1. Lecture 3 MIT 12043, Fundamentals of Programming By: S. Sabraz Nawaz
Decision Structures
Lecture 3
MIT 12043, Fundamentals of Programming
By: S. Sabraz Nawaz

2. Chapter Topics Relational Operators The if Statement
The if-else Statement
Nested if statements
The if-else-if Statement
MIT 12043, By S. Sabraz Nawaz

3. Relational Operators == > < !=
Relational operators in Java are used to compare two variables.
Java provides six relational operators:
The result of any relational operator is “true” or “false”
Equal ==
Greater than
>
Less than
<
Greater than or equal to
>=
Less than or equal to
<=
Unequal !=
MIT 12043, By S. Sabraz Nawaz

4. Boolean Expressions int a = 3; b = 4; a == b will evaluate to false
a != b will evaluate to true
a > b will evaluate to false
a < b will evaluate to true
a >= b will evaluate to false
a <= b will evaluate to true
MIT 12043, By S. Sabraz Nawaz

5. Sequence Structure
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6. Decision Structure
Programs often need more than one path of execution, however. Many algorithms require a program to execute some statements only under certain circumstances. This can be accomplished with a decision structure
In a decision structure’s simplest form, a specific action is taken only when a condition exists. If the condition does not exist, the action is not performed
MIT 12043, By S. Sabraz Nawaz

7. Decision Structure… Simple decision structure logic
MIT 12043, By S. Sabraz Nawaz

8. Decision Structure… Three-action decision structure logic
MIT 12043, By S. Sabraz Nawaz

9. Selection Statements
The control statement that allows alternative actions based upon conditions that are evaluated at run time are generally called selection statements
One way to code a decision structure in Java is with the if statement
MIT 12043, By S. Sabraz Nawaz

10. The if Statement if (boolean expression is true)
The if statement is used to create a decision structure, which allows a program to have more than one path of execution.
The if statement causes one or more statements to execute only when a boolean expression is true.
The if statement decides whether a section of executes or not.
if (boolean expression is true)
execute this statement;
MIT 12043, By S. Sabraz Nawaz

11. The if Statement… if (coldOutside) { wearCoat; wearHat; wearGloves; }
A block if statement may be modeled as:
if (coldOutside) {
wearCoat;
wearHat;
wearGloves; }
Note the use of curly
braces to block several
statements together.
MIT 12043, By S. Sabraz Nawaz

12. if Statements and Boolean Expressions
if (x > y)
System.out.println("X is greater than Y");
if(x == y)
System.out.println("X is equal to Y");
if(x != y) {
System.out.println("X is not equal to Y");
x = y;
System.out.println("However, now it is."); }
MIT 12043, By S. Sabraz Nawaz

13. Programming Style and if Statements
An if statement can span more than one line; however, it is still one statement.
if (average > 95)
grade = ′A′;
is functionally equivalent to
if(average > 95) grade = ′A′;
MIT 12043, By S. Sabraz Nawaz

14. Block if Statements
Conditionally executed statements can be grouped into a block by using curly braces {} to enclose them.
If curly braces are used to group conditionally executed statements, the if statement is ended by the closing curly brace.
if (expression) {
statement1;
statement2; }
Curly brace ends the statement.
MIT 12043, By S. Sabraz Nawaz

15. Only this statement is conditionally executed.
Block if Statements
Remember that when the curly braces are not used, then only the next statement after the if condition will be executed conditionally
if (expression)
statement1;
statement2;
statement3;
Only this statement is conditionally executed.
MIT 12043, By S. Sabraz Nawaz

16. Example 01
Write a Java program that takes a person’s age as input and decides whether that person is eligible to vote
If the person’s age is above 18, he or she is eligible to vote
The program should display a message whether he or she is eligible to vote
MIT 12043, By S. Sabraz Nawaz

17. Example 01…
MIT 12043, By S. Sabraz Nawaz

18. Example 02
Write a Java program that takes a person’s age and his or her name as inputs and decides whether that person is eligible to vote
If the person’s age is above 18, he or she is eligible to vote
The program should display a message with the Name of the person and his or eligibility to vote
MIT 12043, By S. Sabraz Nawaz

19. Example 02
MIT 12043, By S. Sabraz Nawaz

20. if-else Statements
The if-else statement adds the ability to conditionally execute code when the if condition is false.
if (expression)
statementOrBlockIfTrue;
else
statementOrBlockIfFalse;
It works the same way as the if statement except that, when the condition is false, the statement within the else clause executes
MIT 12043, By S. Sabraz Nawaz

