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Chapter 5
Loops
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807
1
Objectives

To write programs for executing statements repeatedly using a while

To develop a program for GuessNumber and SubtractionQuizLoop (§5.2.1).
To follow the loop design strategy to develop loops (§5.2.2).
To develop a program for SubtractionQuizLoop (§5.2.3).
To control a loop with a sentinel value (§5.2.3).
To obtain large input from a file using input redirection rather than typing from the
keyboard (§5.2.4).




loop (§5.2).

To write loops using do-while statements (§5.3).

To write loops using for statements (§5.4).

To discover the similarities and differences of three types of loop statements (§5.5).
To write nested loops (§5.6).
To learn the techniques for minimizing numerical errors (§5.7).
To learn loops from a variety of examples (GCD, FutureTuition,
MonteCarloSimulation) (§5.8).
To implement program control with break and continue (§5.9).
(GUI) To control a loop with a confirmation dialog (§5.10).





Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All rights reserved. 0132130807
2
Program Execution Structure
Compiler executes in three different ways:
Sequences
Selection (If Statement)
Loop ( Repetition /Iteration)
Process 1
Process 2
Process 3
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
3
Motivations
Suppose that you need to print a string
(e.g., "Welcome to Java!") a hundred times.
It would be tedious to have to write the following
statement a hundred times:
System.out.println("Welcome to Java!");
So, how do you solve this problem?
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
4
Java Loops – while, do…While, & for
 There may be a situation when we need to
execute a block of code several number of
times, and is often referred to as a loop.
 Java has very flexible three looping
mechanisms. You can use one of the
following three loops:
while Loop
do...while Loop
for Loop
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
5
Opening Problem
Problem:
System.out.println("Welcome
System.out.println("Welcome
System.out.println("Welcome
System.out.println("Welcome
System.out.println("Welcome
System.out.println("Welcome
100
times
to
to
to
to
to
to
Java!");
Java!");
Java!");
Java!");
Java!");
Java!");
…
…
…
System.out.println("Welcome to Java!");
System.out.println("Welcome to Java!");
System.out.println("Welcome to Java!");
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
6
while Loop

