Chapter 4 Introduction to Control Statements

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Chapter 4Introduction to Control Statements

• Fundamentals of Java

2
Objectives

• Use the increment and decrement operators.
• Use standard math methods.
• Use if and if-else statements to make choices.

3
Objectives (cont.)

• Use while and for loops to repeat a process.
• Construct appropriate conditions for control
  statements using relational operators.
• Detect and correct common loop errors.

4
Vocabulary

• Control statements
• Counter
• Count-controlled loop
• Entry-controlled loop
• Flowchart
• Infinite loop

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Vocabulary (cont.)

• Iteration
• Off-by-one error
• Overloading
• Sentinel
• Task-controlled loop

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Additional Operators

• Extended assignment operators
• Assignment operator combined with arithmetic and
  concatenation operators

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Additional Operators (cont.)

• Increment operator
• Increase value of operand by 1
• Decrement operator --
• Decrease value of operand by 1

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Standard Classes and Methods The Math Class
Table 4-1 Seven methods in the Math class
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Standard Classes and Methods The Math Class
(cont.)

• The two abs() methods are overloaded.
• Overloaded Multiple methods in the same class
  with the same name
• Using sqrt() method example

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Standard Classes and Methods The Math Class
(cont.)

• Math class methods example

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Standard Classes and Methods The Random Class

• Random number generator Returns numbers chosen
  at random from a pre-designated interval

Table 4-2 Methods in the Random class
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A Visit to the Farm
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The if and if-else Statements

• Principal forms

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The if and if-else Statements (cont.)

• Additional forms

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The if and if-else Statements (cont.)

• Better to over-use braces than under-use them
• Can help to eliminate logic errors
• Condition of an if statement must be a Boolean
  expression
• Evaluates to true or false
• A flowchart can be used to illustrate the
  behavior of if-else statements.

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The if and if-else Statements (cont.)
Figure 4-1 Flowcharts for the if and if-else
statements
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The if and if-else Statements (cont.)

• Examples of if statements

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The if and if-else Statements (cont.)
Table 4-3 Relational operators
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The if and if-else Statements (cont.) Checking
Input for Validity
Example 4.1 Computes the area of a circle if
the radius gt 0 or otherwise displays an error
message
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The while Statement

• Provides a looping mechanism
• Executes statements repeatedly for as long as
  some condition remains true

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The while Statement (cont.)
Figure 4-2 Flowchart for a while statement
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The while Statement (cont.)

• Example

• Counter-controlled loop
• cntr is the counter
• Loop repeats a determined number of times

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The while Statement (cont.)

• Tracing Track value of variables through each
  iteration of the loop

Table 4-4 Trace of how variables change on each
iteration through a loop
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The while Statement (cont.)

• Counting backward

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The while Statement (cont.)

• Task-controlled loop Continues to execute until
  some task is accomplished

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The while Statement (cont.)

• Common structure of a while loop

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The while Statement (cont.)
Example 4.2 Compute and display the factorial of
n
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The for statement

• Combines counter initialization, condition test,
  and update into a single expression
• Easy to create count-controlled loops

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The for statement (cont.)

• Example

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The for statement (cont.)

• Count-controlled input

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The for statement (cont.)

• Better to declare the loop control variable
  within the loop header
• Only visible within the loop
• Variable name can be reused later in other loops

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The for statement (cont.)

• Both for loops and while loops are
  entry-controlled loops.
• Condition tested at top of loop on each pass
• Choosing a for loop versus a while loop is often
  a matter of style.
• for loop advantages
• Can declare loop control variable in header
• Initialization, condition, and update in one line
  of code

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Nested Control Statements and the break Statement

• Control statements may be nested.
  

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Nested Control Statements and the break Statement
(cont.)

• break statement Prematurely end a loop

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Nested Control Statements and the break Statement
(cont.)

• Sentinel-controlled input Continue a loop until
  a sentinel variable has a specific value

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Using Loops with Text Files

• Advantages of using text files versus input from
  a human user
• Data sets can be much larger.
• Data input quickly, with less chance of error.
• Data can be used repeatedly.
• Data files can be created by hand in a text
  editor or generated by a program.

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Using Loops with Text Files (cont.)

