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Conditional Statements

Flavor 1:

if ( <some boolean expression> )

{

<some instruction list>

}

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

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IF

if ( )

{

…

}

frontIsClear();

nextToABeeper();

nextToARobot();

facingNorth();

facingSouth();

facingEast();

facingWest();

anyBeepersInBeeperBag();

Robot

“predicates”

either True or False

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What Does it Mean?

public boolean frontIsClear()

• HELP - The word void has been replaced by the word boolean. This Means this method returns either true or false.

• If there is NO wall in between the Robot and the corner directly in front, this method returns true.

• Otherwise (there is a wall), this method returns false.

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Method Definition

public boolean nextToABeeper();

• If there is a (one or more) Beeper on the Robot’s current corner, this method returns true.

• Otherwise (NO Beeper current corner), this method returns false.

• That is, returns true if safe for Robot to pickBeeper

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Method Definition

public boolean nextToARobot();

• If there is another (one or more) Robot on the Robot’s current corner, this method returns true.

• Otherwise (NOT another Robot on current corner), this method returns false.

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Method Definition

public boolean anyBeepersInBeeperBag();

• If this Robot has any (one or more) beepers in its beeper bag, this method returns true.

• Otherwise (NO beepers in beeper bag), this method returns false.

• That is, returns true if safe for Robot to putBeeper

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Method Definition

public boolean facingNorth();

• If this Robots is facing North (up), this method returns true.

• Otherwise (facing South, East or West ), this method returns false.

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Method Definition

public boolean facingSouth();

• If this Robots is facing South (down), this method returns true.

• Otherwise (facing North, East or West ), this method returns false.

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Method Definition

public boolean facingEast();

• If this Robots is facing East (to the right), this method returns true.

• Otherwise (facing North, South or West ), this method returns false.

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Method Definition

public boolean facingWest();

• If this Robots is facing West (to the left), this method returns true.

• Otherwise (facing North, South or East ), this method returns false.

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Indenting by example• How to properly indent while writing if structures by example!

public void someMethod()

{

if ( <someBooleanExpression> )

{

codeGoesHere();

allAdditionalCodeLinesUp();

soAllFirstLettersAreInTheSameColumn();

}

afterClosingBracketNextMethodCallGoesHere();

untilAnotherIfStatementIsEncountered();

}

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A word about indentingHow to properly indent while writing if structures!

• Every time an if is used, each line after the curly bracket, {, should be spaced over to the right a constant amount (say 3 spaces).

– To simplify, the first letter of each method/line-of-code should be under the ( in the if statement.

• When the if is terminated ( closing curly bracket } ), the following lines of code are moved left by the same number of spaces used above (3 was used above).

– To simplify, the first letter of each method/line-of-code should be under the i in the if statement.

Please review previous slide

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A Final Indenting exampleif ( frontIsClear())

{

move();

turnLeft();

}

if ( frontIsClear())

{

move();

turnLeft();

if ( frontIsClear())

{

move();

}

}

turnLeft();

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Sample Codeif (karel.frontIsClear() ) // if (frontIsClear())

{

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

}

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

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Sample Code for Karel10if (frontIsClear() ) // if (frontIsClear())

{

move(); // no danger of hitting wall

}

if (anyBeepersInBeeperBag() ){

putBeeper(); // no danger of error}

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Boolean Operators

Robot (Java) (&&, ||, !) – ! Not– && And– || Or

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! (not)Write this method/** * @returns true if there is a wall * directly in front of the Robot, * false otherwise. */

public boolean isFrontBlocked(){

// enter code

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What Now?How does the Robot tell if the front direction is blocked?

• The front being blocked is the opposite of front is clear.

• We can write isFrontBlock by using the opposite of isFrontClear.

• Therefore isFrontBlock, looks like:

public boolean isFrontBlock(){ return !isFrontClear(); }

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Order of Operations

• () - Parenthesis

• ! - not

• && and

• || - orA && B || C != A && ( B || C )

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Your First Assignment

• In assignment Karel03, you knew that every corner contained a beeper.

• In Karel10, Swiper the Fox has stolen Beepers from several corners in the world.

• Your assignment is to implement a new BeeperSweeper Robot that only picks beepers on corners with beepers.

See handout (Karel_10_part1_if.doc) for more details

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Your Second Assignment• In this assignment, your Robot will follow a secret path

by obeying the following rules.

1. The path is 41 steps long. The final corner will have will have a wall on three sides and a Beeper on the corner.

2. The Robot should step according to the following rule.1. If the Robot’s path is blocked by a wall, the Robot should turn left. (There

will not be a wall blocking the move after turning left).2. If the Robot’s is on a corner with a beeper, the Robot should turn right.

(There will not be a wall blocking the move after turning right).3. If the path is not blocked and no beeper was on the corner, move forward

one corner.

*See handout (Karel_11_part1_if.doc) for more details