Conditional Statements

1
Conditional Statements

• Flavor 1
• if ( ltsome boolean expressiongt )
• ltsome instruction listgt

For now these are method invokations (see next
slide)
2
IF
predicates either True or False
frontIsClear() nextToABeeper() nextToARobot() f
acingNorth() facingSouth() facingEast() facingW
est() anyBeepersInBeeperBag()

• if ( )

Robot
3
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• HELP - The word void has been replaced by the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.

4
Method Definition

• public boolean nextToABeeper()
• If there is a (one or more) Beeper on the Robots
  current corner, this method returns true.
• Otherwise (NO Beeper current corner), this method
  returns false.

5
Method Definition

• public boolean nextToARobot()
• If there is another (one or more) Robot on the
  Robots current corner, this method returns true.
• Otherwise (NOT another Robot on current corner),
  this method returns false.

6
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.

7
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.

8
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.

9
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.

10
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.

11
Indenting by example

• How to properly indent while writing if
  structures by example!
• public void someMethod()
• if ( ltsomeBooleanExpressiongt )
• codeGoesHere()
• allAdditionalCodeLinesUp()
• soAllFirstLettersAreInTheSameColumn()
• AfterClosingBracketNextMethodCallGoesHere()
• untilAnotherIfStatementIsEncountered()

12
A word about indenting

• How 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

13
A Final Indenting example

• if ( frontIsClear())
• move()
• turnLeft()
• if ( frontIsClear())
• move()
• turnLeft()
• if ( frontIsClear())
• move()
• turnLeft()

14
Sample Code

• if (karel.frontIsClear() ) // if
  (frontIsClear())
• karel.move() // no danger of hitting wall

if ( karel.anyBeepersInBeeperBag()
) karel.putBeeper() // no danger of error
15
Boolean Operators

• Robot (Java) (, , !)
• ! Not
• And
• Or

16
! (not)

• Write this method
• /
• _at_returns true if there wall is a
• directly in front of the Robot,
• false otherwise.
• /
• public boolean isFrontBlocked()
• // enter code

17
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()

18
Order of Operations

• () - Parenthesis
• ! - not
• and
• - or
• A B C ! A ( B C )

19
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

20
Your Second Assignment

• In this assignment, our Robot will follow a
  secret path by obeying the following rules.
• The path is 41 steps long. The final corner will
  have will have a wall on three sides and a Beeper
  on the corner.
• If the Robots path is blocked by a wall, the
  Robot should turn left and move. (There will not
  be a wall blocking the move after turning left).
• If the Robots is on a corner with a beeper, the
  Robot should turn right and move. (There will
  not be a wall blocking the move after turning
  right).
• 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