This document discusses different types of loops in Java including while, do-while, and for loops. It explains the syntax and flow control of each loop type. Examples are provided to demonstrate how to use loops appropriately. Common mistakes like infinite loops and off-by-one errors are highlighted. The break and continue keywords are also explained. Finally, the document briefly introduces methods in Java.

1. Lecture#5
Aleeha Iftikhar

2. Repetitions Statements
 while Loops
 do-while Loops
 for Loops
 break and continue

3. The while Statement
 Syntax for the while Statement
 while ( <boolean expression> )
<statement>
while ( number <= 100 ) {
sum = sum + number;
number = number + 1;
}
Statement
(loop body)
Boolean Expression

4. Control Flow of while
int sum = 0, number = 1
number <= 100 ?
false
sum = sum + number;
number = number + 1;
true

5. int age;
Scanner scanner = new Scanner(System.in);
System.out.print("Your Age (between 0 and 130): ");
age = scanner.nextInt( );
while (age < 0 || age > 130) {
System.out.println(
"An invalid age was entered. Please try again.");
System.out.print("Your Age (between 0 and 130): ");
age = scanner.nextInt( );
}
Example: Testing Input Data
Priming Read
For Integer input

6. 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.
 Make sure the loop body contains a statement that will
eventually cause the loop to terminate.
 Make sure the loop repeats exactly the correct number of
times.
 If you want to execute the loop body N times, then
initialize the counter to 0 and use the test condition
counter < N or initialize the counter to 1 and use the test

7. Loop Pitfall - 1
Infinite Loops
Both loops will not
terminate because the
boolean expressions will
never become false.int count = 1;
while ( count != 10 ) {
count = count + 2;
}
2
int product = 0;
while ( product < 500000 ) {
product = product * 5;
}
1

8. Loop Pitfall - 2
• Goal: Execute the loop body 10 times.
count = 1;
while ( count < 10 ){
. . .
count++;
}
1
count = 0;
while ( count <= 10 ){
. . .
count++;
}
3
count = 1;
while ( count <= 10 ){
. . .
count++;
}
2
count = 0;
while ( count < 10 ){
. . .
count++;
}
4

9. The do-while Statement
do {
sum += number;
number++;
} while ( sum <= 1000000 );
do
<statement>
while ( <boolean expression> ) ;
Statement
(loop body)
Boolean Expression

10. Control Flow of do-while
int sum = 0, number = 1
sum += number;
number++;
sum <= 1000000 ?
true
false

11. The for Statement
for ( i = 0 ; i < 20 ; i++ ) {
number = scanner.nextInt();
sum += number;
}
for ( <initialization>; <boolean expression>; <increment> )
<statement>
Initialization
Boolean
Expression
Increment
Statement
(loop body)

12. The for Statement
int i, sum = 0, number;
for (i = 0; i < 20; i++) {
number = scanner.nextInt( );
sum += number;
}
These statements are
executed for 20 times
( i = 0, 1, 2, … , 19).

13. Control Flow of for
i = 0;
false
number = . . . ;
sum += number;
true
i ++;
i < 20 ?

14. More for Loop Examples
for (int i = 0; i < 100; i += 5)1
i = 0, 5, 10, … , 95
for (int j = 2; j < 40; j *= 2)2
j = 2, 4, 8, 16, 32
for (int k = 100; k > 0; k--) )3
k = 100, 99, 98, 97, ..., 1

15. 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.

16. Caution
Adding a semicolon at the end of the for clause
before the loop body is a common mistake, as shown
below:
for (int i=0; i<10; i++);
{
System.out.println("i is " + i);
}
Similarly, the following loop is also 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 {
System.out.println("i is " + i);
i++;
} while (i<10);
Wrong
Correct

17. Loop-and-a-Half Repetition Control
• Loop-and-a-half repetition control can be
used to test a loop’s terminating condition in
the middle of the loop body.
• It is implemented by using reserved words
while, if, and break.

18. Example: Loop-and-a-Half Control
String name;
Scanner scanner = new Scanner(System.in);
while (true){
System.out.print("Your name“);
name = scanner.next( );
if (name.length() > 0) break;
System.out.println("Invalid Entry." +
"You must enter at least one character.");
}

19. Example: Loop-and-a-Half Control
String name;
Scanner scanner = new Scanner(System.in);
while (true){
System.out.print("Your name“);
name = scanner.next( );
if (name.length() > 0) break;
System.out.println("Invalid Entry." +
"You must enter at least one character.");
}

20. The break Keyword
false
true
Statement(s)
Next
Statement
Continuation
condition?
Statement(s)
break

21. The continue Keyword
false
true
Statement(s)
Next
Statement
Continue
condition?
Statement(s)
continue

22. Chapter 4 Methods
 Introducing Methods
 Benefits of methods, Declaring Methods, and Calling Methods
 Passing Parameters
 Pass by Value

23. Introducing Methods
Method Structure
A method is a
collection of
statements that
are grouped
together to
perform an
operation.

24. Introducing Methods, cont.
•parameter profile refers to the
type, order, and number of the
parameters of a method.
•method signature is the
combination of the method name and
the parameter profiles.
•The parameters defined in the
method header are known as formal
parameters.
•When a method is invoked, its

25. DeclaringDefinning Methods
public static int max(int num1, int num2)
{
if (num1 > num2)
return num1;
else
return num2;
}

26. Calling Methods, cont.
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
pass i
pass j

27. Calling Methods, cont.
The main method
i:
j:
k:
The max method
num1:
num2:
result:
pass 5
5
2
5
5
2
5
pass 2
parameters

28. THE END