Use a loop statement if you need to do the same
thing repeatedly.
pseudocode syntax
Java syntax
while (<condition>)
while <condition>
{
<statement(s)>
<statement(s)>
}
The condition is at the bottom of the loop (in contrast to the
while loop, where the condition is at the top of the loop).
The compiler requires putting a ";" at the very end, after the do
loop's condition.
Proper style dictates putting the "while" part on the same line as
the "}"
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
7
while Loop Flow Chart
while (loop-continuation-condition) {
// loop-body;
int count = 0;
while (count < 100) {
System.out.println("Welcome to Java!");
Statement(s);
count++;
} // end of while
} // end of while
count = 0;
Loop
Continuation
Condition?
true
Statement(s)
(loop body)
(A)
false
(count < 100)?
false
true
System.out.println("Welcome to Java!");
count++;
(B)
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
8
while Loop
A while loop is a control structure that allows you to repeat a task
a certain number of times.
Using loop statement means tell the computer to print a string a
hundred times without coding:
int count = 0;
while (count < 100) {
System.out.println("Welcome to Java");
count++;
} // end of while
public class Welcome {
public static void main (String [ ] args){
int count = 0;
while (count < 100) {
System.out.println("Welcome to Java");
count++;
} // end of while
} // end of main method
} // end of class
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
9
Here key point of the while loop is that the loop might not ever
run. When the expression is tested and the result is false, the loop
body will be skipped and the first statement after the while loop will
be executed. Syntax
while(Boolean_expression)
{ //Statements
}
Example:
public class Test1 {
public static void main(String args[]){
int x= 10;
while( x < 20 ){
System.out.print("value of x :" + x);
x++;
System.out.print("\n");
} //end of while
} // end of main
} //end Liang,
ofIntroduction
class
to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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10
while Loop
a main method that finds the sum of userentered integers where -99999 is a sentinel value.
 Write
public static void main(String[] args)
{
Scanner stdIn = new Scanner(System.in);
int sum = 0;
// sum of user-entered values
int x;
// a user-entered value
System.out.print("Enter an integer (-99999 to quit): ");
x = stdIn.nextInt();
while (x != -99999)
{
sum = sum + x;
System.out.print("Enter an integer (-99999 to quit): ");
x = stdIn.nextInt();
} // end of while
System.out.println("The sum is " + sum);
} // end main
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
import java.util.Scanner;
public class SentinelValue {
/** Main method */
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Read an initial data
System.out.print(
"Enter an int value (the program exits if the input is 0): ");
int data = input.nextInt();
// Keep reading data until the input is 0
int sum = 0;
while (data != 0) {
sum += data;
// Read the next data
System.out.print(
"Enter an int value (the program exits if the input is 0): ");
data = input.nextInt();
} //end of while
System.out.println("The sum is " + sum);
} //end of main
} //end of class
Run
Enter an int value (the program exits if the input is 0): 2
Enter an int value (the program exits if the input is 0): 3
Enter an int value (the program exits if the input is 0): 4
Enter an int value (the program exits if the input is 0): 0
The sum is 9
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
12
animation
Trace while Loop
int count = 0;
Initialize count
while (count < 2) {
System.out.println("Welcome to Java!");
count++;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
13
animation
Trace while Loop, cont.
int count = 0;
(count < 2) is true
while (count < 2) {
System.out.println("Welcome to Java!");
count++;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
14
animation
Trace while Loop, cont.
int count = 0;
Print Welcome to Java
while (count < 2) {
System.out.println("Welcome to Java!");
count++;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
15
animation
Trace while Loop, cont.
int count = 0;
Increase count by 1
count is 1 now
while (count < 2) {
System.out.println("Welcome to Java!");
count++;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
16
animation
Trace while Loop, cont.
int count = 0;
(count < 2) is still true since count
is 1
while (count < 2) {
System.out.println("Welcome to Java!");
