Transcript Chapter 3 Control Methods

Chapter 3 Control Statements
Selection
Statements
–Using if and if...else
–Nested if Statements
–Using switch Statements
–Conditional Operator
Repetition
Statements
–Looping: while, do-while, and for
–Nested loops
–Using break and continue
Repetitions
 while

do-while Loops
 for

Loops
Loops
break and continue
while Loop Flow Chart
while (continuation-condition) {
// loop-body;
}
Continuation
condition?
true
Statement(s)
Next
Statement
false
while Loop Flow Chart, cont.
i = 0;
int i = 0;
while (i < 100) {
System.out.println(
"Welcome to Java!");
i++;
}
(i < 100)
false
true
System.out.println("Welcoem to Java!");
i++;
Next
Statement
Example 3.2: Using while Loops
// TestWhile.java: Test the while loop
import javax.swing.JOptionPane;
public class TestWhile {
/** Main method */
public static void main(String[] args) {
int data;
int sum = 0;
// Read an initial data
String dataString = JOptionPane.showInputDialog(null,
"Enter an int value, \nthe program exits if the input is 0",
"Example 3.2 Input", JOptionPane.QUESTION_MESSAGE);
data = Integer.parseInt(dataString);
// Keep reading data until the input is 0
while (data != 0) {
sum += data;
// Read the next data
dataString = JOptionPane.showInputDialog(null,
"Enter an int value, \nthe program exits if the input is 0",
"Example 3.2 Input", JOptionPane.QUESTION_MESSAGE);
data = Integer.parseInt(dataString);
}
JOptionPane.showMessageDialog(null, "The sum is " + sum,
"Example 3.2 Output", JOptionPane.INFORMATION_MESSAGE);
System.exit(0);
}
}
Caution
Don’t use floating-point values for
equality checking in a loop control. Since
floating-point values are approximations,
using them could result in imprecise
counter values and inaccurate results. This
example uses int value for data. If a
floating-point type value is used for data,
(data != 0) may be true even though data is
0.
// data should be zero
double data = Math.pow(Math.sqrt(2), 2) - 2;
if (data == 0)
System.out.println("data is zero");
else
do-while Loop
do {
Statement(s)
// Loop body;
} while (continue-condition);
true
Continue
condition?
false
Next
Statement
for Loops
for (initial-action; loop-continuation-condition;
action-after-each-iteration) {
//loop body;
}
int i = 0;
while (i < 100) {
System.out.println("Welcome to Java! ” + i);
i++;
}
Example:
int i;
for (i = 0; i < 100; i++) {
System.out.println("Welcome to Java! ” + i);
}
for Loop Flow Chart
for (initial-action;
loop-continuation-condition;
action-after-each-iteration) {
//loop body;
}
Action-AfterEach-Iteration
Initial-Action
Continuation
condition?
true
Statement(s)
(loop-body)
Next
Statement
false
for Loop Example
int i;
for (i = 0; i<100; i++) {
System.out.println(
"Welcome to Java");
}
i=0
i++
i<100?
false
true
System.out.println(
“Welcom to Java!”);
Next
Statement
Which Loop to Use?
The three forms of loop statements, while, do, and for, are
expressively equivalent; that is, you can write a loop in
any of these three forms.
I recommend that you use the one that is most intuitive
and comfortable for you. In general, a for loop may be
used if the number of repetitions is known, as, for
example, when you need to print a message 100 times. A
while loop may be used if the number of repetitions is not
known, as in the case of reading the numbers until the
input is 0. A do-while loop can be used to replace a while
loop if the loop body has to be executed before testing the
continuation condition.
Caution
Adding a semicolon at the end of the
for clause before the loop body is a
common mistake, as shown below:
Wrong
for (int i=0; i<10; i++);
{
System.out.println("i is " + i);
}
Caution, cont.
Similarly, the following loop is also
wrong:
Wrong
int i=0;
while (i<10);
{
System.out.println("i is " + i);
i++;
}
In the case of the do loop, the
following semicolon is needed to end
the loop.
int i=0;
do {
Correct
System.out.println("i is " + i);
i++;
The break Keyword
Continuation
condition?
true
Statement(s)
break
Statement(s)
Next
Statement
false
The continue Keyword
Continue
condition?
true
Statement(s)
continue
Statement(s)
Next
Statement
false
Using break
Example for using the break keyword: 3.5
// TestBreak.java: Test the break keyword in the loop
public class TestBreak {
/** Main method */
public static void main(String[] args) {
int sum = 0;
int item = 0;
while (item < 5) {
item ++;
sum += item;
//if (sum >= 6) break;
}
System.out.println("The sum is " + sum);
}
}
Using continue
Example for using the continue keyword:
// TestContinue.java: Test the continue keyword
public class TestContinue {
/** Main method */
public static void main(String[] args) {
int sum = 0;
int item = 0;
while (item < 5) {
item++;
if (item == 2) continue;
sum += item;
}
System.out.println("The sum is " + sum);
}
}
Review of Terms: Deena’s Class
The 3 essential elements of a programming loop:
priming, test, and update
 User-controlled loops vs Program-controlled loops
 Some user-controlled loops use sentinels.
 Accumulator and Marker variables: how are these
used in loops?
 What is a flag variable? When do you use it?
