cs320n –visual programming random numbers and random motion (slides 6-3) thanks to wanda dann,...

CS320n –Visual Programming

Random Numbers and Random Motion

(Slides 6-3)

Thanks to Wanda Dann, Steve Cooper, and Susan Rodger for slide ideas.

Visual Programming Random Numbers and Random Motion

2

What We Will Do Today• Look at the use of random numbers in

programs

• Start the material on repetition


3

Random Numbers

• Random numbers are used in certain kinds of computer programs

• Examples:– 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


4

Built-in Functions• Alice provides built-in functions for

generating random numbers.


5

Example• from assignment 4• The knight has a class level method

• replaced distance and revolutions with random number function


6

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


7

What will knight’s behavior be?


8

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


9

Improved Random Movement


10

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.


11

Random Hopping• Rabbit hops (moves up and then down) a

random amount

• Predictions on behavior?


12

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


13

Variables – in a Method• A variable in a method

– 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


14

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


15

Using Variables• Demo of the bunny hop


16

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.


17

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:


18

Storyboard• Only three move instructions are needed:

– 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, maxDo 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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randomMotion

• The minimum distance of the move forward instruction is 0 (the goldfish always moves forward).


20

Demo• To call the randomMotion method, the min and

max values must be specified.

cs320n –visual programming random numbers and random motion (slides 6-3) thanks to wanda dann,...

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