CS320n –Visual Programming
Random Numbers and Random Motion
(Slides 6-3)
Thanks to Wanda Dann, Steve Cooper, and Susan Rodger
for slide ideas.
What We Will Do Today
• Look at the use of random numbers in
programs
• Start the material on repetition
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Random Numbers and Random
Motion
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Random Numbers
• Random numbers are used in certain kinds of
computer programs
• Examples:
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security for web applications
encryption for satellite transmissions
gaming programs
scientific simulations
• In this session, we will look at examples of
how to use random numbers in animations
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Random Numbers and Random
Motion
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Built-in Functions
• Alice provides built-in functions for
generating random numbers.
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Random Numbers and Random
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Example
• from assignment 4
• The knight has a class level method
• replaced distance and revolutions with
random number function
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The random number function
• World’s random number function
• returns a fractional value between 0 and 1
• uniformly distributed
– each number has an equal chance of being
picked
• Side track: generation of random numbers
is a whole area of study in computer
science
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Motion
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What will knight’s behavior be?
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Picking from Ranges of Values
• Default random numbers are between 0 and 1
• Can alter the min and max value
• Currently knight always turns left
– how can this be changed?
• Alter the range for the distance moved forward
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Motion
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Improved Random Movement
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Demo: Integers (whole numbers)
• To generate a random integer value (having no
decimal point or digits to the right of a decimal
point), select integerOnly from the more…
options and make it true.
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Random Numbers and Random
Motion
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Random Hopping
• Rabbit hops (moves up and then down) a
random amount
• Predictions on behavior?
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Remembering Things
• Parameters were used to pass information
into a method or function
• The parameter could take many different
values
– who -> wizard, dragon, troll, princess
– target -> any object in the world
• The parameter can be used over and over
in the method
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Variables – in a Method
• A variable in a method
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stores a value
has an initial value
can have its value changed
can only be used in the method it
is declared in
• To create a variable
• To use the value in a variable
– drag the variable into place
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Motion
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Setting Variable to other Value
• To give a variable a numeric value can assign it a value
at top of method
• to give it the result of a function call drag variable into
method and choose set value
• pick dummy value and then replace with random
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Random Numbers and Random
Motion
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Using Variables
• Demo of the bunny hop
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Random Motion
• In some animations, we want an object to move
to a random location.
• We call this random motion.
• For example, the goldfish in this world is to swim
in a random motion.
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Six possible directions
• Of course, six move directions are possible
– forward, backward, left, right, up, down
• We can eliminate backward because goldfish
do not swim backward.
• To simplify the code, we can take advantage of
negative numbers.
– For example, this instruction actually moves the
goldfish right:
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Random Numbers and Random
Motion
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Storyboard
• Only three move instructions are needed:
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up (will move down if number is negative)
left (will move right if number is negative)
forward (no backward motion)
• Two parameters (min, max) will be used to restrict the
motion of the fish to a nearby location-- to look like
swimming.
fish.randomMotion:
Parameters: min, max
Do together
fish move up a random number distance
fish move left a random number distance
fish move forward a random number distance
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Motion
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randomMotion
• The minimum distance of the move forward
instruction is 0 (the goldfish always moves
forward).
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Demo
• To call the randomMotion method, the min and
max values must be specified.
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