Chapter 4 Introduction to Control Statements

Chapter 4Introduction to Control

Statements

Fundamentals of Java

Fundamentals of Java 2

Objectives

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

choices.


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.


Vocabulary

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


Vocabulary (cont.)

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


Additional Operators

Extended assignment operators– Assignment operator combined with

arithmetic and concatenation operators


Additional Operators (cont.)

Increment operator: ++– Increase value of operand by 1

Decrement operator: --– Decrease value of operand by 1


Standard Classes and Methods: The Math Class

Table 4-1: Seven methods in the Math class


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:


Standard Classes and Methods: The Math Class (cont.)

Math class methods example:


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


A Visit to the Farm


The if and if-else Statements

Principal forms:


The if and if-else Statements (cont.)

Additional forms:



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.



Figure 4-1: Flowcharts for the if and if-else statements



Examples of if statements:



Table 4-3: Relational operators


The if and if-else Statements (cont.): Checking Input for Validity

Example 4.1: Computes the area of a circle if the radius >= 0 or otherwise displays an error message


The while Statement

Provides a looping mechanism – Executes statements repeatedly for as long as

some condition remains true


The while Statement (cont.)

Figure 4-2: Flowchart for a while statement



Example:

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



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



Counting backward:



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



Common structure of a while loop:



Example 4.2: Compute and display the factorial of n


The for statement

Combines counter initialization, condition test, and update into a single expression

Easy to create count-controlled loops


The for statement (cont.)

Example:



Count-controlled input:



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



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 headerInitialization, condition, and update in one line of

code


Nested Control Statements and the break Statement

Control statements may be nested.


Nested Control Statements and the break Statement (cont.)

break statement: Prematurely end a loop


Nested Control Statements and the break Statement (cont.)

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


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.


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



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



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


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


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.



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



Debugging loops:– If an error is suspected, make sure that:

Variables are initialized before entering the loopThe terminating condition stops the iterations

when the test variables have reached the intended limit

The statements in the body are correct



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


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


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)



Example 4.5: CircleArea with dialog I/O



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



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.



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

preferred size is optional.


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.


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.


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.


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.


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.


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

Documents

Chapter 4 Introduction to Control Statements