3. S/E with Control Structures

1
3. S/E with Control Structures

• 3.1 Relational Operators and Expressions
• 3.2 If and if-else Statements
• 3.3 The Type Double
• 3.4 Program Design with the While Loop
• 3.5 Debugging
• 3.6 Getting Started with Objects and
• Applets Drawing Shapes

2
Objectives

• Learn the basic sequence, selection, and
  repetition statements
• Identify objects and their responsibilities
• Simple debugging techniques
• Handle floating-point numbers
• Study applets, draw various shapes

3
3.1 Relational Operators and Expressions

• Relational operators include
• lt, lt, gt, gt, , and !
• Each relational operator takes two operands and
  gives a boolean value true or false
• Be careful not to confuse with
• Relational operators have precedence lower than
  arithmetic operators but higher than assignment
  operators

4
Figure 3.2 Java relational and equality operators
5
Control Flow

• The order Java executes the statements of a
  program is sequential if not told otherwise
• int item1 25
• int item2 12
• item2 item1 15
• Conditional statement like the if and if-else
  statements allow us to make decision
• They change the default control flow

6
Entry
int item1 25
int item2 12
Item2 item115
Exit
Figure 3.3 The sequence control flow
7
3.2 The if and if-else statements

• The syntax
• if ( condition )
• if_true_statement
• else
• if_false_statement
• condition must be boolean true or false
• If condition is true, if_true_statement is
  executed if the condition is false,
  if_false_statement is executed
• One or the other will be executed, but not both

8
condition
if_true_statement
False
Figure 3.4 Control flow for the if statement
9
The if Statement Example

• hours Integer.parseInt(input)
• if (hours gt 40)
• System.out.println(You worked overtime
• this week)
• System.out.println(You worked hours
• hours)

10
hours Integer.parselnt(input)
True
Hoursgt40
System.out.println(You worked overtime
this week)
False
System.out.println( you worked hours
hours)
Figure 3.5 Control Flow for Example 3.2
11
True
False
Condition
if_true_statement
if_false_statement
Figure 3.6 Flow chart for the if-else statement
12
The if-else Statement Example

• if (hours lt 40)
• wage hourlyRate hours
• else
• wage hourlyRate 40
• hourlyRate 1.5 (hours - 40)

13
Block Statements

• Several statements can be grouped together into a
  block statement
• A block is delimited by braces ( )
• A block statement can be used wherever a
  statement is called for in the Java syntax
• For example, in an if-else statement, the
  if-portion, or the else-portion, or both, could
  be block statements

14
True
Z lt 10
x 2 y 7
False
Figure 3.7 Flow chart for if statement with
block, step 1
15

True
Z lt 10
x 2
False
y 7
Figure 3.8 Flow chart if statement with block,
step 2
16
Block Statements

• Use a consistent style for blocks

if (x lt 10) y 5 z 8 else y
9 z -2
if (x lt 10) y 5 z 8 else y
9 z -2
17
3.3 The Type Double

• Scientific (floating-point) notation for real
  numbers 123.45 and 0.0012345 can be written
  respectively as
• 0.12345E3
• 0.12345E-2
• 0.12345 is the mantissa, 3 and -2 are the
  exponents, with the implicit base of 10
• Scientific notation is good for numbers with very
  small or very large absolute value
• Java has type double that provides 16 decimal
  digits accurately.

18

• public class Triangle
• private double side1, side2, side3
• public Triangle(double a, double b, double c)
• side1 a side2 b side3 c
• public double circumference()
• return side1 side2 side3
• public double area()
• double s circumference() / 2.0
• return Math.sqrt(s(s - side1)(s -
  side2)(s - side3))

19
Double Value Output

• When printed using print or println methods of
  System.out, Java prints double values using the
  most convenient format
• Scientific format for numbers greater than
  10,000,000 or less than -10,000,000, and for
  numbers between -.001 and .001.
• Java treats decimal literals as type double
• Java does not print trailing zeroes
• Type double is accurate to 16 significant digits
• The 17th digit may be rounded

20
Input and Formatted Output

• import javax.swing.
• import java.text.
• public static void main(String args)
• NumberFormat nf NumberFormat.getCurrentcyInst
  ance()
• String s JOptionPane.showInputDialog(Hours
  worked? )
• double hours Double.parseDouble(s)
• JOptionPane.showMessageDialog
• (null, Your pay is nf.format(hours
  5.5))
• System.exit(0)

21
Formatted Output

• The NumberFormat class has static methods that
  return formatter object
• getCurrencyInstance()
• getPercentInstance()
• Each formatter object has a method called format
  that returns a string with the specific
  information in the appropriate format
• The DecimalFormat class can be used to format a
  floating point value in generic way
• The DecimalFormat class takes a string that
  represents a pattern for the formatted number

22
Data Conversions

• Sometimes it is convenient to convert data from
  one type to another
• Conversion must be handled carefully to avoid
  losing information
• Widening conversions are safest because they tend
  to go from a small data type to a larger one
  (such as a short to an int)
• Narrowing conversions can lose information
  because they tend to go from a larger data type
  to a smaller one (such as an int to a short)

