intervals, "on-tick" handlers

46. June 28 2009TeachScheme, ReachJava 200946 A more complex animation Write an animation of an image that moves 5 pixels to the right every second We'll need a function that takes in an image and returns the same image 5 pixels to the right Follow the design recipe!

47. June 28 2009TeachScheme, ReachJava 200947 move-right-5 ; move-right-5 : image -> image

48. June 28 2009TeachScheme, ReachJava 200948 move-right-5 ; move-right-5 : image -> image (check-expect (move-right-5 calendar) (image-beside (rectangle 5 0 "solid" "white") calendar)) (check-expect (move-right-5 (circle 3 "solid" "red")) (image-beside (rectangle 5 0 "solid" "white") (circle 3 "solid" "red")))

49. June 28 2009TeachScheme, ReachJava 200949 move-right-5 ; move-right-5 : image -> image (define (move-right-5 old-pic) ) (check-expect (move-right-5 calendar) (image-beside (rectangle 5 0 "solid" "white") calendar)) (check-expect (move-right-5 (circle 3 "solid" "red")) (image-beside (rectangle 5 0 "solid" "white") (circle 3 "solid" "red")))

50. June 28 2009TeachScheme, ReachJava 200950 move-right-5 ; move-right-5 : image -> image (define (move-right-5 old-pic) ; old-pican image ) (check-expect (move-right-5 calendar) (image-beside (rectangle 5 0 "solid" "white") calendar)) (check-expect (move-right-5 (circle 3 "solid" "red")) (image-beside (rectangle 5 0 "solid" "white") (circle 3 "solid" "red")))

51. June 28 2009TeachScheme, ReachJava 200951 move-right-5 ; move-right-5 : image -> image (define (move-right-5 old-pic) ; old-pican image (image-beside (rectangle 5 0 "solid" "white") old-pic)) (check-expect (move-right-5 calendar) (image-beside (rectangle 5 0 "solid" "white") calendar)) (check-expect (move-right-5 (circle 3 "solid" "red")) (image-beside (rectangle 5 0 "solid" "white") (circle 3 "solid" "red")))

52. June 28 2009TeachScheme, ReachJava 200952 Running the animation (run-animation 500 50 calendar 1 (on-tick move-right-5))

53. June 28 2009TeachScheme, ReachJava 200953 Other kinds of event handlers tick-handler : image -> image Specify with on-tick mouse-handler : image number(x) number(y) event -> image Specify with on-mouse key-handler : image key -> image Specify with on-key I'm lying: they're actually more general than this. Note: on-tick, on-mouse, and on-key all take in a function name as an argument.

54. June 28 2009TeachScheme, ReachJava 200954 Another animation Write an animation of a calendar that moves with the mouse on a 500x300 yellow background. Need a mouse-handling function Contract must be ; handle-mouse : image num(x) num(y) event -> image We don't know what an "event" is yet; ignore it.

55. June 28 2009TeachScheme, ReachJava 200955 calendar-at-mouse ; calendar-at-mouse : image num(x) num(y) event -> image

56. June 28 2009TeachScheme, ReachJava 200956 calendar-at-mouse ; calendar-at-mouse : image num(x) num(y) event -> image (check-expect (calendar-at-mouse schemelogo 47 218 "dummy") (place-image calendar 47 218 (rectangle 500 300 "solid" "yellow"))) (check-expect (calendar-at-mouse stick-figure 350 12 "event") (place-image calendar 350 12 (rectangle 500 300 "solid" "yellow")))

57. June 28 2009TeachScheme, ReachJava 200957 calendar-at-mouse ; calendar-at-mouse : image num(x) num(y) event -> image (check-expect (calendar-at-mouse schemelogo 47 218 "dummy") (place-image calendar 47 218 (rectangle 500 300 "solid" "yellow"))) (check-expect (calendar-at-mouse stick-figure 350 12 "event") (place-image calendar 350 12 (rectangle 500 300 "solid" "yellow"))) (define (calendar-at-mouse old-pic x y event) )