count++;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
17
animation
Trace while Loop, cont.
int count = 0;
Print Welcome to Java
while (count < 2) {
System.out.println("Welcome to Java!");
count++;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
18
animation
Trace while Loop, cont.
int count = 0;
Increase count by 1
count is 2 now
while (count < 2) {
System.out.println("Welcome to Java!");
count++;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
19
animation
Trace while Loop, cont.
int count = 0;
(count < 2) is false since count is 2
now
while (count < 2) {
System.out.println("Welcome to Java!");
count++;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
20
animation
Trace while Loop
int count = 0;
The loop exits. Execute the next
statement after the loop.
while (count < 2) {
System.out.println("Welcome to Java!");
count++;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
21
Caution
Don’t use floating-point values for equality checking in a loop control.
Since floating-point values are approximations for some values, using
them could result in imprecise counter values and inaccurate results.
Consider the following code for computing 1 + 0.9 + 0.8 + ... + 0.1:
double item = 1; double sum = 0;
while (item != 0) { // No guarantee item will be 0
sum += item;
item -= 0.1;
}
System.out.println(sum);
Variable item starts with 1 and is reduced by 0.1 every time the loop
body is executed. The loop should terminate when item becomes 0.
However, there is no guarantee that item will be exactly 0, because the
floating-point arithmetic is approximated. This loop seems OK on the
surface, but it is actually an infinite loop.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
22
do-while Loop
Statement(s)
(loop body)
The do-while loop is a variation
of the while loop.
The syntax below:
true
do {
// Loop body;
Loop
Continuation
Condition?
false
Statement(s);
} while (loop-continuation-condition);
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
23
do While Loop
 When
to use a do loop:
– If you know that the repeated thing will
always have to be done at least one time.
 Syntax:
do
{
<statement(s)>
} while (<condition>);
– Note: The condition is at the bottom of the loop (in contrast to
the while loop, where the condition is at the top of the loop).
– The compiler requires putting a ";" at the very end, after the do
loop's condition.
– Proper style dictates putting the "while" part on the same line
as the "}"
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
24
The do...while Loop
If the Boolean expression is true, the flow of control jumps back up
to do, and the statements in the loop execute again.
This process repeats until the Boolean expression is false.
public class Test2 {
public static void main(String args[]){
int x= 10;
do {
System.out.print("value of x : " + x );
x++;
System.out.print("\n");
} while( x < 20 );
} // end of main
} //end of class
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
25
do while Loop
// FloorSpace.java – Calculates total floor space in a house
import java.util.Scanner;
public class FloorSpace {
public static void main(String[] args)
{
Scanner stdIn = new Scanner(System.in);
double length, width;
// room dimensions
double floorSpace = 0;
// house's total floor space
String response;
// user's y/n response
do
{
System.out.print("Enter the length: ");
length = stdIn.nextDouble();
System.out.print("Enter the width: ");
width = stdIn.nextDouble();
floorSpace += length * width;
System.out.print("Any more rooms? (y/n): ");
response = stdIn.next();
} while (response.equalsIgnoreCase("y"));
System.out.println("The total floor space is " + floorSpace);
} // end main
} // end class
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
26
for Loops
for (initial-action; loopcontinuation-condition;
action-after-each-iteration) {
// loop body;
Statement(s);
}
Initial-Action
Loop
Continuation
Condition?
int i;
for (i = 0; i < 100; i++) {
System.out.println(
"Welcome to Java!");
}
i=0
false
(i < 100)?
true
Statement(s)
(loop body)
true
System.out.println(
"Welcome to Java");
Action-After-Each-Iteration
i++
(A)
(B)
false
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
27
for Loop
 When
to use a for loop:
– If you know the exact number of loop iterations
before the loop begins.
 For
example, use a for loop to:
– Print this countdown from 10.
Sample session:
10 9 8 7 6 5 4 3 2 1 Liftoff!
– Find the factorial of a user-entered number.
Sample session:
Enter a whole number: 4
4! = 24
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
28
for Loop