23
Data Conversions

• In Java, data conversions can occur in three
  ways
• assignment conversion
• arithmetic promotion
• casting
• Assignment conversion occurs when a value of one
  type is assigned to a variable of another type
• Only widening conversion can happen via
  assignment
• Arithmetic promotion happens automatically when
  operators in expressions converts their operands
• Casting is the most powerful, and dangerous,
  technique for conversion
• result (float) total / count

24
2.54
Original expression

361
2.54
After Conversion

361.0
Figure 3.9 Conversion of mixed-mode expression
25
3.4 The While Loop

• Repetition statements allow us to execute a
  statement multiple times repetitively
• They are often referred to as loops
• Like if-else statements, they are controlled by
  boolean expressions
• Java has three kinds of repetition statements
  the while loop, the do loop, and the for loop
• The while loop has this syntax
• while ( condition )
• while_true_statement

26
The While Loop

• If the condition is true, the statement is
  executed
• Then the condition is evaluated again
• The statement is executed repetitively until the
  condition becomes false
• If the condition is false initially, then the
  statement is never executed
• The body of a while loop must eventually make the
  condition false
• If not, it is an infinite loop, which is a common
  logical error unless you are absolutely sure it
  is not

27
False
True
Condition
while_true_statement
Figure 3.10 Flow chart for the while loop
28
Problem Solving

• The purpose of writing a program is to solve a
  problem
• The general steps in problem solving are
• Understand the problem
• Dissect the problem
• Design a solution
• Consider alternatives to the solution and refine
  it
• Implement the solution
• Test the solution and fix and problems that exist

29
Problem Solving

• Many software development project failed because
  the developers didnt understand the problems to
  be solved
• Avoid assumptions and clarify ambiguities
• As problems and their solutions becomes larger,
  we must organize our development into manageable
  pieces
• We will dissect our solutions into pieces called
  classes and objects, taking an object-oriented
  approach

30
Program Development

• The creation of software involves four basic
  activities
• establishing the requirements
• creating a design
• implementing the code
• testing the implementation
• Real development process is much more involved
  than this, but these four steps are a good
  starting point

31
Requirements

• Requirements specify the tasks a program must
  accomplish (what to do, not how to do it)
• They often include description of the user
  interface
• The initial requirements must be critiqued,
  modified, and expanded
• It is often difficult to establish detailed,
  unambiguous, and complete requirements
• Careful attention to the requirements can save
  significant time and money in the overall project

32
Design

• An algorithm is a step-by-step process for
  solving a problem
• A program follow one or more algorithms
• The design of a program specifies the algorithms
  and data needed
• In object-oriented development, the design
  establishes the classes, their data and methods
• The details of a method may be expressed in
  pseudocode, which is code-like, but does not
  necessarily follow any specific syntax

33
Implementation

• Implementation is the process of translating a
  design into source code
• Most novice programmers think that writing code
  is the heart of software development, but it is
  actually the least creative step
• Almost all important decisions are made during
  requirements analysis and design
• Implementation should focus on coding details,
  including guidelines, clarity, maintainability,
  expandability, and documentation

34
Testing

• A program should be executed multiple times with
  various input in an attempt to find errors
• Debugging is the process of discovering the cause
  of a problem and fix it
• Debugging can only indicate the presence of
  errors (bugs), but not the absence
• Dont ever think there is only one more bug to
  fix
• tests should focus on design details as well as
  overall requirements

35
Read the quantity of scores while (count lt
quantity) Read the next score Add the score
to the total so far Increment the count of
scores Display the quantity and the total
Figure 3.11 Pseudocode for the sum of test scores
problem
36
3.5 Debugging

• Syntax error
• caught by the compiler
• easy to fix
• Run-time error
• reported at run-time by interpreter
• often depending on input and environment
• usually cause program to terminate immaturely
• Logical error
• program runs but results are wrong
• can be very difficult to find and/or fix
• Requirement error
• disastrous

37
3.6 Getting Started with Applets

• A java application is a stand-alone program with
  a main method
• An applet is a Java program that is intended to
  be transported over the web and executed using a
  web browser
• An applet doesnt have a main method
• Instead, there are several special methods that
  serve specific purposes
• The paint method, for instance, is automatically
  executed and is used to draw the applets contents

38
Drawing Shapes

• The paint method accepts a parameter that is an
  object of the Graphics class
• A Graphics object defines a graphical context on
  which we can draw shapes and text
• The Graphics class has several methods for
  drawing shapes
• The class that defines the applet extends the
  Applet class
• This makes use of inheritance, an object-oriented
  concept to be explored later

39
(70,80)
(130,230)
Figure 3.12 Drawing a line
40
200
(50,50)
100
Figure 3.13 Drawing a rectangle
41
Figure 3.14 An oval with its bounding rectangle
42
90º
0º
180º
270º
Figure 3.15 Degree measure around a circle
43
90º
45º
Figure 3.16 An arc from the oval of Figure 3.14
44
Figure 3.17 The structure of a rounded rectangle
45
Dimension d getSize() int w d.width int
h d.height g.drawRect(2w/3,0,w/3,h/3)
Figure 3.19 Drawing a rectangle relative to the
applet's size