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Karel – Chapter 5Conditionally Executing

Instructions

Note: Original slides provided by www.apComputerScience.com and modified for Mr. Smith’s AP Computer Science A class

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CH5 – Conditional Statements

Version 1:

if ( <some boolean expression> ){

<some instruction list>}

For now: these are method invocations (see next slide)

Boolean expressions return true or false

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if statement

if ( ){ … }

frontIsClear()

nextToABeeper()

nextToARobot()

facingNorth()

facingSouth()

facingEast()

facingWest()

anyBeepersInBeeperBag()

Robot

“predicates”

either true or false

Let’s check the API to see our options

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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

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Examples from a client program

if ( karel.frontIsClear() ){

karel.move(); // no danger of hitting wall

}

if ( karel.anyBeepersInBeeperBag() ){ karel.putBeeper(); // no danger of error}

The client has to refer to the object it is acting on

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Examples from an object class

if ( frontIsClear() ){

move(); // no danger of hitting wall}

if (anyBeepersInBeeperBag() ){ putBeeper(); // no danger of error}

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Extending Robot

public class SmartBot extends Robot{

public boolean beeperIsToLeft() {…}

public boolean twoBeepersOrMoreOnCorner() {…}public void faceEast() {…}

}Draw the

Inheritance Hierarchy

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Creating a boolean method

public boolean methodName(){ // other instructions could go here if ( <some boolean expression> ) {

return true; }

return false; }

The method must return either true or false

Returns true and ends the method

Returns false and ends the method

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public boolean beeperIsToLeft(){

turnLeft();move();if ( nextToABeeper() ){

turnLeft(); turnLeft(); move(); turnLeft(); return true;}turnLeft(); turnLeft(); move(); turnLeft();return false;

}

MUST put world back in initial

situation that it was in BEFORE the method was

invoked

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SmartBotCreate 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.

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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:

UrRobot

Robot

SmartBot

What annoying thing (should have) happened to you while coding the last

few examples?Yep, you wrote (or wanted to) turnAround() and maybe even turnRight() AGAIN! ANNOYING!

Solution(s)?

BetterRobot

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What if there is only one statement in the THEN

clause?if ( frontIsClear()){

move();}

is the same as …..

if ( frontIsClear()) move();

if ( frontIsClear()){

move(); turnLeft();

}

is NOT the same as …..

if ( frontIsClear()) move(); turnLeft();

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Nested IF(IF statement inside an IF statement)

if ( frontIsClear()){

move(); if ( nextToABeeper()) { pickBeeper(); }

}

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public boolean exactlyOneBeeperOnCorner() {

if (nextToABeeper()){

pickBeeper();if (nextToABeeper()){

putBeeper();return false;

}putBeeper();return true;

}return false;

}

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0Beepers

1Beeper

2Beepers

public boolean exactlyOneBeeperOnCorner()

{

if (nextToABeeper())

{

pickBeeper();

if (nextToABeeper())

{

putBeeper();

return false;

}

putBeeper();

return true;

}

return false;

}

Check the lines of code that would execute for each scenario

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0Beepers

1Beeper

2Beepers

public boolean exactlyOneBeeperOnCorner()

{

if (nextToABeeper())

{

pickBeeper();

if (nextToABeeper())

{

putBeeper();

return false;

}

putBeeper();

return true;

}

return false;

}

Check the lines of code that would execute for each scenario

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Boolean Operators

• Java uses same boolean operators as C++

(&&, ||, !)• && means AND• || means OR• ! means NOT

• Example: if (! frontIsClear() || facingSouth())

{ turnLeft();

}move();

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if (frontIsClear() && nextToABeeper())

frontIsClear() nextToABeeper()true truetrue falsefalse truefalse false

if (frontIsClear() || nextToABeeper())

frontIsClear() nextToABeeper()true truetrue falsefalse truefalse false

resulttruefalsefalsefalse

resulttruetruetruefalse

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IF - ELSEVersion 2:if ( <boolean expression> ){

<statements>}else{

<statements – somewhat different>}

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IF - ELSEExample:if ( beeperIsToLeft() ){

turnLeft(); move(); pickBeeper();}else{

move();}

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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() {

}

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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.

Initial Situation End Situation

Look at ex. 9 in the book (pages 132-133)

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IF – ELSE Simplifications

simplify:if ( frontIsClear() ){

return true;}else{

return false;}

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IF – ELSE Simplifications

simplify:if ( frontIsClear() ){

return true;}else{

return false;}

One option:if ( frontIsClear() ){

return true;}return false;

Or even better:return frontIsClear();

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Simplify

if ( ! leftIsBlocked() ){

return true;}else{

return false;}

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Possible Simplifications

if ( ! leftIsBlocked() )

{return true;

}else{

return false;}

One option:if ( leftIsBlocked() ){

return false;}return true;

Or even better:return ! leftIsBlocked();

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Simplify – bottom factoring

if ( facingSouth() )

{turnLeft();move();

}

else{

turnRight();move();

}

if ( facingSouth() )

{

turnLeft();

}

else

{

turnRight();

}

move();

move();

move();

move();

move();

move();

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Simplify – top factoringif ( beeperOnLeft() ){

move();turnLeft();

}else{

move();turnRight();

}

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Top factoring does not work here

if ( beeperOnLeft() ){

move();turnLeft();

}else{

move();turnRight();

}

move();if ( beeperOnLeft()

){

turnLeft();}else{

turnRight();}

Moves to new corner before checking for

beeper

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However, top factoring does work here

if ( nextToABeeper() )

{turnLeft();

pickBeeper();}else{

turnLeft();move();

}

turnLeft();if

(nextToABeeper() )

{pickBeeper();

}else{

move();} turnLeft() does not affect

whether robot is next to a beeper

turnLeft();

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Being redundant again and again and againha ha

if ( facingNorth() ){

move(); pickTwoBeepers(); if (facingNorth()) {

turnLeft(); }

}

This if statement is redundant

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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();

}