• Text file format
• If data is numerical, numbers must be separated
  by white space.
• Must be an end-of-file character
• Used by program to determine whether it is done
  reading data
• When an error occurs at run-time, the JVM throws
  an exception.
• IOException thrown if error occurs during file
  operations

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Using Loops with Text Files (cont.)
Example 4.3 Computes and displays the average of
a file of floating-point numbers
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Using Loops with Text Files (cont.)
Example 4.4 Inputs a text file of integers and
writes these to an output file without the zeroes
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Errors in Loops

• May occur in initializing statements, terminating
  conditions, body statements, or update statements
• Initialization error Failure to initialize or
  initializes to incorrect value
• Off-by-one error Loop iterates one too many or
  one too few times

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Errors in Loops (cont.)

• Infinite loop Error in the terminating condition
• Loop never terminates
• Errors in loop body affect whether the loop
  completes its task correctly or at all.
• Update error Update statements in wrong place
  may affect calculations
• Failing to update at all results in an infinite
  loop.

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Errors in Loops (cont.)

• Effects of limited floating-point precision When
  using floating-point variables as loop control
  variables, you must understand that not all
  values can be represented.
• Better not to use or ! in condition statement
  under these conditions

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Errors in Loops (cont.)

• Debugging loops
• If an error is suspected, make sure that
• Variables are initialized before entering the
  loop
• The terminating condition stops the iterations
  when the test variables have reached the intended
  limit
• The statements in the body are correct

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Errors in Loops (cont.)

• Debugging loops (cont.)
• If an error is suspected, make sure that
• The update statements are positioned correctly
  and modify the test variables so that they
  eventually pass the limits tested in the
  terminating condition

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Graphics and GUIs I/O Dialog Boxes and Loops

• A convenient way to accept input from a user is
  to pop up an input dialog box.
• Use JOptionPane.showInputDialog().

Figure 4-4 Input dialog box
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Graphics and GUIs I/O Dialog Boxes and Loops
(cont.)

• If expected input is a number, must use
  Integer.parseInt() or Double.parseDouble()
• To output a message, use a message dialog box.
• JOptionPane.showMessageDialog( anObserver,
  aString)

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Graphics and GUIs I/O Dialog Boxes and Loops
(cont.)
Example 4.5 CircleArea with dialog I/O
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Graphics and GUIs I/O Dialog Boxes and Loops
(cont.)
Figure 4.5 Dialog I/O user interface for the
circle area program
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Graphics and GUIs I/O Dialog Boxes and Loops
(cont.)

• Setting up multiple panels
• Can use a loop to initialize and install panels
• Setting the preferred size of a panel
• Use JPanel classs setPreferredSize() method.
• JFrame classs pack() method will cause the
  window to adjust its size to exactly fit the
  preferred size of any contained panels.

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Graphics and GUIs I/O Dialog Boxes and Loops
(cont.)
Example 4-7 Color panel whose background is a
color provided by the client. A client-specified
preferred size is optional.
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Design, Testing, and Debugging Hints

• Most errors involving selection statements and
  loops are not syntax errors.
• The presence or absence of the symbols can
  seriously affect the logic of a selection
  statement or loop.
• When testing programs that use if or if-else
  statements, use test data that forces the program
  to exercise all logical branches.

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Design, Testing, and Debugging Hints (cont.)

• Use an if-else statement rather than two if
  statements when the alternative courses of action
  are mutually exclusive.
• When testing a loop, be sure to use limit values
  as well as typical values.
• Be sure to check entry conditions and exit
  conditions for each loop.

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Design, Testing, and Debugging Hints (cont.)

• For a loop with errors, use debugging output
  statements to verify the values of the control
  variable on each pass through the loop.
• Text files are convenient to use when the data
  set is large, when the same data set must be used
  repeatedly with different programs, and when
  these data must be saved permanently.

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Summary

• Java has operators for extended assignment and
  for increment and decrement.
• The Math class provides several useful methods,
  such as sqrt and abs.
• The Random class allows you to generate random
  integers and floating-point numbers.
• if and if-else statements are used to make
  one-way and two-way decisions.

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Summary (cont.)

• The comparison operators, such as , lt, and gt,
  return Boolean values that can serve as
  conditions for control statements.
• The while loop allows the program to run a set of
  statements repeatedly until a condition becomes
  false.
• The for loop is a more concise version of the
  while loop.

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Summary (cont.)

• Other control statements, such as an if
  statement, can be nested within loops.
• A break statement can be used with an if
  statement to terminate a loop early.
• Many kinds of logic errors can occur in loops.
• Off-by-one error
• Infinite loop