presentationpower point _chapter_04.ppt

Chapter 4
Introduction 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

5
Vocabulary (cont.)
 Iteration
 Off-by-one error
 Overloading
 Sentinel
 Task-controlled loop

6
Additional Operators
 Extended assignment operators
– Assignment operator combined with
arithmetic and concatenation operators

7
Additional Operators (cont.)
 Increment operator: ++
– Increase value of operand by 1
 Decrement operator: --
– Decrease value of operand by 1

8
Standard Classes and Methods:
The Math Class
Table 4-1: Seven methods in the Math class

9
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:

10
The Math Class (cont.)
 Math class methods example:

11
The Random Class
 Random number generator: Returns
numbers chosen at random from a pre-
designated interval
Table 4-2: Methods in the Random class

12
A Visit to the Farm

13
The if and if-else Statements
 Principal forms:

14
(cont.)
 Additional forms:

15
(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.

16
(cont.)
Figure 4-1: Flowcharts for the if and if-else statements

17
(cont.)
 Examples of if statements:

18
(cont.)
Table 4-3: Relational operators

19
(cont.): Checking Input for Validity
Example 4.1: Computes the area of a circle if the
radius >= 0 or otherwise displays an error message

20
The while Statement
 Provides a looping mechanism
– Executes statements repeatedly for as long as
some condition remains true

21
The while Statement (cont.)
Figure 4-2: Flowchart for a while statement

22
 Example:
 Counter-controlled loop:
– cntr is the counter
– Loop repeats a determined number of times

23
 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

24
 Counting backward:

25
 Task-controlled loop: Continues to
execute until some task is accomplished

26
 Common structure of a while loop:

27
Example 4.2: Compute and display the factorial of n

28
The for statement
 Combines counter initialization, condition test,
and update into a single expression
 Easy to create count-controlled loops

29
The for statement (cont.)
 Example:

30
 Count-controlled input:

31
 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

32
 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

33
Nested Control Statements and the
break Statement
 Control statements may be nested.

34
break Statement (cont.)
 break statement: Prematurely end a loop

35
break Statement (cont.)
 Sentinel-controlled input: Continue a loop
until a sentinel variable has a specific value

36
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.

37
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

38
Example 4.3: Computes and displays the average
of a file of floating-point numbers

39
Example 4.4: Inputs a text file of integers and
writes these to an output file without the zeroes

40
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

41
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.

42
 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

43
 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

44
 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

45
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

46
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)

47
Example 4.5: CircleArea with dialog I/O

48
Figure 4.5: Dialog I/O user interface for the circle area program

49
 Setting up multiple panels:
– Can use a loop to initialize and install panels
 Setting the preferred size of a panel:
– Use JPanel class’s setPreferredSize()
method.
– JFrame class’s pack() method will cause the
window to adjust its size to exactly fit the
preferred size of any contained panels.

50
Example 4-7: Color panel whose background is a
color provided by the client. A client-specified
preferred size is optional.

51
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.

52
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.

53
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.

54
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.

55
Summary (cont.)
 The comparison operators, such as ==, <=,
and >=, 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.

56
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

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