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Karel – Chapter 5 Conditionally Executing Instructions

Karel – Chapter 5 Conditionally Executing Instructions. Note: Original slides provided by www.apComputerScience.com and modified for Mr. Smith’s AP Computer Science A class. CH5 – Conditional Statements. Boolean expressions return true or false. Version 1:

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Karel – Chapter 5 Conditionally Executing Instructions

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  1. Karel – Chapter 5Conditionally Executing Instructions Note: Original slides provided bywww.apComputerScience.comand modified for Mr. Smith’s AP Computer Science A class

  2. CH5 – Conditional Statements Boolean expressions return true or false Version 1: if ( <some boolean expression> ) { <some instruction list> } For now: these are method invocations (see next slide)

  3. “predicates” either true or false if statement frontIsClear() nextToABeeper() nextToARobot() facingNorth() facingSouth() facingEast() facingWest() anyBeepersInBeeperBag() if ( ) { … } Robot Let’s check the API to see our options

  4. Robot Class public class Robot extends UrRobot { public boolean frontIsClear() {…} public boolean nextToABeeper() {…} public boolean nextToARobot() {…} etc… } Now I have a brain! again, you don’t write this class

  5. Examples from a client program if ( karel.frontIsClear() ) { karel.move(); // no danger of hitting wall } The client has to refer to the object it is acting on if ( karel.anyBeepersInBeeperBag() ) { karel.putBeeper(); //no danger of error }

  6. Examples from an object class if ( frontIsClear() ) { move(); // no danger of hitting wall } if (anyBeepersInBeeperBag() ) { putBeeper(); //no danger of error }

  7. Extending Robot public class SmartBot extends Robot { public boolean beeperIsToLeft() {…} public boolean twoBeepersOrMoreOnCorner() {…} public void faceEast() {…} } Draw the Inheritance Hierarchy

  8. Creating a booleanmethod The method must return either true or false public boolean methodName() { // other instructions could go here if ( <some boolean expression> ) { return true; } return false; } Returns true and ends the method Returns false and ends the method

  9. MUST put world back in initial situation that it was in BEFORE the method was invoked public booleanbeeperIsToLeft() { turnLeft(); move(); if ( nextToABeeper() ) { turnLeft(); turnLeft(); move(); turnLeft(); return true; } turnLeft(); turnLeft(); move(); turnLeft(); return false; }

  10. SmartBot Create a SmartBot class that extends Robot You write these methods: beeperIsToLeft()(see previous slide) beeperIsToRight() (this method will return true if at least one beeper is on corner right of robot) twoBeepersOrMoreOnCorner()(this method will return true if at least two beepers are on corner)Note: you may have to nest if statements – look at page 118 in Karel textbook faceEast(), faceNorth(), faceSouth(), faceWest() (these methods will force the robot to turn in the indicated direction)Note: Look at page 112 in Karel textbook Also use the SmartBotTester to test SmartBot and these new methods. You should download and use the world file named smartbotWorld.txt.

  11. Paying Attention? • For the last several slides, we’ve been using a new robot class. By now you’ve probably figured out that our Inheritance Structure looks like this: What annoying thing (should have) happened to you while coding the last few examples? UrRobot Robot Yep, you wrote (or wanted to) turnAround() and maybe even turnRight() AGAIN! ANNOYING! SmartBot BetterRobot Solution(s)?

  12. What if there is only one statement in the THEN clause? if ( frontIsClear()) { move(); turnLeft(); } is NOT the same as ….. if ( frontIsClear()) move(); turnLeft(); if ( frontIsClear()) { move(); } is the same as ….. if ( frontIsClear()) move();

  13. Nested IF(IF statement inside an IF statement) if ( frontIsClear()) { move(); if ( nextToABeeper()) { pickBeeper(); } }

  14. exactlyOneBeeperOnCorner() publicboolean exactlyOneBeeperOnCorner() { if (nextToABeeper()) { pickBeeper(); if (nextToABeeper()) { putBeeper(); returnfalse; } putBeeper(); returntrue; } returnfalse; }

  15. if (exactlyOneBeeperOnCorner()) Check the lines of code that would execute for each scenario

  16. if (exactlyOneBeeperOnCorner()) Check the lines of code that would execute for each scenario l l l l l l l l l l l l l l l l l l l l l l l l l l l l

  17. Boolean Operators • Java uses same boolean operators as C++ (&&, ||, !) • && means AND • || means OR • ! means NOT • Example: if (! frontIsClear() || facingSouth()) { turnLeft(); } move();

  18. Truth Tables if (frontIsClear() && nextToABeeper()) frontIsClear()nextToABeeper() true true true false false true false false if (frontIsClear() || nextToABeeper()) frontIsClear()nextToABeeper() true true true false false true false false result true false false false result true true true false

  19. IF - ELSE Version 2: if ( <boolean expression> ) { <statements> } else { <statements – somewhat different> }

  20. IF - ELSE Example: if ( beeperIsToLeft() ) { turnLeft(); move(); pickBeeper(); } else { move(); }

  21. Practice Using && and || Write this method which could be put in SmartBot /* returns true if there is at least one beeper on both sides of bot, false otherwise */ public boolean beeperOnLeftAndRight() { }

  22. MazeWalker Look at ex. 9 in the book (pages 132-133) Initial Situation End Situation Create a single followWallRight() method to handle each of these situations. Hint: Before coding, look at the four situations and see what is the same and different for each. Start with the initial situation for each robot. How could you use an if statement to determine if they are in a specific situation? This can be done with an if statement that includes nested if statements.

  23. IF – ELSE Simplifications simplify: if ( frontIsClear() ) { return true; } else { return false; }

  24. IF – ELSE Simplifications One option: if ( frontIsClear() ) { return true; } return false; Or even better: return frontIsClear(); simplify: if ( frontIsClear() ) { return true; } else { return false; }

  25. Simplify if ( ! leftIsBlocked() ) { return true; } else { return false; }

  26. Possible Simplifications One option: if ( leftIsBlocked() ) { return false; } return true; Or even better: return ! leftIsBlocked(); if ( ! leftIsBlocked() ) { return true; } else { return false; }

  27. move(); Simplify – bottom factoring move(); if ( facingSouth() ) { turnLeft(); } else { turnRight(); } if ( facingSouth() ) { turnLeft(); move(); } else { turnRight(); move(); } move(); move(); move(); move();

  28. Simplify – top factoring if ( beeperOnLeft() ) { move(); turnLeft(); } else { move(); turnRight(); }

  29. Top factoringdoes not work here move(); if ( beeperOnLeft() ) { turnLeft(); } else { turnRight(); } if ( beeperOnLeft() ) { move(); turnLeft(); } else { move(); turnRight(); } Moves to new corner before checking for beeper

  30. turnLeft(); However, top factoringdoes work here turnLeft(); if (nextToABeeper() ) { pickBeeper(); } else { move(); } if ( nextToABeeper() ) { turnLeft(); pickBeeper(); } else { turnLeft(); move(); } turnLeft() does not affect whether robot is next to a beeper

  31. Being redundant again and again and againha ha if ( facingNorth() ) { move(); pickTwoBeepers(); if (facingNorth()) { turnLeft(); } } This if statement is redundant

  32. Here is better code(unless one of the instructions can cause a change in direction) if ( facingNorth() ) { move(); pickTwoBeepers(); if (facingNorth()) { turnLeft(); } } if ( facingNorth() ) { move(); pickTwoBeepers(); turnLeft(); }

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