for loop syntax
for loop example
for (<initialization>;
<condition>; <update>)
{
<statement(s)>
}
for (int i=10; i>0; i--)
{
System.out.print(i + " ");
}
System.out.println("Liftoff!");
for loop semantics:


Before the start of the first loop iteration, execute the initialization component.
At the top of each loop iteration, evaluate the condition component:



If the condition is true, execute the body of the loop.
If the condition is false, terminate the loop (jump to the statement below the loop's
closing brace).
At the bottom of each loop iteration, execute the update component and then
jump to the top of the loop.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
29
for Loop
 Trace
this code fragment with an input value of 3.
Scanner stdIn = new Scanner(System.in);
int number;
// user entered number
double factorial = 1.0; // factorial of user entry
System.out.print("Enter a whole number: ");
number = stdIn.nextInt();
for loop index variables are often,
but not always, named i for “index.”
for (int i=2; i<=number; i++)
{
Declare for loop index variables
factorial *= i;
within the for loop heading.
}
System.out.println(number + "! = " + factorial);
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
30
The for loop
A for loop is a repetition control structure that allows you to
efficiently write a loop that needs to execute a specific number of
times.
A for loop is useful when you know how many times a task is to be
repeated.
public class Test3 {
public static void main(String args[]) {
for(int x = 10; x < 20; x = x+1){
System.out.print("value of x : " + x );
System.out.print("\n");
} //end of for loop
} //end of main method
} // end of class
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
31
Here is the flow of control in a for loop:
The initialization step is executed first, and only once. This step allows
you to declare and initialize any loop control variables.
for(int x = 10; x<20; x = x + 1);
Next, the Boolean expression is evaluated. If it is true, the body of
the loop is executed.
x < 20;
// true, then
System.out.print("value of x : " + x ); // body of the loop
System.out.print("\n"); //body of the for loop
// increment the statement and back to checking the condition
If it is false, the body of the loop does not execute and flow of control
jumps to the next statement past the for loop.
 After the body of the for loop executes, the flow of control jumps back
up to the update statement.
 The Boolean expression is now evaluated again. If it is true, the loop
executes and the process repeats itself (body of loop, then update step,then
Boolean expression). After the Boolean expression is false, the for loop
terminates. Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
32
For Loop vs. While Loop
The only thing that can make it faster would be to have less nesting
of loops, and looping over less values.
The only difference between a for loop and a while loop is the
syntax for defining them. There is no performance difference at all.
int i = 0;
while (i < 20){
// do stuff
i++;
}
Is the same as:
for (int i = 0; i < 20; i++){
// do Stuff
}
(Actually the for-loop is a little better because the i will be out of scope
after the loop while the i will stick around in the while loop case.)
A for loop is just a syntactically prettier way of looping
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
33
Problem: Guessing Numbers
Write a program that randomly generates an
integer between 0 and 100, inclusive. The program
prompts the user to enter a number continuously
until the number matches the randomly generated
number. For each user input, the program tells the
user whether the input is too low or too high, so
the user can choose the next input intelligently.
Here is a video and sample run:
Video Link: Problem Guessing Numbers
GuessNumber
Run
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
34
import java.util.Scanner;
public class GuessNumber {
public static void main(String[] args) {
// Generate a random number to be guessed
int number = (int)(Math.random() * 101);
// static double, random() Returns a double value with a positive sign, greater than or
Scanner input = new Scanner(System.in); // scanner breakdown formatting/allocate input and
their data type
System.out.println("Guess a magic number between 0 and 100");
int guess = -1;
while (guess != number) {
// Prompt the user to guess the number
System.out.print("\nEnter your guess: ");
guess = input.nextInt();
if (guess == number)
System.out.println("Yes, the number is " + number);
else if (guess > number)
System.out.println("Your guess is too high");
else
System.out.println("Your guess is too low");
} // End of loop
}
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
35
Problem: An Advanced Math Learning Tool
The Math subtraction learning tool program
generates just one question for each run. You can
use a loop to generate questions repeatedly. This
example gives a program that generates five
questions and reports the number of the correct
answers after a student answers all five questions.
Video Link: Problem SubtractionQuizLoop
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
36
import java.util.Scanner;
public class SubtractionQuizLoop {
public static void main(String[] args) {
final int NUMBER_OF_QUESTIONS = 5; // Number of questions
int correctCount = 0;
// Count the number of correct answers
int count = 0;
// Count the number of questions
long startTime = System.currentTimeMillis();
String output = "";
// output string is initially empty
Scanner input = new Scanner(System.in);
while (count < NUMBER_OF_QUESTIONS) {
// 1. Generate two random single-digit integers
int number1 = (int)(Math.random() * 10);
int number2 = (int)(Math.random() * 10);
// 2. If number1 < number2, swap number1 with number2
if (number1 < number2) {
int temp = number1;
number1 = number2;
number2 = temp;
}
// 3. Prompt the student to answer “What is number1 – number2?”
System.out.print(
"What is " + number1 + " - " + number2 + "? ");
int answer = input.nextInt();
// 4. Grade the answer and display the result
if (number1 - number2 == answer) {
System.out.println("You are correct!");
correctCount++;
}
else
System.out.println("Your answer is wrong.\n" + number1
+ " - " + number2 + " should be " + (number1 - number2));
// Increase the count
count++;
output += "\n" + number1 + "-" + number2 + "=" + answer +
((number1 - number2 == answer) ? " correct" : " wrong");
}
long endTime = System.currentTimeMillis();
long testTime = endTime - startTime;
SubtractionQuizLoop
System.out.println("Correct count is " + correctCount +
"\nTest time is " + testTime / 1000 + " seconds\n" + output);
}
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Run
37
Ending a Loop with a Sentinel Value
Another technique for lop control is to designate a
special value. Often the number of times a loop is