58. June 28 2009TeachScheme, ReachJava 200958 calendar-at-mouse ; calendar-at-mouse : image num(x) num(y) event -> image (check-expect (calendar-at-mouse schemelogo 47 218 "dummy") (place-image calendar 47 218 (rectangle 500 300 "solid" "yellow"))) (check-expect (calendar-at-mouse stick-figure 350 12 "event") (place-image calendar 350 12 (rectangle 500 300 "solid" "yellow"))) (define (calendar-at-mouse old-pic x y event) ; old-pican image ; xa number ; ya number ; eventwhatever )

59. June 28 2009TeachScheme, ReachJava 200959 calendar-at-mouse ; calendar-at-mouse : image num(x) num(y) event -> image (check-expect (calendar-at-mouse schemelogo 47 218 "dummy") (place-image calendar 47 218 (rectangle 500 300 "solid" "yellow"))) (check-expect (calendar-at-mouse stick-figure 350 12 "event") (place-image calendar 350 12 (rectangle 500 300 "solid" "yellow"))) (define (calendar-at-mouse old-pic x y event) ; old-pican image ; xa number ; ya number ; eventwhatever (place-image calendar x y (rectangle 500 300 "solid" "yellow")))

60. June 28 2009TeachScheme, ReachJava 200960 calendar-at-mouse ; calendar-at-mouse : image num(x) num(y) event -> image (define BACKGROUND (rectangle 500 300 "solid" "yellow")) (check-expect (calendar-at-mouse schemelogo 47 218 "dummy") (place-image calendar 47 218 BACKGROUND)) (check-expect (calendar-at-mouse stick-figure 350 12 "event") (place-image calendar 350 12 BACKGROUND)) (define (calendar-at-mouse old-pic x y event) ; old-pican image ; xa number ; ya number ; eventwhatever (place-image calendar x y BACKGROUND))

61. June 28 2009TeachScheme, ReachJava 200961 Running the animation (run-animation 500 300 calendar 999 (on-mouse calendar-at-mouse))

62. June 28 2009TeachScheme, ReachJava 200962 Numbers Remember, my students are numerophobic. We haven't seen an arithmetic operator yet. (Week 4 of a non-major course) Arithmetic operators follow the same syntax rule as every other function: (operation argument …) The +, -, *, / operations accept 2 or more arguments Example: 3 + 4*5 + 6 becomes (+ 3 (* 4 5) 6) Other built-in functions with obvious names: sqrt, cos, sin, abs, …

63. June 28 2009TeachScheme, ReachJava 200963 Examples of arithmetic 3+4 becomes (+ 3 4) 3+(4*5) becomes (+ 3 (* 4 5)) 1+2+3+4+5 becomes (+ 1 2 3 4 5) 3x-7 becomes (- (* 3 x) 7) (presumably x is an already-defined variable) 3*4+5/6 becomes (+ (* 3 4) (/ 5 6)) (but you have to know order of operations to understand the former; the latter is unambiguous)

64. June 28 2009TeachScheme, ReachJava 200964 Exercise Convert the following from "standard" algebraic notation to Scheme notation: 3+cos(0) 2/(x+1) (-b+√(b 2 -4ac))/2a where a, b, c are variables

65. June 28 2009TeachScheme, ReachJava 200965 Writing functions on numbers ; cube : number -> number (define (cube num) ; num a number (* num num num)) (check-expect (cube 0) 0) (check-expect (cube 5) 125) (check-expect (cube -6) -216) (check-expect (cube (/ 2 3)) (/ 8 27))

66. June 28 2009TeachScheme, ReachJava 200966 Kinds of numbers Try (cube (cube (cube 1234567890))) Integers behave correctly Try (+ (/ 2 3) (/ 3 4)) Fractions behave correctly (can choose output in either fraction or repeating-decimal form) Abbreviations: -4 = (- 4), 2/3 = (/ 2 3), etc. as long as there are no spaces

67. June 28 2009TeachScheme, ReachJava 200967 Kinds of numbers Try (sqrt 2) Result is marked as inexact, as are results of subsequent computations using it Note (sqr (sqrt 2)) ≠ 2 (check-expect (sqr (sqrt 2)) 2) will fail! (check-within (sqr (sqrt 2)) 2 0.001) passes the test Use check-within whenever function result might be inexact.

