Decision Structures

transcript

Decision Structures

Lecture 3MIT 12043, Fundamentals of Programming

By: S. Sabraz Nawaz

Chapter TopicsoRelational Operatorso The if Statemento The if-else StatementoNested if statementso The if-else-if 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, 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

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

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 of

executes or not.

if (boolean expression is true)execute this statement;

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.

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.");

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′;

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.

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

if-else Statements• The if-else statement adds the ability to

conditionally execute code when the if condition is false.

if (expression)statementOrBlockIfTrue;

elsestatementOrBlockIfFalse;

• It works the same way as the if statement except that, when the condition is false, the statement within the else clause executes

if-else Statement Flowcharts

Wear a coat.

YesIs it cold

outside?

Wear shorts.

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

BS, otherwise 3,000 of the BSo 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

Solution…

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

Alignment and Nested if Statements

if (expression)

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.30,0002. The customer must have held his or her current job for

at least 02 years

Example 04…

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

if-else-if Syntaxif (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…Operato

rMeaning 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 Meaningx > 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

Will be continue in next lecture…

Decision Structures

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