executed is not predetermined. You may use an
input value to signify the end of the loop. Such a
special input values is known as a sentinel value.
Write a program that reads and calculates the sum
of an unspecified number of integers. The input 0
signifies the end of the input.
SentinelValue
Run
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
38
animation
Trace for Loop
Declare i
int i;
for (i = 0; i < 2; i++) {
System.out.println(
"Welcome to Java!");
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
39
animation
Trace for Loop, cont.
int i;
for (i = 0; i < 2; i++) {
System.out.println(
"Welcome to Java!");
}
Execute initializer
i is now 0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
40
animation
Trace for Loop, cont.
int i;
for (i = 0; i < 2; i++) {
System.out.println( "Welcome to Java!");
}
(i < 2) is true
since i is 0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
41
animation
Trace for Loop, cont.
Print Welcome to Java
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
42
animation
Trace for Loop, cont.
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
}
Execute adjustment statement
i now is 1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
43
animation
Trace for Loop, cont.
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
}
(i < 2) is still true
since i is 1
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
44
animation
Trace for Loop, cont.
Print Welcome to Java
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
45
animation
Trace for Loop, cont.
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
}
Execute adjustment statement
i now is 2
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
46
animation
Trace for Loop, cont.
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
}
(i < 2) is false
since i is 2
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
47
animation
Trace for Loop, cont.
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
}
Exit the loop. Execute the next
statement after the loop
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
48
Note
The initial-action in a for loop can be a list of zero or more
comma-separated expressions. The action-after-eachiteration in a for loop can be a list of zero or more commaseparated statements. Therefore, the following two for
loops are correct. They are rarely used in practice,
however.
for (int i = 1; i < 100; System.out.println(i++));
for (int i = 0, j = 0; (i + j < 10); i++, j++) {
// Do something
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
49
Note
If the loop-continuation-condition in a for loop is omitted,
it is implicitly true. Thus the statement given below in (a),
which is an infinite loop, is correct. Nevertheless, it is
better to use the equivalent loop in (b) to avoid confusion:
for ( ; ; ) {
// Do something
}
(a)
Equivalent
while (true) {
// Do something
}
(b)
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
50
Caution
Adding a semicolon at the end of the for clause before
the loop body is a common mistake, as shown below:
Logic
Error
for (int i=0; i<10; i++);
{
System.out.println("i is " + i);
}
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Caution, cont.
Similarly, the following loop is also wrong:
int i=0;
Logic Error
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 {
System.out.println("i is " + i);
i++;
Correct
} while (i<10);
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For Loop vs. While Loop
The only thing that can make it faster would be to have less nesting
of loops, and looping over less values.
The only difference between a for loop and a while loop is the
syntax for defining them. There is no performance difference at all.
int i = 0;
while (i < 20){
// do stuff
i++;
}
Is the same as:
for (int i = 0; i < 20; i++){
// do Stuff
}
(Actually the for-loop is a little better because the i will be out of scope
after the loop while the i will stick around in the while loop case.)
A for loop is just a syntactically prettier way of looping
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Which Loop to Use?
The three forms of loop statements, while, do-while, and for, are
expressively equivalent; that is, you can write a loop in any of these
three forms. For example, a while loop in (a) in the following figure
can always be converted into the following for loop in (b):
while (loop-continuation-condition) {
// Loop body
}
Equivalent
for ( ; loop-continuation-condition; ) {
// Loop body
}
(a)
(b)
A for loop in (a) in the following figure can generally be converted into the
following while loop in (b) except in certain special cases (see Review Question
3.19 for one of them):
for (initial-action;
loop-continuation-condition;
action-after-each-iteration) {
// Loop body;
}
(a)
Equivalent
initial-action;
while (loop-continuation-condition) {
// Loop body;
action-after-each-iteration;
}
(b)
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Recommendations
Use the one that is most intuitive and comfortable for
you.
o 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.
o 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 (use counter).
o 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.
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55
Loop Comparison
for loop:
do loop:
while loop:
When to use
Template
If you know, prior to
the start of loop, how
many times you want
to repeat the loop.
for (int i=0; i<max; i++)
{
<statement(s)>
}
If you always need to
do the repeated thing at
least one time.
do
{
If you can't use a for
loop or a do loop.
<prompt - do it (y/n)?>
while (<response == 'y'>)
{
<statement(s)>
<prompt - do it again (y/n)?>
}
<statement(s)>
<prompt - do it again (y/n)?>
} while (<response == 'y'>);
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Nested Loops
 Nested
loops = a loop within a loop.
 Example – Write a program that prints a
rectangle of characters where the user
specifies the rectangle's height, the
rectangle's width, and the character's value.
Sample session:
Enter height: 4
Enter width: 3
Enter character: <
<<<
<<<
<<<
<<<
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Nested Loops
import java.util.Scanner;
public class RecAngle1
{
public static void main(String[] args)
{
Scanner stdIn = new Scanner(System.in);
int height, width;
// rectangle's dimensions
char printCharacter; // this character forms the rectangle
System.out.print("Enter height: ");
height = stdIn.nextInt();
System.out.print("Enter width: ");
width = stdIn.nextInt();
System.out.print("Enter character: ");
printCharacter = stdIn.next().charAt(0);
for (int row=1; row<=height; row++)
{
for (int col=1; col<=width; col++)
{
System.out.print(printCharacter);
} //end of nested for
System.out.println();
} //end of for
} // end main
} // end class Rectangle
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Boolean Variables