21. if-else Statement Flowcharts
Wear a coat.
Yes
Is it cold
outside?
Wear shorts. No
MIT 12043, By S. Sabraz Nawaz

22. Example 03
Write a Java program that takes a person’s age and his or her name as inputs and decides whether that person is eligible to vote or not
If the person’s age is above 18, he or she is eligible to vote
The program should display a message with the Name of the person and his or eligibility to vote
MIT 12043, By S. Sabraz Nawaz

23. Example 03…
MIT 12043, By S. Sabraz Nawaz

24. Exercise A company’s payment structure to its employees is as follows
If the Basic salary exceeds 30,000/=, the Dearness Allowance 40% the BS, otherwise 50% of the BS
If the BS exceeds 25,000/=, the House Rent Allowance is 20% of the BS, otherwise 3,000 of the BS
Income Tax is calculated at the rate of 12% if the Total Salary exceeds 50,000/=
Write a Java program that takes BS as input, calculate DA, HRA, IT, Total Salary, Net Salary, and display them all on screen
MIT 12043, By S. Sabraz Nawaz

25. Solution
MIT 12043, By S. Sabraz Nawaz

26. Solution…
MIT 12043, By S. Sabraz Nawaz

27. Nested if Statements
To test more than one condition, an if statement can be nested inside another if statement
If an if statement appears inside another if statement (single or block) it is called a nested if statement
The nested if is executed only if the outer if statement results in a true condition
MIT 12043, By S. Sabraz Nawaz

28. Alignment and Nested if Statements
if (expression) {
statement; }
else
This if and else
go together.
This if and else
go together.
MIT 12043, By S. Sabraz Nawaz

29. Example 04
For example, consider a banking program that determines whether a bank customer qualifies for a special, low interest rate on a loan
To qualify, two conditions must exist:
The customer’s salary must be at least Rs.30,000
The customer must have held his or her current job for at least 02 years
MIT 12043, By S. Sabraz Nawaz

30. Example 04…
MIT 12043, By S. Sabraz Nawaz

31. if-else-if The if-else-if statement tests a series of conditions
It is often simpler to test a series of conditions with the if-else-if statement than with a set of nested if-else statements
MIT 12043, By S. Sabraz Nawaz

32. if-else-if Syntax
if (expression_1) {
statement;
etc. }
else if (expression_2)
Insert as many else if clauses as necessary
else
If expression_1 is true these statements are executed, and the rest of the structure is ignored.
Otherwise, if expression_2 is true these statements are executed, and the rest of the structure is ignored.
These statements are executed if none of the expressions above are true.
MIT 12043, By S. Sabraz Nawaz

33. Example 05
Your lecturer has asked you to write a program that will allow a student to enter a test score and then display the grade for that score
MIT 12043, By S. Sabraz Nawaz

35. Logical Operators
MIT 12043, By S. Sabraz Nawaz

36. Logical Operators
Logical operators connect two or more relational expressions into one or reverse the logic of an expression.
Java provides two binary logical operators,
&& and ||
used to combine two boolean expressions into a single expression
It also provides the unary ! Operator
reverses the truth of a boolean expression
MIT 12043, By S. Sabraz Nawaz

37. Logical Operators… Operator Meaning Effect && AND
Connects two boolean expressions into one. Both expression must be true for the overall expression to be true || OR
Connects two boolean expressions into one. One or both expressions must be true for the overall expression to be true. It is only necessary for one to be true, and it does not matter which one. !
NOT
Reverses the truth of a boolean expression. If it is applied to an expression that is true, the operator returns false. If it is applied to an expression that is false, the operator returns true
MIT 12043, By S. Sabraz Nawaz

38. Logical Operators… Expression Meaning x > y && a < b
Is x greater than y AND is a less than b?
x == y || x == z
Is x equal to y OR is x equal to z?
!(x > y)
Is the expression x > y NOT true?
MIT 12043, By S. Sabraz Nawaz

39. Truth table for the && operator
Expression
Value of the Expression
true && false
false
false && true
false && false
true && true
true
MIT 12043, By S. Sabraz Nawaz

40. Truth table for the ||operator
Expression
Value of the Expression
true || false
true
false || true
false || false
false
true || true
MIT 12043, By S. Sabraz Nawaz

41. Truth table for the ! operator
Expression
Value of the Expression
!true
false
!false
true
MIT 12043, By S. Sabraz Nawaz

42. Example 06 – Improved Ex 05

44. Will be continue in next lecture…
MIT 12043, By S. Sabraz Nawaz