1. Control Structures

2. Program Flow
Java will execute the statements in your code in a specific sequence, or "flow".
The "flow" of the program and can be described through a "flow diagram":
a more sophisticated program
statement
statement
statement
statement
a simple program
statement
statement
statement
statement
statement
statement

3. What are Control Structures?
Control structures alter the flow of the program, the sequence of statements that are executed in a program.
They act as "direction signals" to control the path a program takes.
Two types of control structures in Java:
decision statements
loops

4. Decision Statements
A decision statement allows the code to execute a statement or block of statements conditionally.
Two types of decisions statements in Java:
if statements
switch statements

5. If Statement if (expression) { statement; } rest_of_program;
expression must evaluate to a boolean value, either true or false
If expression is true, statement is executed and then rest_of_program
If expression is false, statement is not executed and the program continues at rest_of_program

6. If Statement Flow Diagram
execute
statement
rest_of_program
Is expression
true?
yes no
The if decision statement executes a statement if an expression is true
if (expression) {
statement1; }
rest_of_program

7. If-Else Statement
if (expression) { statement1; } else{ statement2; } next_statement;
Again, expression must produce a boolean value
If expression is true, statement1 is executed and then next_statement is executed.
If expression is false, statement2 is executed and then next_statement is executed.

8. If-Else Flow Diagram is
“expression”
true?
execute
statement1
rest_of_program no
yes
statement2
The if-else decision statement executes a statement if an expression is true and a different statement if it is not true.
if (expression){
statement1;
} else {
statement2; }
rest_of_program

9. Chained If-Else Statements
if (grade == 'A')
System.out.println("You got an A.");
else if (grade == 'B')
System.out.println("You got a B.");
else if (grade == 'C')
System.out.println("You got a C.");
else
System.out.println("You got an F.");

10. Switch Statements
The switch statement enables you to test several cases generated by a given expression.
For example: switch (expression) { case value1: statement1; case value2: statement2; default: default_statement; } Every statement after the true case is executed
The expression must evaluate to a char, byte, short or int, but not long, float, or double.

11. y n y n expression equals value1? Do value1 thing switch (expression){
case value1:
// Do value1 thing
case value2:
// Do value2 thing
...
default:
// Do default action }
// Continue the program n y
expression
equals
value2?
Do value2 thing n
Do default action
Continue the
program

12. Break Statements in Switch Statements
The break statement tells the computer to exit the switch statement
For example:
switch (expression) {
case value1:
statement1;
break;
case value2:
statement2;
default:
default_statement; }

13. expression
equals
value1?
value2?
do default action
Do value1 thing
Do value2 thing
break
Continue the
program y n
switch (expression){
case value1:
// Do value1 thing
break;
case value2:
// Do value2 thing
...
default:
// Do default action }
// Continue the program

14. Remember the Chained If-Else . . .
if (grade == 'A')
System.out.println("You got an A.");
else if (grade == 'B')
System.out.println("You got a B.");
else if (grade == 'C')
System.out.println("You got a C.");
else
System.out.println("You got an F.");

15. This is how it is accomplished with a switch:
if-else chains can be sometimes be rewritten as a “switch” statement.
switches are usually simpler and faster
switch (grade) { case 'A': System.out.println("You got an A."); break; case 'B': System.out.println("You got a B."); break; case 'C': System.out.println("You got a C."); break; default: System.out.println("You got an F."); }

16. Loops
A loop allows you to execute a statement or block of statements repeatedly.
Three types of loops in Java:
1. while loops
2. for loops
3. do-while loops (not discussed in this course)

17. The while Loop while (expression){ statement }
This while loop executes as long as the given logical expression between parentheses is true. When expression is false, execution continues with the statement following the loop block.
The expression is tested at the beginning of the loop, so if it is initially false, the loop will not be executed at all.

18. For example: What is the value of sum?
int sum = 0;
int i = 1;
while (i <= 10){
sum += i;
i++; }
What is the value of sum?
= 55

19. The for Loop
for (init_expr; loop_condition; increment_expr) { statement; }
The control of the for loop appear in parentheses and is made up of three parts:
The first part, the init_expression,sets the initial conditions for the loop and is executed before the loop starts.
Loop executes so long as the loop_condition is true and exits otherwise.
The third part of the control information, the increment_expr, is usually used to increment the loop counter. This is executed at the end of each loop iteration.

20. For example: What is the value of sum?
int sum = 0;
for (int i = 1; i <= 10; i++) {
sum += i; }
What is the value of sum?
= 55

21. What will this for loop do?
Example 3:
for(int div = 0; div < 1000; div++){
if(div % 2 == 0) {
System.out.println("even: " + div);
} else {
System.out.println("odd: " + div); }
What will this for loop do?
prints out each integer from 0 to 999,
correctly labeling them even or odd

22. If there is more than one variable to set up or increment they are separated by a comma.
for(i=0, j=0; i*j < 100; i++, j+=2) { System.out.println(i * j); }
You do not have to fill all three control expressions but you must still have two semicolons.
int n = 0;
for(; n <= 100;) { System.out.println(++n);

23. The for loop Initialize count The while loop Test condition
is true?
Execute loop
statement(s)
Increment
count
New statement
The for loop y n
The while loop
Test condition
is true?
Execute loop
statement(?)
Next statement y n

24. The continue Statement
The continue statement causes the program to jump to the next iteration of the loop.
/**
* prints out "5689" */
for(int m = 5; m < 10; m++) {
if(m == 7) {
continue; }
System.out.print(m);

25. Another continue example:
int sum = 0;
for(int i = 1; i <= 10; i++){
if(i % 3 == 0) {
continue; }
sum += i;
What is the value of sum?
= 37

26. The break Statement
We have seen the use of the break statement in the switch statement.
You can also use the break statement to exit the loop entirely.
// prints out numbers unless
// num is ever exactly 400
while (num > 6) {
if(num == 400) {
break; }
System.out.println(num);
num -= 8;

27. Nested Loops
You can nest loops of any kind one inside another to any depth. What does this print?
for(int i = 10; i > 0; i--) {
if (i > 7) {
continue; }
while (i > 3) {
if(i == 5) {
break;
System.out.println(--i);
System.out.println(i); 6 5 4 3 2 1

28. POP QUIZ
In the switch statement, which types can expression evaluate to?
What must be used to separate each section of a for statement.
Which statement causes a program to skip to the next iteration of a loop.
Write a for loop that outputs in reverse sequence.
Write a for loop that outputs all numbers that are divisible by 3 between 0-50.
char, byte, short, int
semicolons
continue
Answers:
From slide “Switch Statements”: char, byte, short or int
From slide “The for loop”: a semi-colon
From slide “The continue Statement”: continue
for(int i=100; i>0; i--)
System.out.println(i);
for(int i=0; i<=50; i++) {
if (i%3 == 0) }