68. June 28 2009TeachScheme, ReachJava 200968 Exercises Define a function f that takes in two numbers x and y and returns 3x-2y Define a function discriminant that takes in three numbers a, b, and c and returns b 2 - 4ac As usual, follow the design recipe!

69. June 28 2009TeachScheme, ReachJava 200969 Animations revisited Previous animations computed the "new image" from the "old image" Often makes more sense to compute on something other than an image -- a "model" "Model" can be an image or a number (or really any data type you choose)

70. June 28 2009TeachScheme, ReachJava 200970 Kinds of event handlers tick-handler : model -> model Specify with on-tick mouse-handler : model number(x) number(y) event -> model Specify with on-mouse key-handler : model key -> model Specify with on-key redraw-handler : model -> image Specify with on-redraw unless model is an image, in which case you can skip it run-animation : number(width) number(height) model number(tick-interval) handler … -> boolean Note: on-tick, on-mouse, on-key, and on-redraw all take in a function name as an argument.

71. June 28 2009TeachScheme, ReachJava 200971 Animations with numeric models Write an animation of a blue circle that grows in radius by 1 pixel per half second Need a tick handler and a redraw handler Follow the design recipe for both There happens to already be an add1 function which will work as the tick handler

72. June 28 2009TeachScheme, ReachJava 200972 Growing-circle animation ; blue-circle-of-size : number -> image (check-expect (blue-circle-of-size 0) (circle 0 "solid" "blue")) (check-expect (blue-circle-of-size 12) (circle 12 "solid" "blue")) (define (blue-circle-of-size r) (circle r "solid" "blue")) (run-animation 200 200 0 1/2 (on-tick add1) (on-redraw blue-circle-of-size))

73. June 28 2009TeachScheme, ReachJava 200973 Exercise: parametric equations Write an animation that displays a small dot at x coordinate 100+50*cos(t/20) and y coordinate 100+30*sin(t/20) where t is the number of time steps so far. (Set the tick interval fairly short, e.g. 1/10 second.) Hint: write helper functions x(t) and y(t). Play with the formulae and see what different patterns you can get.

74. June 28 2009TeachScheme, ReachJava 200974 Exercise Write an animation that displays a digital counter, in 18-point blue numerals. It should start at 0 and increase by 1 every second. Hint: there's a built-in function number->string that converts a number to its decimal representation. Then use text to convert the string to an image.

75. June 28 2009TeachScheme, ReachJava 200975 Discussion break How is this different from what you've done in the past? How much explaining would it take for your students?

76. June 28 2009TeachScheme, ReachJava 200976 Animations with numeric models Can now assign animations in which model is a number Examples: radius, x coordinate, or y coordinate, number of sides, … Model can increase & decrease in response to ticks, mouse actions, & keyboard actions

77. June 28 2009TeachScheme, ReachJava 200977 Animations with randomness (random 8) returns a random integer from 0 through 7 Use this to write more fun and unpredictable animations

78. June 28 2009TeachScheme, ReachJava 200978 Strings ; string-append : string … -> string ; string-length : string -> number ; substring : string number [number] -> string ; string->number : string -> number ; number->string : number -> string Can now write animations with strings as the model (e.g. adding or chopping characters)

79. June 28 2009TeachScheme, ReachJava 200979 Booleans ; = : number number -> boolean ; >, =, <= : similar ; string=? : string string -> boolean ; image=? : image image -> boolean ; not : boolean -> boolean ; and : boolean … -> boolean ; or : boolean … -> boolean

80. June 28 2009TeachScheme, ReachJava 200980 A Boolean-valued function ; 18-to-25? : number -> boolean (define (18-to-25? age) ; age a number (and (>= age 18) (<= age 25))) (check-expect (18-to-25? 17) false) (check-expect (18-to-25? 18) true) (check-expect (18-to-25? 22) true) (check-expect (18-to-25? 25) true) (check-expect (18-to-25? 26) false)

81. June 28 2009TeachScheme, ReachJava 200981 Stopping an Animation There's another kind of handler: (stop-when function) where function has contract model -> boolean

82. June 28 2009TeachScheme, ReachJava 200982 Stopping an Animation Example: a growing disk that stops growing when the radius reaches 100 ; model is a number representing radius ; over-100? : number -> boolean (define (over-100? r) (> r 100)) (run-animation 200 200 0 1/4 (on-tick add1) (on-redraw blue-circle-of-size) (stop-when over-100?))

