Parameters and World-level methods

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Parameters and World-level methods

• Alice

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Our Dragon world

• The dragon must to take off and fly, to carry the
  princess

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Storyboards

• We can make the dragon takeOff and fly.

Dragon.takeOff Do together
dragon move up 1 meter dragon flap
its wings
Dragon.fly Do together dragon
move forward 1 meter dragon flap its
wings
Note that we would need very similar methods to
get the dragon to land (by flying down) or to fly
in other directions
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A Better Solution

• Multiple versions of very similar code seems a
  bit tedious. The only things that change is the
  direction the dragon is to fly.
• A better solution is to write a more flexible
  method.

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Parameters

• Built-in methods provide flexibility by providing
  parameters such as distance and direction.
• Parameters allow you to pass in values
  (arguments).
• Example
• Parameters direction, distance, duration
• Arguments left, 0.05 revolutions, 0.5 seconds

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Kinds of Parameters

• Alice provides several kinds of parameters that
  can be used in your own methods.

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The storyboard
In this example, we can write just one method
and use a parameter to specify which direction
the dragon is to fly.

• Dragon.flyInAnyDirection
• Parameter whichDirection
• Do together
• dragon move in whichDirection
• dragon flap its wings

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Demo

• Concepts illustrated
• Enter name and select the type of each parameter
• whichDirection is a Direction parameter
• A parameter acts as a placeholder in the
  instruction
• Arguments are passed to the parameter in
  the call to the method

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A Number parameter
Add a Number parameter to specify the distance
the dragon travels as it is flying. Note
that the call to the method must now include an
argument for the distance.
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World-level methods

• It is also possible to create world-level methods
• These are used when there are references to
  multiple objects in the world

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Example

• Consider our dragon world
• The dragon has taken off
• Now it must fly to the princess.
• Such a method will involve 2 objects the dragon
  and the princess

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Demo Starting a new method

• First, to associate the new method with the World
• select the World tile in the Object Tree
• select the methods tab in the details area
• click on the "create new method" button