Week 3

Interactive Programming, Conditional Logic, and While Loops

Interactive Programming in Alice

Control of flow

• Flow control determines the sequence of statements executed when a program runs
  • which statements are executed and in what order
• In movie-style programs (Alice text chapters 1-4) the sequence of actions is completely determined by the programmer
  • every execution of a given program is the same
  • does not depend on any runtime data or events

Program input

• In most programs the flow of control is determined by runtime input
• The programmer determines how the program responds to input, but the input is not determined until the program runs
• There may be countless variations on how a given program behaves
  • this makes testing programs much harder!

Two kinds of input

• Data input is read by the program when it wants it
  • we'll do lots of this using Python
• User event input is generated when the program runs and the program responds (almost) immediately
  • this is called interactive programming
  • Alice makes this easy (within its framework)

Two kinds of animation

• Movie animations are not interactive at all
  • no input of any kind
  • every movie run is the same
• Animated games are highly interactive
  • game events trigger short animated sequences
  • usually there is no action at all, or only something simple and repetitive, unless there has been a recent event
  • no two game runs are the same

Alice events

• Alice programs can respond to a variety of user generated events:
  • clicking the mouse
  • pressing a key on the keyboard
  • moving the mouse
  • starting the program
• In response to events, Alice may
  • call a method
  • move an object
  • move the camera or change its orientation
• We'll use the bold blue features above

Event handling methods

• A method that is called in response to an event is called an event handler
• When an event is linked to a method that performs an action, a program behavior is created
• You can change the method invoked by the When the world starts event so it is no longer World.my first method
  • if World.my first method is not called at any other time, you need to deleted it (drag to trash bin), or you will get a warning message when the program starts indicating that the method cannot be reached

Which of the following cannot cause an event in Alice?

1. mouse click
2. keyboard key press
3. network activity
4. mouse motion
5. none (Alice can respond to any of the above)

Answer C

Which of the following is not true of functions

1. functions return values
2. functions can take arguments
3. functions can be used as instructions
4. functions in Alice are associated with objects

Answer: C

The most important consideration in program style is usually

1. runtime efficiency
2. clarity
3. brevity
4. originality

Answer: B

Demo: spotlight control

• We start with spotlightScene.a2w
  • the light object brightness property is turned down to 25% so illumination from the dimmer stageSpotlight object is visible
• Alice visual rendering supports a large number of light sources
• What is most unnatural about Alice light modeling?
• Demo scenario: create an interactive program that allows "manual" control of the spotlight aiming and provision for flashing it red

Demo interactive program specification

• Use the arrow keys to move the spotlight left, right, up, and down in small increments
  • each of these events requires calling only one method (belonging to the stageSpotlight object)
• Have the spotlight flash red when the mouse clicks on it
  • first set the stageSpotlight color property to red
  • then set the stageSpotlight color property back to no color (white)

spotlightControl.a2w

• World.my first method and initial When the world starts event that invokes it have been deleted, since nothing is done when the program starts

Basic event programming in Alice

• Events are created in the Events editor frame
  • use the create new event button, which creates When statements that only appear in this editor frame
• A method is called to handle each event
  • the event may be built-in or added to the program
  • only World methods show up in the create new event button drop-down menu, but any method call may be dragged into the event statement

Parameters and Event-Handler Methods in Alice

Demo scenario and program structure

• Add to the spotlight controller a control panel with buttons that flash different colors when pressed
• Code reuse: use an event method parameter so only one event method is needed for all the flash buttons

Demo: spotlightControlPanel.a2w

Demo concepts

• Parameters allow one event handling method to handle four different events
• Use multiple light sources for more precise control of lighting
  • in the demo the world light is turned off (0 brightness)
  • a lightBulb object provides light for the control panel
  • the pointLight range property appears to have no effect

Testing interactive programs

• When parameters are used in interactive programming, it is especially important to test that all possible parameter values work as expected
• If event actions have significant interaction with one another, it is necessary to test many possible permutations (combinations and orders) of events in order to catch interaction bugs
  • there are not significant interactions in the last demo
• There may be so many permutations that it is impossible to test them all, in which case you have to be especially careful to program with the best possible correctness and style

Functions in Alice

Demo: throwing a ball

• Scenario: animate the motion of a ball thrown in the air

  • doing this with any physical accuracy requires calculating the effect of gravity
  • the ball motion program in the Alice text section 6.1 does not do this

• Physics: if the initial velocity is v, the velocity at time t is

  v + at

  where a is the acceleration

  • the acceleration due to gravity is -9.8 meters per second squared

Demo: throwBall.a2w

New loop concept

• A loop may have a index variable, the value of which changes each time the body of the loop is executed
  • in Alice, click the show complicated version loop statement button to expose this variable
  • an index variable starts with an initial value and is incremented by some amount until it get up to (but in Alice not including) a given final value
    • in our example, the variable index takes on the value 0, .02, .04,...,.98

How a function works

• A function receives value(s), performs some computation on the value(s), and returns (sends back) a value

Types of functions

• The type of a function is the type of value it returns
  • a number
  • a specific object
  • a color
  • a Boolean (true or false)
  • other property values...