83. June 28 2009TeachScheme, ReachJava 200983 Conditionals (cond [boolean-expr-1 answer-1] [boolean-expr-2 answer-2] … [boolean-expr-n answer-n]) tries each boolean-expr in turn. As soon as one of them evaluates to true, it evaluates and returns the corresponding answer.

84. June 28 2009TeachScheme, ReachJava 200984 Functions with conditionals Write a function reply that takes in one of the strings "good morning", "good afternoon", or "good night", and returns "I need coffee!", "I need a nap!", or "Bed time!" respectively.

85. June 28 2009TeachScheme, ReachJava 200985 Functions with conditionals Contract & data analysis ; reply : string -> string Data analysis: the input falls into three categories: "good morning", "good afternoon", and "good night". The output likewise falls into three categories: "I need coffee!", "I need a nap!", or "Bed time!"

86. June 28 2009TeachScheme, ReachJava 200986 Functions with conditionals Test cases Need a test case for each category of input, and each category of output. Conveniently, they match up one-to-one in this example. (check-expect (reply "good morning") "I need coffee!") (check-expect (reply "good afternoon") "I need a nap!") (check-expect (reply "good night") "Bed time!")

87. June 28 2009TeachScheme, ReachJava 200987 Functions with conditionals Skeleton & inventory Since there are three categories of input (and output), we'll probably need a 3-branch cond: (define (reply greeting) ; greeting ; a string (cond [ question answer ] [ question answer ] [ question answer ] ) )

88. June 28 2009TeachScheme, ReachJava 200988 Functions with conditionals Body Fill in either all three answers, or all three questions, whichever is easier. In this case, the answers. (define (reply greeting) ; greeting ; a string (cond [ question "I need coffee!" ] [ question "I need a nap!" ] [ question "Bed time!" ] ))

89. June 28 2009TeachScheme, ReachJava 200989 Functions with conditionals Body Then do the other of (questions, answers). (define (reply greeting) ; greeting ; a string (cond [ (string=? greeting "good morning") "I need coffee!" ] [ (string=? greeting "good afternoon") "I need a nap!" ] [ (string=? greeting "good night") "Bed time!" ] ))

90. June 28 2009TeachScheme, ReachJava 200990 Functions with conditionals Error-checking Quibble: this isn't idiot-proof. What happens if input isn't one of the three recognized inputs? Answer: ugly error message. Solution: revise data analysis (and everything that depended on it) Input is "good morning", "good afternoon", "good night", or anything else. Output is "I need coffee!", "I need a nap!", "Bed time!", or "Huh?" Add one more test case (check-expect (reply "buenas noches") "Huh?")

91. June 28 2009TeachScheme, ReachJava 200991 Functions with conditionals Error-checking Add one more cond case: (define (reply greeting) ; greeting ; a string (cond [ (string=? greeting "good morning") "I need coffee!" ] [ (string=? greeting "good afternoon") "I need a nap!" ] [ (string=? greeting "good night") "Bed time!" ] [ else "Huh?" ] ))

92. June 28 2009TeachScheme, ReachJava 200992 Isomorphism The shape of the data determines the shape of the code and tests. Say this ten times before bed.

