boolean_expression Block statement Branching Statements Simple if statement if-else statement if-elseif-else statement Nested if statements switch statement

1. 1
CS120
Lecture 4: Selection
Jubail University College
Computer and Information Technology Department

2. 2
Outline
 boolean_expression
 Block statement
 Branching Statements
 Simple if statement
 if-else statement
 if-elseif-else statement
 Nested if statements
 switch statement

3. 3
Simple if Statement …
 General structure:
 If the boolean_expression is true, then the block_statement is
executed. Otherwise it will NOT execute.
if ( boolean_expression )
block_statement
System.out.print(“Enter negative number: “);
int x = ;
if ( x > 0 ){
System.out.println(“negative number is
required.”);
x = x * -1;
}

4. 4
-- Simple if Statement …
Control Flow of if:
x > 0
System.out.println(
“negative number is
required.”);
x = x * -1
true
false
Continue remaining
of the program

5. 5
-- if-else Statement …
 The if-else statement is used if an alternative action need to
be done in case the condition is false.
 General structure:
if ( boolean_expression ){
if_true_block
}
else {
else_block
}
Executed when
boolean_expression
true
Executed when
boolean_expression false

6. 6
-- if-else Statement …
 Example:
 What is printed if n1 = n2?
System.out.print(“Enter two numbers: “);
int n1 = ;
int n2 = ;
if ( n1 > n2 ){
System.out.println(n1 + “ is greater than ” + n2);
}
else {
System.out.println(n1 + “ is not greater than ” + n2);
}

7. 7
-- if-else Statement …
n1 > n2
System.out.println
(n1 + “is greater
than” + n2”);
true
Continue remaining
of the program
System.out.println(n1
+ “is not greater
than” + n2”);
false
Control Flow of if-else:

8. 8
Block Statement
 A block statement consists of one or more Java statements
enclosed in braces.
 Example of a block statement:
{
statement 1;
statement 2;
…
statement n;
}
 It is used to group a number of statements to be done under
certain condition.
 A block statement is treated like a single statement.

9. 9
Block Statements
 Use braces if the <then> or <else> block has more than 1
statement. If only one statement is there braces are
optional.
System.out.print(“Enter two numbers: “);
int x = , y = , d;
if ( x > y )
d = x – y;
System.out.println(“The difference = “ + d);
else
d = y – x;
System.out.println(“The difference = “ + d);
if ( x > y )
d = x – y;
else
d = y – x;
System.out.println(“The difference = “ + d);
Error
Correct

10. 10
-- if-elseif-else Statement …
 This statement is used for multiple alternative actions
based on different conditions. It is used when a single block
need to be executed from several blocks.
 General structure:
if (condition-1)
BLOCK1
else if (condition-2)
BLOCK2
else if (condition-3)
BLOCK3
...
else if (condition-n)
BLOCKn
else
BLOCKn+1
Optional
Blocki is
executed if its
condition-i is true
It is executed if all
conditions are
false
conditions are
evaluated from
top to bottom

11. 11
if-elseif-else
System.out.print(“Enter your score: “);
int score = k.nextInt();
if (score >= 85)
System.out.println(”Grade is A");
else if (score >= 75)
System.out.println(”Grade is B");
else if (score >= 65)
System.out.println(”Grade is C");
else if (score >= 50)
System.out.println(”Grade is D");
else
System.out.println(”Grade is N");
Test Score Grade
85  score A
75  score  85 B
65  score  75 C
50  score  65 D
score  50 N
 The following code prints appropriate message based on
the following table:

12. 12
- Nested if Statements …
 An if statement can be inside another if statement.
 The inner if statement is executed when the enclosing block
statement is executed.
 Always use indentation with nested if statements for better
readibilty.
if (salary >= 5000) {
if (months >= 3) {
System.out.println("You get 50000
loan");
} else {
System.out.println("You get 20000
loan");
}
} else {
System.out.println("You cannot get a loan");
}

13. 13
Nested simple-if
if (x < y) {
if (x < z)
System.out.println("Hello");
} else {
System.out.println("Good bye");
}
Equivalent to
if (x < y) {
if (x < z) {
System.out.println("Hello");
}
} else {
System.out.println("Good bye");
}
This if is executed only
when x < y is true.

14. 14
Nested if-else
Equivalent to
 In nested if statements when there is no braces, else is
associated with the closest if.
if (x < y)
if (x < z)
System.out.println("Hello");
else
System.out.println("Good bye");
if (x < y) {
if (x < z) {
System.out.println("Hello");
}
else {
System.out.println("Good bye");
}
}
This if-else is executed
only when x < y is
true.

15. 15
Nested if-else
 Convert the following diagram into Java code.
pH > 7
pH < 12
pH is 7
pH > 2
True
False
‘Very
acidic’
False
False
‘Acidic’
True
‘Neutral’
True
‘Very
alkaline’
False ‘Alkaline’
True

16. 16
Nested if-else
 Convert the following diagram into Java code.
pH > 7
pH < 12
pH is 7
pH > 2
True
False
‘Very
acidic’
False
False
‘Acidic’
True
‘Neutral’
True
‘Very
alkaline’
False ‘Alkaline’
True

17. 17
- The switch Statement …
 The switch statement is another kind of multiway branching.
switch (Controlling_Expression) {
case Label1:
BLOCK1
break;
case Label2:
BLOCK2
break;
...
case Labeln:
BLOCKn
break;
default:
Default_BLOCK
}
Optional
Evaluate to char,
short, byte or int
and each label is of
the same type.
case i is executed if
Labeli = Cont_Exp.
Stops switch
statement. If omitted,
execution continues to
next case.
It is executed if all
Labels ≠ Cont_Exp.

18. 18
… -- The switch Statement
double y = 30, z = 20;
Scanner kb = new Scanner(System.in);
System.out.println("1. add ");
System.out.println("2. Subtract ");
System.out.println("3. Multiply ");
System.out.print("Enter a value:");
int choice = kb.nextInt();
switch (choice) {
case 1: System.out.println(z + y);
break;
case 2: System.out.println(z - y);
case 3: System.out.println(z * y);
break;
default: System.out.println("Wrong Choice.");
break;
}
break;
Enter a value: 2
-10
Enter a value: 65
Wrong Choice.
If
omitted Enter a value: 2
-10
600

19. 19
Exercises
1. Write a Java program that reads an integer using
Scanner then tests if it is odd or even. The
format of the input and output should be as
follows:
Please enter a number: 10
10 is even.
2. Write a program that reads three integers and
prints them in increasing order. Example:
Please enter three integers: 9, 5, 21
The integers in increasing order are: 5 9 21

20. 20
Exercises
3. Write a program that reads an integer between 0
and 10 and then prints the word corresponding
to the number. Example:
Please enter a number between 0 and 10: 7
You entered: seven
Read section 3.1