Lecture 3 MIT 12043, Fundamentals of Programming … · Boolean Expressions int a = 3; b = 4; a ==...

Decision Structures

Lecture 3

MIT 12043, Fundamentals of Programming

By: S. Sabraz Nawaz

3-2

Chapter Topics

o Relational Operators

o The if Statement

o The if-else Statement

o The if-else-if Statement

o Nested if statements

o The switch Statement

MIT 12043, By S. Sabraz Nawaz

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 3

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


Sequence Structure


Decision Structure

• Programs often need more than one path of execution

• 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


Decision Structure…

• Simple decision structure logic


Decision Structure…

• Three-action decision structure logic


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


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The if Statement

• The if statement is used to create a decision structure, which

allows a program to have more than one path of execution

o The if statement causes one or more statements to execute only when a boolean

expression is true.

• The if statement decides whether a section executes or not

if (boolean expression is true)

execute this statement;


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The if Statement…

• A block if statement may be modeled as:

if (coldOutside)

{

wearCoat;

wearHat;

wearGloves;

}

Note the use of curly

braces to block several

statements together.


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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.");

}


3-13

Programming Style and if Statements

• An if statement can span more than one line; however, it is still

one statement.

if (average > 85)

grade = ′A′;

is functionally equivalent to

if(average > 95) grade = ′A′;


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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.


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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.


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


Example 01…


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


• The program should display a message with the Name of the

person and his or eligibility to vote


Example 02


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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


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if-else Statement Flowcharts

Wear a coat.

YesIs it cold

outside?

Wear t-Shirt

No


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


• The program should display a message with the Name of the

person and his or eligibility to vote


Example 03…


Exercise

• A company’s payment structure to its employees is as

followso If the Basic salary exceeds 30,000/=, the Dearness Allowance 40% the BS,

otherwise 50% of the BS

o If the BS exceeds 25,000/=, the House Rent Allowance is 20% of the BS,

otherwise 3,000 of the BS

o 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


Solution

25

Solution…


3-27

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


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Alignment and Nested if Statements

if (expression)

{

if (expression)

{

statement;

}

else

{

statement;

}

}

else

{

statement;

}

This if and else

go together.This if and

else

go together.


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:

1. The customer’s salary must be at least Rs.30,000

2. The customer must have held his or her current job for at

least 02 years


Example 04…


if-else-if Statements

• 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


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if-else-if Syntax

if (expression_1)

{

statement;

statement;

etc.

}

else if (expression_2)

{

statement;

statement;

etc.

}

Insert as many else if clauses as necessary

else

{

statement;

statement;

etc.

}

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.


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


Logical Operators


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 ! Operatoro reverses the truth of a boolean expression



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

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?


Logical Operators…

Truth table for the && operator

Expression Value of the Expression

true && false false

false && true false

false && false false

true && true true


Truth table for the ||operator


true || false true

false || true true

false || false false

true || true true


Truth table for the ! operator


!true false

!false true


Example 06 – Improved Ex 05

42

The % Operator

• The above modulus operator returns the remainder

• Eg.o 3 % 2 1 (remainder is one)

o 15 % 4 3 (remainder is three)

• Odd or Even number finder


Comparing String Objects

• You cannot use relational operators to compare String objects

• To compare, you should use the String class’s equals method.

• The general form of the method is as follows:

StringReference1.equals(StringReference2)

o StringReference1 is a variable that references a String object, and StringReference2 is another variable that references a String object

• The method returns true if the two strings are equal, or false if they are not equal. Here is an example:

if (name1.equals(name2))

• The expression in the if statement will return true if they are the same, or false if they are not the same


Example 07

• Note: the equal method is case sensitive; if you want to ignore the case

case, use string. equalsIgnoreCase() method


Classroom Exercise

A car rental company has three types of cars; Premio, Alien, andCivic. The rental scheme is as follows.

Write a Java program to input Vehicle Type, Number of DaysRented, and Kilometres run to calculate the Total Rent Payableand display on the screen.


Type Premio Alien Civic

Fixed Rent per day 1,500/- 2,000/- 2,500/-

Rate per KM 20/- 30/- 40/-

Free Allowance per day 80/- 60/- 50/-

• For decisions involving many possible paths and values we use the switch statement

• The switch statement can have a number of possible execution paths

• At the end of each sequence we insert a breakstatement that means ‘go to the end of the switch’

• A switch works with the byte, short, char, andint primitive data types. It works with the String class as well.

The switch Statement

48

The switch Statement

switch (variable)

{

case value1:

statement;

statement;

break;

case value2:

statement;

statement;

break;

...

...

default:

statement;

break;

}

Don’t forget breaks!

switch

case

default

break

Reserved words

49

• You can have any number of case statements within a switch.

Each case is followed by the value to be compared to and a

colon

• The value for a case must be the same data type as the variable

in the switch, and it must be a constant or a literal (character)

• When the variable being switched on is equal to a case, the

statements following that case will execute until a break

statement is reached

Rules apply to a switch statement

50

• When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement

• Not every case needs to contain a break. If no break appears, the flow of control will fall through to subsequent cases until a break is reached

• A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case

Rules apply to a switch statement

51

• The same case can have two or more labels. For

example:

switch (num)

{

case 1:

case 2:System.out.println (“Stand up and sing a song");

break;

case 3:

...

}

switch - Example

52

53

Month Finder

54

Switch with Multiple case labels

Next Chapter: Looping


Lecture 3 MIT 12043, Fundamentals of Programming … · Boolean Expressions int a = 3; b = 4; a ==...

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