1. Lecture Notes – Week 3 Lecture-2
Chapter 5 (Loops)

2. Outline Introduction to loops Three forms of loops To do list while
do-while
for
To do list

3. Introduction to Loops Motivating example:
Example: Print "Welcome to Java!” 100 times.
How would we do it?
Write the following statement 100 times?
System.out.println("Welcome to Java!");
That would be too tedious!
We would need to write a loop.

4. Loops
Loops are constructs that control repeated execution of a block of statements.
int count = 0;
while (count < 100) {
System.out.println("Welcome to Java!");
count++; }
A while loop starts with keyword while, followed by a Boolean expression for the condition, then followed by a statement body.
We also need to declare/initialise a variable for counting.

5. while Statements } while (Boolean-expression) { statement(s);
int count = 0;
while (count < 100) {
System.out.println(”Welcome to Java!”);
count++; }
while (Boolean-expression) {
statement(s); } 5 5

6. Trace while Loop Initialize count to 0 int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; } 6 6 6

7. While (count < 2) is true
Trace while Loop
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
While (count < 2) is true
(count is 0 at moment) 7 7 7 7

8. Print “Welcome to Java!”
Trace while Loop
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Print “Welcome to Java!” 8 8 8 8 8

9. (count is increased to 1 now)
Trace while Loop
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Increase count by 1
(count is increased to 1 now) 9 9 9 9 9 9

10. While (count < 2) is true
Trace while Loop
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
While (count < 2) is true
(count is 1 at moment) 10 10 10 10 10

11. Print “Welcome to Java!”
Trace while Loop
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Print “Welcome to Java!” 11 11 11 11 11 11

12. (count is increased to 2 now)
Trace while Loop
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Increase count by 1
(count is increased to 2 now) 12 12 12 12 12 12 12

13. While (count < 2) is false
Trace while Loop
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
While (count < 2) is false
(count is 2) 13 13 13 13 13 13

14. Exit the loop and execute the next statement after the loop.
Trace while Loop
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Exit the loop and execute the next statement after the loop. 14 14 14 14 14 14 14 14

15. Increment and decrement operators15

16. do-while Statements Now compare it to: do { statement(s);
} while (Boolean-expression);
int count = 0;
do {
System.out.println(”Welcome to Java!”);
count++;
} while (count < 100);
Now compare it to:
int count = 0;
while (count < 100) {
System.out.println(”Welcome to Java!”);
count++; } 16 16 16

17. for Statements
for (initial-action; Boolean-expression; action-after) {
statement(s); };
int i;
for (i = 0; i < 100; i++) {
System.out.println(”Welcome to Java!”); }; 17 17 17

18. Trace for Loop Declare variable i int i; for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
} ; 18 18 18

19. Trace for Loop int i; for (i = 0; i < 2; i++) { Initialise I to 0
System.out.println("Welcome to Java!");
} ;
Initialise I to 0
(i is 0) 19 19 19 19

20. Trace for Loop int i; for (i = 0; i < 2; i++) { (i < 2) is true
System.out.println("Welcome to Java!");
} ;
(i < 2) is true
(i is 0) 20 20 20 20 20

21. Print “Welcome to Java!”
Trace for Loop
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
} ;
Print “Welcome to Java!” 21 21 21 21 21 21 21

22. Trace for Loop int i; Increase i by 1 for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
} ;
Increase i by 1
(i is increased to 1) 22 22 22 22 22 22 22

23. Trace for Loop int i; for (i = 0; i < 2; i++) { (i < 2) is true
System.out.println("Welcome to Java!");
} ;
(i < 2) is true
(i is 1) 23 23 23 23 23

24. Print “Welcome to Java!”
Trace for Loop
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
} ;
Print “Welcome to Java!” 24 24 24 24 24 24 24

25. Trace for Loop int i; Increase i by 1 for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
} ;
Increase i by 1
(i is increased to 2) 25 25 25 25 25 25 25

26. Trace for Loop int i; for (i = 0; i < 2; i++) { (i < 2) is false
System.out.println("Welcome to Java!");
} ;
(i < 2) is false
(i is 2) 26 26 26 26 26 26

27. Exit the loop and execute the next statement after the loop.
Trace for Loop
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!");
} ;
Exit the loop and execute the next statement after the loop. 27 27 27 27 27 27 27

28. Exercise
Write a program that displays the squares of the numbers from 0 to 15.
Using For Loop
While Loop
Do-While Loop
Output should be as :

29. To Do List Before Next Lecture
Read Week 3 lecture slides.
Read and run Week 3 program examples.
Selectively read those sections in Chapter 5 that cover the topics in this lecture.
Attend your practical session in Week 4, consisting of a tutorial and a set of programming exercises.
Glance through those sections in Chapter 6 that cover the topics in Week 4’s lecture.