Flow of Control Recitation

1
Flow of Control Recitation
09/(11,12)/2008
CS 180 Department of Computer Science, Purdue
University
2
Project 2

• Now posted on the class webpage.
• Due Wed, Sept. 17 at 10 pm.
• Start early. All questions on the class
  newsgroup.
• Evening consulting hours.
• LWSN B146. MTW 7-10 pm.

3
Statements in Braces

• If and else parts in braces are preferred for
  even a single statement.
• if (count lt 3)
• total 0
• is better than
• if (count lt 3)
• total 0

4
Statements in Braces

• Benefits to do this
• Decreases the chance of introducing a bug when
  adding a new statement.
• Makes the program a little more
  self-documenting (more readable and easier to
  maintain).

5
Is NOT Always Equal

• should not be used to determine if two objects
  have the same value, including two strings
• Even though (s1 s2) might appear to work, do
  not use this.
• Use (s1.equals(s2)) instead to compare two
  strings.

6
Multibranch if-else Statements

• if-else if-else is different from a series of if
  statements.

if (score gt 60) result P else if
(score gt 0) result F else result
Z
if (score gt 60) result P if (score gt
0) result F else result Z
7
Short-circuit Evaluation

• Can prevent problems.
• if ((number ! 0) (sum/number gt 5))
• Can produce problems!
• if ((x.evaluate()) (y.reassign()))
• How to run y.reassign() every time?
• Use and to replace and .
• if ((x.evaluate()) (y.reassign()))
• But notice and are different!

8
Input Output of Boolean Values

• Example
• boolean booleanVar false
• System.out.println(booleanVar)
• System.out.println("Enter a boolean value")
• Scanner keyboard new Scanner(System.in)
• booleanVar keyboard.nextBoolean()
• System.out.println("You entered " booleanVar)

9
Input Output of Boolean Values

• Dialog
• false
• Enter a boolean value true
• You entered true

10
The switch Statement

• Syntax
• switch (Controlling_Expression)
• case Case_Label
• Statement(s)
• break
• case Case_Label
• default

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The switch Statement

• View sample program Listing 3.4 class
  MultipleBirths

Sample screen output
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The switch Statement

• The action for each case typically ends with the
  word break.
• The optional break statement prevents the
  consideration of other cases.
• The controlling expression can be anything that
  evaluates to an integral type.

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The switch Statement

• Example without break
• switch (grade)
• case P
• System.out.print(You have)
• case F
• System.out.print(No, you have not)
• default
• System.out.println(passed.)

14
Enumerations

• Consider a need to restrict contents of a
  variable to certain values.
• An enumeration lists the values a variable can
  have.
• Exampleenum MovieRating E, A, BMovieRating
  ratingrating MovieRating.A

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Enumerations

• Now possible to use in a switch statement

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Enumerations

• An even better choice of descriptive identifiers
  for the constants.enum MovieRating
  EXCELLENT, AVERAGE, BADrating
  MovieRating.AVERAGEcase EXCELLENT ...

17
Specifying a Drawing Color

• When drawing a shape inside an applets paint
  method, think of the drawing being done with a
  pen that can change colors.
• The method setColor changes the color of the
  "pen."
• canvas.setColor(Color.YELLOW)
• Drawings done later appear on top of drawings
  done earlier.

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Specifying a Drawing Color

• View sample program, Listing 3.5class YellowFace

Sample screen output
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Specifying a Drawing Color

• Predefined Colors for the setColor Method

20
Dialog Box for a Yes/No Question

• Used to present the user with a yes/no question.
• The window contains
• The question text
• Two buttons labeled yes and no.

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Dialog Box for a Yes/No Question

• Example
• int answer
• JOptionPane.showConfirmDialog(null,
• "End program?",
• "Click Yes or No", JOptionPane.YES_NO_
  OPTION)
• if (answer JOptionPane.YES_OPTION)
• System.exit(0)
• else if (answer JOptionPane.NO_OPTION)
• System.out.println("One more time")

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Dialog Box for a Yes/No Question

• A Yes-or No-Dialog Box