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For Wednesday

For Wednesday. Read Becker, chapter 4, sections 3-5 Complete WebCT quiz. Program 2. Any questions?. Factoring out Differences. What if I want a CollectionBot? What do I need to change? How do I avoid duplicating code?. Helper Methods. What is a helper method?

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For Wednesday

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  1. For Wednesday • Read Becker, chapter 4, sections 3-5 • Complete WebCT quiz

  2. Program 2 • Any questions?

  3. Factoring out Differences • What if I want a CollectionBot? • What do I need to change? • How do I avoid duplicating code?

  4. Helper Methods • What is a helper method? • Who should call a helper method? • How do we enforce that?

  5. Access Modifiers • Public • Access from anywhere • Use for the services a robot provides to others • Protected • Access from methods in the class or its subclasses • Use for methods that may be overridden but don’t need to be public • Private • Access from only methods in the class • Use for anything that doesn’t need to be public or protected

  6. Control Structures • Allow programs to make decisions • Why the name “control structure”?

  7. Motivation • So far, we’ve written programs that • executed one statement after another (pattern?) • executed all statements in a method and returned • Some problems can’t be solved this way • Have a robot move forward until it reaches a wall. • Have a robot pick up things from intersections when not all intersections have things on them • Other examples? • Robot • Non-robot

  8. Different Kinds of Decisions • Should this group of statements be executed once, or not at all? if (karel.canPickThing()) { karel.turnLeft(); } karel.move(); • Should this group of statements be executed again? while (karel.canPickThing()) { karel.turnLeft(); } karel.move();

  9. Tracing an if statement if (karel.frontIsClear()) { karel.move(); } karel.turnLeft();

  10. Tracing a while statement while (karel.frontIsClear()) { karel.move(); } karel.turnLeft();

  11. What’s the Pattern?

  12. Pseudocode and Decisions

  13. Questions Robots Can Ask • Can I pick up a Thing from this intersection? • How many Things are in my backpack? • Is the path in front of me clear? • What avenue am I on? • What street am I on? • What direction am I facing? • What is my speed? • What is my current label?

  14. Predicates • Questions with the answers yes or no (actually true or false) • Can be directly used in if and while statements. • Return values of type boolean.

  15. Negatives • Sometimes we want to test the opposite of a predicate • Use the symbol !

  16. Integer Queries if (karel.getStreet() == 1) { karel.turnAround(); } while (karel.countThingsInBackpack < 4) { karel.pickThing(); }

  17. Comparison Operators • == • != • < • > • <= • >=

  18. Practice • Write code to move two spaces forward, picking up any things in any space the robot enters.

  19. Problem • Write a method to make a Robot go completely around the inside of a box created by walls.

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