Programs often need to keep track of the state of some
condition.
For example, if you're writing a program that simulates the
operations of a garage door opener, you'll need to keep track
of the state of the garage door's direction and the direction
up or down?
You need to keep track of the direction "state" because the
direction determines what happens when the garage door
opener's button is pressed.
If the direction state is up, then pressing the garage door
button causes the direction to switch to down.
If the direction state is down, then pressing the garage door
button causes the direction to switch to up.
To implement the state of some condition, use a boolean variable.
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Nested Loops
Problem: Write a program that uses nested for loops to
print a multiplication table.
MultiplicationTable
public class MultiTable {
/** Main method */
public static void main(String[] args) {
// Display the table heading
System.out.println("
Multiplication Table"); // table title – display a title on the first line in the output
// Display the number title
System.out.print(" ");
for (int j = 1; j <= 9; j++)
System.out.print(" " + j);
//this for loop display the numbers 1 through 9 on the second line.
System.out.println("\n-----------------------------------------"); // the dash (-) line displayed on the third line
}
}
// Print table body
for (int i = 1; i <= 9; i++) {
//outer loop with the control variable i in the outer loop
System.out.print(i + " | ");
for (int j = 1; j <= 9; j++) {
// Display the product and align properly //inner loop with the control variable j in the inner loop
System.out.printf("%4d", i * j);
// for each I, the product I * j is displayed on a line in the inner loop.
}
//with j begin 1, 2, 3, …9.
System.out.println();
}
Run
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Minimizing Numerical Errors
Numeric errors involving floating-point numbers are
expected. This section discusses how to minimize such
errors through an example. Listing 4.7.
Here is an example that sums a series that starts with 0.01
and ends with 1.0. The numbers in the series will increment
by 0.01, as follows: 0.01 + 0.02 + 0.03 and so on.
public class TestSum {
public static void main(String[] args) {
// Initialize sum
float sum = 0;
Video Link: Problem TestSum
// Add 0.01, 0.02, ..., 0.99, 1 to sum
for (float i = 0.01f; i <= 1.0f; i = i + 0.01f)
sum += i;
// Display result
System.out.println("The sum is " + sum);
}
}
TestSum
Run
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Problem:
Finding the Greatest Common Divisor
Problem: Write a program that prompts the user to enter two positive
integers and finds their greatest common divisor.
Solution: Suppose you enter two integers 4 and 2, their greatest
common divisor is 2. Suppose you enter two integers 16 and 24, their
greatest common divisor is 8. So, how do you find the greatest
common divisor? Let the two input integers be n1 and n2. You know
number 1 is a common divisor, but it may not be the greatest
commons divisor. So you can check whether k (for k = 2, 3, 4, and so
on) is a common divisor for n1 and n2, until k is greater than n1 or n2.
GreatestCommonDivisor
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import java.util.Scanner;
public class GreatestCommonDivisor {
/** Main method */
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Prompt the user to enter two integers
System.out.print("Enter first integer: ");
int n1 = input.nextInt();
System.out.print("Enter second integer: ");
int n2 = input.nextInt();
int gcd = 1;
int k = 2;
while (k <= n1 && k <= n2) {
if (n1 % k == 0 && n2 % k == 0)
gcd = k;
k++;
}
System.out.println("The greatest common divisor for " + n1 +
" and " + n2 + " is " + gcd);
}
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}
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Problem: Predicating the Future Tuition- Listing 4.9
double tuition = 10000; int year = 1 // Year 1
tuition = tuition * 1.07; year++;
// Year 2
tuition = tuition * 1.07; year++;
// Year 3
tuition = tuition * 1.07; year++;
// Year 4
...
FutureTuition
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Problem: Predicating the Future Tuition
Problem: Suppose that the tuition for a university is $10,000 this year
and tuition increases 7% every year. In how many years will the
tuition be doubled?
Before you write this program try to solve this problem first by hand.
public class FutureTuition {
public static void main(String[] args) {
double tuition = 10000; // Year 1
int year = 1;
while (tuition < 20000) {
tuition = tuition * 1.07;
year++;
}
System.out.println("Tuition will be doubled in "
+ year + " years");
FutureTuition
}
}
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Problem: Monte Carlo Simulation
The Monte Carlo simulation refers to a technique that uses random
numbers and probability to solve problems. This method has a wide
range of applications in computational mathematics, physics,
chemistry, and finance. This section gives an example of using the
Monte Carlo simulation for estimating . Assume the radius of
the Circle is 1, area is  and the square area is 4.
circleArea / squareArea =  / 4.
y
1
-1
1
-1
x
 can be approximated as 4 *
numberOfHits / 1000000.
MonteCarloSimulation
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Problem: Monte Carlo Simulation
Web Link: About Monte Carlo Simulation - Introduction
public class MonteCarloSimulation {
public static void main(String[] args) {
final int NUMBER_OF_TRIALS = 10000000;
int numberOfHits = 0;
for (int i = 0; i < NUMBER_OF_TRIALS; i++) {
double x = Math.random() * 2.0 - 1;
double y = Math.random() * 2.0 - 1;
if (x * x + y * y <= 1)
numberOfHits++;
} //end of while
double pi = 4.0 * numberOfHits / NUMBER_OF_TRIALS;
System.out.println("PI is " + pi);
} // end of main method
} //end of class
Web link: Class Math
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Using break and continue
Examples for using the break and continue
keywords that can be used in loop statements to
provide additional controls.