93. June 28 2009TeachScheme, ReachJava 200993 Another example ; pepper-scale : number -> string ("serrano", "cayenne", "thai", "habanero") ; Scoville 5000-25000 -> serrano ; Scoville 30000-50000 -> cayenne ; Scoville 65000-90000 -> thai ; Scoville 100000-up -> habanero ; Data analysis: input is a number, but falls into 4 categories: ; 5000-25000, 30000-50000, 65000-90000, 100000-up. ; Output is likewise four categories: "serrano", "cayenne", "thai"., "habanero"

94. June 28 2009TeachScheme, ReachJava 200994 Another example ; Test cases therefore need to include all 4 categories plus borderlines. (check-expect (pepper-scale 5000) "serrano") (check-expect (pepper-scale 16500) "serrano") (check-expect (pepper-scale 25000) "serrano") (check-expect (pepper-scale 30000) "cayenne") (check-expect (pepper-scale 42000) "cayenne") (check-expect (pepper-scale 50000) "cayenne") (check-expect (pepper-scale 65000) "thai") (check-expect (pepper-scale 85000) "thai") (check-expect (pepper-scale 90000) "thai") (check-expect (pepper-scale 100000) "habanero") (check-expect (pepper-scale 120000) "habanero")

95. June 28 2009TeachScheme, ReachJava 200995 Another example: skeleton & inventory There are four categories, hence a four-branch cond: (define (pepper-scale scoville) ; scoville a number (cond [ q a ] [ q a ] [ q a ] [ q a ] ))

96. June 28 2009TeachScheme, ReachJava 200996 Another example: body Fill in the answers (define (pepper-scale scoville) ; scoville a number (cond [ q "serrano" ] [ q "cayenne" ] [ q "thai" ] [ q "habanero" ] ))

97. June 28 2009TeachScheme, ReachJava 200997 Another example: body Fill in the questions (define (pepper-scale scoville) ; scoville a number (cond [ (and (>= scoville 5000) ( = scoville 30000) ( = scoville 65000) ( = scoville 100000) "habanero" ] ))

98. June 28 2009TeachScheme, ReachJava 200998 Another example ; rough-age : number -> string ("child", "teenager", or "adult") ; Data analysis: input is a number, but falls into 3 categories: ; under 13, 13-19, and over 19. ; Output is likewise three categories: "child", "teenager", "adult". ; Test cases therefore need to include all 3 categories plus borderlines. (check-expect (rough-age 7) "child") (check-expect (rough-age 13) "teenager") (check-expect (rough-age 16.3) "teenager") (check-expect (rough-age 19) "teenager") (check-expect (rough-age 20) "adult")

99. June 28 2009TeachScheme, ReachJava 200999 Functions with conditionals ; rough-age : number -> string ("child", "teenager", or "adult") ; Data analysis: input is a number, but falls into 3 categories: ; under 13, 13-19, and over 19. (define (rough-age age) ; age a number (cond [( = age 13) ( age 19) "adult"])) (check-expect (rough-age 7) "child") (check-expect (rough-age 13) "teenager") (check-expect (rough-age 16.3) "teenager") (check-expect (rough-age 19) "teenager") (check-expect (rough-age 20) "adult")

100. June 28 2009TeachScheme, ReachJava 2009100 We could have written… (define (rough-age age) ; age a number (cond [(< age 13) "child"] [(<= age 19) "teenager"] [else "adult"])) by relying on the fall-through behavior of the conditional. Advantage: less typing. Advantage: save a few nanoseconds of run time (maybe). Disadvantage: you can't tell when a particular branch will happen just by looking at that condition; you have to also look at all the previous ones Disadvantage: branches of conditional can no longer be reordered without changing function behavior. Disadvantage: the isomorphism to the input data type is less clear. Use else only when you really mean "anything else".

101. June 28 2009TeachScheme, ReachJava 2009101 Animations with conditionals Can now assign animations that decide among a finite set of cases, e.g. slide show of a sequence of pictures stop light that cycles red, green, yellow, red…

102. June 28 2009TeachScheme, ReachJava 2009102 So far we've… covered the first six weeks of my non-majors' programming course learned four syntax rules –function call –variable definition –function definition –conditional gotten some practice writing, composing, and re-using functions, following a concrete step-by-step recipe learned to write interactive animations with model/view framework and callbacks (will do the same in Java)