CS320n%20

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CS320n Visual Programming

• Execution Control with If / Else and Boolean
  Functions (Slides 6-2-1)

Thanks to Wanda Dann, Steve Cooper, and Susan
Rodger for slide ideas.
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What We Will Do Today

• Learn about
• using if / else statements to control the flow of
  a program
• using boolean functions

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Adding More Complexity to Worlds

• Initially programs were not interactive
• program ran and user watched
• Then learned how to make programs interactive
• add events to program
• when event occurs an event handler method is
  called
• adds variability to programs
• not always the same thing happening

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Event Driven Programming

• a whole philosophy of programming called event
  driven programming
• flow of program controlled by external events as
  opposed to things internal to the program
• very important in user interfaces
• the parts of programs that interact with real
  users
• think about using a computer
• the operating system ( Windows XP, Mac OS X, etc.
  ) has a large component that is event driven
• responding to your actions. (Move mouse, click,
  type)

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Internal Decision Making

• To build more complex programs and animations
  external decision making is needed in the form of
  events
• But also need internal decision making
• important in many types of programs
• good examples are simulations and games

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Examples of Decisions

• A race car simulation
• driver provides input from steering wheel, gas
  pedal, and brake
• if car stays on road display next section of road
• if car hits another car, crash
• if car goes too far off road, crash
• if pass checkpoints, more time

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Logical Expressions

• Program must make decision based on current
  conditions
• on road? time left? intersected another car?
• Conditions are checked in a logical expression
  (also called a boolean expression)
• the logical expression evaluates to true or false
• car on road -gt true
• car passed finish line -gt false
• car intersected any other cars -gt false

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If/Else

• In Alice, a logical expression is used as the
  condition in an If/Else control structure .
• Decisions (using If/Else) are used in
• functions
• methods

IF
ELSE
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Example Boolean Question

• Suppose you are building a simulation system used
  to train flight controllers.
• One of the tasks of a flight controller is to be
  alert for possible collisions in the flight
  space.

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Problem Description

• two aircraft
• biplane and helicopter
• as biplane moves towards helicopter make sure
  they do not collide
• if they are too close, they need to adjust
  altitude (height) or course (direction of travel)

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Problem Description

• two factors in determining whether two aircraft
  are in danger of collision
• total distance between them
• center point to center point
• vertical distance between them
• differences in height
• both distances must be greater than some safe
  distance otherwise aircraft are too close

red line total distance green line vertical
distance
a method or a function? class level or world
level?
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tooCloseByTotalDistance
isTooCloseByTotalDistance Parameters
aircraft1, aircraft2, minDistance return true if
the distance between aircraft1 andaircraft2 is
less than minDistance else return false

• Whether the distance between aircraft1 and
  aircraft2 is less than minDistance is an example
  
• of a boolean expression.
• It is either true of false
• return result of expression

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Steps in building function

• Select the world object and click on the
  functions tab
• select new function

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Steps in building function

• type in name of function
• pick Boolean for the return type (function will
  return true or false)
• add parameters for 2 objects (the aircraft) and a
  number (min distance)

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Steps in building function

• function returns true
• want to return if distance between aircraft is
  less than minDistance
• select World functions
• pick a lt b and drag to replace true
• example of a relational operator

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Steps in building function

• replace true
• fill in a and b with dummy values (like 1)

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Steps in building function

• would like to drag aircraft1 parameter and call
  distance to function
• cant ?
• select biplane and the distance to function
• drag that to replace 1

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Steps in building function

• replace dummy objects with parameters aircraft1
  and aircraft2
• replace second 1 with minDistance

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Completed Method
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Using if / else

• in World.my first method
• check to see if the aircraft are too close
• IF they are then avoid a collision
• hard code motion OR
• create a method to avoid the collision
• to use if / else in program drag if /else from
  bottom

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Using if / else

• if statement tests a boolean expression
• can be a relation operator
• OR a function that returns a boolean
• OR a boolean value (true / false)

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Completing the if / else

• replace parameters with biplane and helicopter
• call method avoid collision if the condition is
  true
• nothing to do if they are not to close

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Completing avoid collision

• assume if aircraft are too close one moves up and
  one moves down to increase the vertical distance
  between them
• elevation separation
• does it matter which moves up and which moves
  down?

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using if / else is avoid collision

• Move the aircraft that is above the other up

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isTooCloseVerticalDistance

• check to see if aircraft are in the same strata
• to find difference in altitude use the built-in
  distance above function
• dont know which aircraft is above the other
• to avoid possible negative value, use the
  absolute value of the distance (World level
  function)

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Adding Motion

• create a method that moves an aircraft forward
  some distance
• if the two aircraft are closer than twice the
  distance of the move
• avoid collision if they are the vertical distance
  between them is too small
• move aircraft forward twice the distance

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forward And Check Collision
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check For Height Collision