TestBreak.java
// you have to used the keyword break in a switch statement
// you can use break in a loop to immediately terminate the loop.
TestBreak

Run
TestContinue.java
TestContinue
Run
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68
Guessing Number Problem Revisited
Here is a program for guessing a number. You can
rewrite it using a break statement. Listing 4.11
public class TestBreak {
public static void main(String[] args) {
int sum = 0;
int number = 0;
while (number < 20) {
number++;
sum += number;
if (sum >= 100)
break; // goes to println
}
System.out.println("The number is " + number);
System.out.println("The sum is " + sum);
GuessNumberUsingBreak
}
}
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Guessing Number Problem Revisited
Here is a program for guessing a number. You can
rewrite it using a continue statement. Listing 4.12
public class TestContinue {
public static void main(String[] args) {
int sum = 0;
int number = 0;
while (number < 20) {
number++;
if (number == 10 || number == 11) continue;
sum += number;
} //end of while
System.out.println("The sum is " + sum);
} //end of main method
} //end of class
TestContinue
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Problem: Displaying Prime Numbers
Problem: Write a program that displays the first 50 prime numbers in
five lines, each of which contains 10 numbers. An integer greater than
1 is prime if its only positive divisor is 1 or itself. For example, 2, 3,
5, and 7 are prime numbers, but 4, 6, 8, and 9 are not.
Solution: The problem can be broken into the following tasks:
•For number = 2, 3, 4, 5, 6, ..., test whether the number is prime.
•Determine whether a given number is prime.
•Count the prime numbers.
•Print each prime number, and print 10 numbers per line.
PrimeNumber
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Exercise 4_22
Financial application: Loan repayment schedule
Please refer to companion website for the program.
Video Link: Exercise 4_22
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(GUI) Controlling a Loop with a
Confirmation Dialog
A sentinel-controlled loop can be implemented using a confirmation
dialog. The answers Yes or No to continue or terminate the loop. The
template of the loop may look as follows:
int option = 0;
while (option == JOptionPane.YES_OPTION) {
System.out.println("continue loop");
option = JOptionPane.showConfirmDialog(null, "Continue?");
}
SentinelValueUsingConfirmationDialog
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