How function are like methods

• Like methods, functions may be associated with any class, including World
  • so like methods, functions may be exported with their class, and the same limitations apply on references to methods and functions outside of the exported class
    • no references to other objects
    • no references to world-level functions you have written (built-in world-level functions are fine to use)
• Like methods, functions may have any number of parameters

How functions differ from methods

• The body of a function contains only optional comment lines followed by a return statement
  • the body of a method contains any number of statements
  • the return statement contains an expression, whose value is returned when the function is called
  • in Alice, return statements are not allowed in methods
• A function call is an expression, whereas a method call is a statement

In Alice, method calls may have parameters

1. in event handlers but not method bodies
2. in event handlers and method bodies
3. in only in method bodies
4. nowhere

Answer: B

In Alice return types are associated with

1. functions
2. methods
3. functions and methods
4. niether functions or methods, only variables

Answer: A

In Alice

1. functions and methods both may have return statements
2. functions must have return statements but they are optional in methods
3. functions always have return statements and methods never do
4. functions and methods always have return statements

Answer: C

Bouncing ball demo main points

• Scenario: ball is thrown up (and a bit to the right), comes down under the force of gravity, and bounces up again when it reaches the ground
• A Y velocity variable is modified each time around the loop and when the ball bounces
  • rather than modifying a time variable
• The program loops indefinitely
• A conditional test (if statement) determines when the ball hits the ground

Demo: bounceBall.a2w

bounceBall demo concepts

• Gravity and initial velocity variables are never assigned after their initialization
  • such variables are called constants
  • constants avoid the use of magic numbers in program text
    • they are magic because it unless they are given names it is hard to see what they are for
• An infinite loop never ends
• yVelocity is now a property (a variable belonging to an object, in this case the ball)
  • updated with each move
• Conditional test used to reset yVelocity when ball hits ground
• The test uses a distance above function call and the relational operator < (less than)

Fundamental programming concept: variable

• A variable is a container that holds a value and has an associated name
• A variable name appearing as an expression is called a variable reference
  • its value is the value in the associated container
• An assignment statement names a variable and has a subexpression
  • the value of the expression is placed in a associated variable
  • this is how a variable's value varies

Kinds of variables

• There are three kinds of variables: parameters, local variables, and properties
• Parameters and local variables
  • are created when a function or method is called and destroyed when the function or method returns
  • can only be referred to within the associated method or function
• Properties are contained in objects and live as long as the objects do

Initialization of variables

• The initial value of a local variable or a property is the value of an expression in its declaration
• The initial value of a parameter is the value of the corresponding argument expression of the function or procedure call that creates the variable

Variables in Alice

• Alice variables are created with a create new variable button
  • if the button is in an editor window, a local variable is created
  • if the button is in the properties detail tab, a property variable is created
• Alice variable and parameter values are assigned by dragging their declaration into the method body as an action (statement)
  • see Alice text section 10.1

Demo: looseBall.a2w

looseBall demo concepts

• Scenario: ball gets lost by falling in a hole
  • the hole is represented by a circle
• A conditional statement nested in the then-part of another conditional statement detects if the ball is near the (center of) the circle, in which case it becomes invisible
• The loop continues indefinitely, even though there is no visible change after the ball falls into the hole
  • we'll fix this problem in a bit

Demo: spotlightColorCycler.a2w

Else-if control pattern

• Else-if: If statement nested in the Else part of another If statement
• Else-if nesting happens repeatedly when there are many conditions, as in the last demo
• Python and many other languages, but not Alice, have a special kind of statement that avoids this deep nesting
• The "then" part associated with the first true condition is executed, and then the outer If is finished
• There may be statements in the Else part of the innermost else-if, which are executed if none of the conditions are true

Else-if control picture

Demo scenario

• We extend spotlightColorCycler.a2w so that after each stage light color change, the White Rabbit performs an act that depends on the color and chance
• The rabbit's act is to simultaneously:
  • turn around, if the color is not red, with 50% probability
  • jump if the color is red or blue
  • otherwise do nothing

Demo: rabbitAct.a2w

Demo concept: Boolean operators

• and
  • true only if both sub-expressions are true
• either ... or ... or both
  • just called or in most computer languages
  • true if either sub-expression is true, or both
• not: reverses the value of its Boolean argument
  • true becomes false and false becomes true
  • analogous to numeric negation

