Improving the Crab Mrs. C. Furman August 19, 2010

Adding Random Behavior  Currently, our crab moves from one edge to another in a straight line, turning in the same way each time.  Is this how crabs move???

int getRandomNumber (int limit)  We will be using this method to generate a random number. Return typeMethod NameFormal Parameter List

Greenfoot.getRandomNumber (20)  The above is a method call.  It returns a value from 0 – 19. That is 20 values to choose from. Class name method name Actual Parameter dot operator

Method Calls classname.method (parameter list)  This is the general form of a method call.  When we used move() or turn() we didn’t use a class name or object name, because move() and turn() are methods of our superclass.  If you are using methods of another class, you need to specify. or object

move() and turn()  To continue the general form of: object.methodname(parameters) object.methodname(parameters)  We can use the keyword this when we are calling methods in our own classes.  So, instead of: move() try this.move();

Exercise  Modify the act method in little-crab-1 by adding this and dot in front of each call to turn() and move().  Your act method should look as follows: public void act() public void act() { if ( this.atWorldEdge() ) if ( this.atWorldEdge() ) { this.turn(17); this.turn(17); } this.move(); this.move(); }

Random turn  Say we want to turn our crab occasionally on a random call to act. if (something is true) { this.turn(5); this.turn(5);}

What can we use to replace the something is true?  Say we want to turn 10% of the time.  So if we generate 100 random numbers, we want to call this.turn(5), when the numbers 0 – 9 are generated.

Relationship Operators  < less than  > greater than  <= less than or equal  >= greater than or equal  != not equal  == equal  note: to ask if two things are equal you need 2 = signs.

Boolean Expression  An expression that can be determined to be either true or false.  To find 10%, we can generate 100 numbers and if the number is less than 10 (0 – 9),then it would be in the range of the 10% of the 100 numbers. Greenfoot.getRandomNumber (100) < 10 Greenfoot.getRandomNumber (100) < 10

Modify your code public void act() { if(this.atWorldEdge()) if(this.atWorldEdge()) { this.turn(17); this.turn(17); } if (Greenfoot.getRandomNumber(100) < 10) if (Greenfoot.getRandomNumber(100) < 10) { this.turn(5); this.turn(5); } this.move(); this.move();}

Exercise Instead of turning 5 each time, modify your program to turn a random amount.

Exercise  Our crab is still only turning to the right. That’s not normal behavior for a crab, so let’s fix this. Modify your code so that the crab turns either left or right up to 45 degrees each time it turns.

Exercise  Run your project with multiple crabs. Do they all turn the same way at the same time?