Boolean operators truth table

Demo concept: randomness

• Pseudo-random number generators allow randomness in program behavior
  • it is call "pseudo" because it is not truly random
  • most languages have one or more kinds of random number generators
• In Alice, the World has two randomness functions
  • choose true probability of the time
    • used in demo with probability 50%
  • random number
    • number returned is randomly chosen in the range specified (0 to 1 by default)
    • optionally, may return integers (whole numbers) only

While: Indefinite Loops in Alice

Demo: looseBallandStop.a2w

• The program is made to stop when the ball falls into the hole by replacing the infinite loop with a while loop with a test that calls the ball's isShowing function
  • the loop body is unchanged

While statement flow of control

• In words:

  While condition is true
      execute statement(s)
  

• If the condition is never true, the statements are never executed

• If the condition is always true, the loop is infinite (it never ends)

Avoiding infinite loops

• Though sometimes infinite loops are desired, they frequently are the result of programming errors
• A while loop will be infinite unless the statements in its body change something so that makes the condition false
  • assuming the condition is true to begin with
  • and assuming no events or other external control events do not make a change the condition depends on

Loop how many times?

• Infinite loop: infinitely many times
  • in Alice, use Loop infinity times
    • another way: While true
• Definite loop: a fixed number of times
  • known when the program is written, when the loop begins
  • in Alice, use a Loop statement
• Indefinite loop: an unknown number of times
  • determined by some condition that occurs while the loop is running
  • in Alice, use a While statement

Demo scenario

• Repeat twice the following simultaneous actions
  • blueBird moves forward 5 meters
  • blueBird repeats 4 wing flaps
  • blueBird turns left and then right

Demo: flying.a2w

Demo concept: nested loops

• Loops can be nested: one loop in the body of another
• if the outer loop iterates n times and the inner loop always iterates m times, the body of the inner loop executes a total of n * m times
• there is no special rule to allow loops and other kinds of statements to be nested in any particular way
• a loop is a statement and the body of a loop is statements of any kind

Two means of repetition

• Iteration: "looping" control structures
  • in Alice: Loop and While statements
  • in most languages, including Python: while and for statements
• Recursion: procedures (functions or methods in Alice) that call themselves
  • recursive calls may be direct or indirect (from other procedures called before the recursive procedure returns)
  • Alice text chapter 8 (optional for this course)
  • in most languages (including Alice and Python), do not use when the number of repetitions will be large (hundreds or more)

In the Alice editor, where are boolean logic operators found?

1. functions tab of any object's details
2. methods tab of any object's details
3. events editor frame
4. functions tab of World's details

Answer: D

Multiple logical conditions may be checked using:

1. only nested if statements
2. only boolean operators
3. only multiple loops
4. nested if statements or boolean operators

Answer: D

Using Alice, my early experience with programming has been

1. more enjoyable than expected
2. less enjoyable than expected
3. about as enjoyable as expected

I would rather

1. continue to program in Alice
2. switch now to another language like Python that is more practical
3. have skipped Alice and started with Python

Onward to Python!

• Python vs. Alice
• Python vs. other languages

Python vs. Alice: In general

• Python is a general-purpose language
  • Python is suitable for a tremendous variety of real-world uses
  • Alice is special purpose language for simple 3D animations
• Python is easier to learn than most languages
  • Alice is even easier to learn, but very limited
• Python is good for writing small and medium sized programs
  • Alice is good for small programs
    • medium sized programs are possible but much more difficult

Python vs. Alice: Syntax

• Like almost all languages, Python requires constant attention to avoid syntax errors
  • Python's syntax is simpler than that of most languages
  • Python's editor provides coloring and automatic indentation that helps with syntax
• Alice's editor prevents syntax errors
  • you still must have some understanding of syntax to know what the editor will allow you to do

Python vs. Alice: Types

• In Python types are not checked until runtime
  • no type declarations in programs
  • very flexible, but requires constant attention to types when writing programs
• Alice's editor prevents most type errors before runtime
  • every variable and parameter declaration includes its type
  • due to type problems, the editor may not allow you do to some things you know are Ok

Python vs. Alice: Bottom line

• Alice is a great language for introducing many programming concepts
  • it prevents many beginner errors
  • it's lots of fun
  • but it's highly specialized (not practical)
• Python is a great language for learning to do practical programming
  • it makes it easy to do many practical things
  • I think it's more fun than other general-purpose languages

Python vs. other languages

• Some other languages, such as Java, are often better for writing large programs
  • large: many thousands of lines of code
• Some other languages, such as Visual Basic, are more popular
  • but they are harder to learn, harder to use, and much less fun!
  • most are much less versatile
• Most are similar to Python in many ways
  • so they are much easier to learn if you already know Python