selection control structures

Control Structures 1:

Selection

Chapter Goals

Be able to use the selection control structure

Be able to solve problems involving repetition.

Understand the difference among various selection & loop structures.

Know the principles used to design effective selection & loops (next topic).

Improve algorithm design skills.

3 Types Flow of Control

Sequential (we had learn in previous topic)The statements in a program are

executed in sequential order Selection

allow the program to select one of multiple paths of execution.

The path is selected based on some conditional criteria (boolean expression)

Repetition (we will learn in next topic)

Flow of Control: Sequential Structures

statement1

statement2

statement3

If the boolean expression evaluates to true, the statement will be executed. Otherwise, it will be skipped.

Flow of Control: Selection Structures

There are 3 types of Java selection structures:

if statementif-else statementswitch statement

Flow of Control: Selection Structures

The if Statement The if statement has the following syntax:

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if ( condition ) statement;

if is a Javareserved word

The condition must be aboolean expression. It mustevaluate to either true or false.

If the condition is true, the statement is executed.If it is false, the statement is skipped.

Logic of an if statement

conditionevaluated

statement

truefalse

if Statement

if (amount <= balance) balance = balance - amount;

Boolean Expressions A condition often uses one of Java's

equality operators or relational operators, which all return boolean results:

== equal to!= not equal to< less than> greater than<= less than or equal to>= greater than or equal to

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The if Statement

if (total > MAX) charge = total * MAX_RATE;System.out.println ("The charge is " + charge);

First the condition is evaluated -- the value of total is either greater than the value of MAX

If the condition is true, the assignment statement is executed -- if it isn’t, it is skipped.

Either way, the call to println is executed next

Java code example

class Count

{

public static void main (String args[])

{

double y=15.0;

double x=25.0;

if (y!=x)

System.out.println("Result : y not equal x");

}

}

OutputResult : y not equal x

Block Statements Several statements can be grouped

together into a block statement delimited by braces

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if (total > MAX){ System.out.println ("Error!!"); errorCount++;}

Block Statementif (amount <= balance){

balance = balance - amount; System.out.println(“Acct new balance = “ + balance);

}

COMPARE WITH

if (amount <= balance) balance = balance - amount; System.out.println(“Acct new balance = “ + balance);

Logical Operators

Expressions that use logical operators can form complex conditions

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if ((income > MIN_LEVEL ) && (age <50)) System.out.println (“Can Apply Loan");

All logical operators have lower precedence than the relational operators

Logical NOT has higher precedence than logical AND and logical OR

Logical (Boolean) Operation in Java

Precedence of Operators

Logical Operators

if ((amount <= 1000.0) && (amount <= balance)){

balance = balance - amount; System.out.println(“Acct new balance = “ + balance);

}

EXAMPLE:New withdrawal condition:Withdrawal amount of more than RM1000.00 is not allowed.

The if-else Statement (2 way selection) An else clause can be added to an if

statement to make an if-else statement

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if ( condition ) statement1;else statement2;

If the condition is true, statement1 is executed; if the condition is false, statement2 is executed

One or the other will be executed, but not both

Logic of an if-else statement

conditionevaluated

statement1

true false

statement2

if/else Statement

if/else Statementif (amount <= balance)

balance = balance - amount;else

balance = balance - OVERDRAFT_PENALTY;

Purpose:

To execute a statement when a condition is true or false

Block Statement

if (amount <= balance){

balance = balance - amount; System.out.println(“Acct new balance = “ + balance);

}else{ balance = balance - OVERDRAFT_PENALTY;

System.out.println(“TRANSACTION NOT ALLOWED”);}

Combine with Boolean operators

if ((age >= 25) && (age <= 50)){

System.out.println(“You are qualified to apply”);}else { System.out.println(“You are NOT qualified to apply”); }

EXAMPLE:Loan Processing. Can apply if age is between 25 to 50.

Multiple Selection (nested if)

Syntax:if (expression1)

statement1

else

if (expression2)

statement2

else

statement3

Java code (multiple selection)if (a>=1){

System.out.println ("The number you enter is :" + a); System.out.println ("You enter the positive number");

}else if (a<0){

System.out.println ("The number you enter is :" + a); System.out.println ("You enter the negative number");

}else {

System.out.println ("The number you enter is :" + a); System.out.println ("You enter the zero number");

}

OutputEnter the number : 15The number you enter is :15You enter the positive number Enter the number : -15The number you enter is :-15You enter the negative number Enter the number : 0The number you enter is :0You enter the zero number  

Multiple SelectionsExample The grading scheme for a course is given as below:

Mark Grade

90 - 100 A

80 – 89 B

70 – 79 C

60 – 69 D

0 - 59 F

Read a mark & determine the grade.

Multiple Selectionsif (mark >= 90)

grade = ‘A ’;

else if (mark >= 80)

grade = ‘B’;

else if (mark >= 70)

grade = ‘C’;

else if (mark >= 60)

grade = ‘D ’;

else

grade = ‘F ’;

Equivalent code with series of if statements

if ((mark >= 90) && (mark <=100))

grade = ‘A ’;

if ((mark >= 80) && (mark >= 89))

grade = ‘B’;

if ((mark >= 70) && (mark >= 79))

grade = ‘C’;

if ((mark >= 60) && (mark >= 69))

grade = ‘D ’;

if ((mark >= 0) && (mark >= 59))

grade = ‘F ’;

switch Structures (multiple selection)

switch (expression){case value1: statements1

break;case value2: statements2

break; ...case valuen: statementsn

break;default: statements}

Expression is also known as selector.

Value can only be integral.

If expressionmatches value2,control jumpsto here

switch Structures

The switch Statement

Often a break statement is used as the last statement in each case's statement list

A break statement causes control to transfer to the end of the switch statement

If a break statement is not used, the flow of control will continue into the next case

Control flow of switch statement with and without the break statements

Switch/Break Examples

int m = 2;switch (m){ case 1 : System.out.println(“m=1”); break; case 2 : System.out.println(“m=2”); break; case 3 : System.out.println(“m=3”); break; default: System.out.println(“default”);}

int m = 2;switch (m){ case 1 : System.out.println(“m=1”); break; case 2 : System.out.println(“m=2”); break; case 3 : System.out.println(“m=3”); break; default: System.out.println(“default”);}

Output: m=2

char ch = ‘b’;switch (ch){ case ‘a’ : System.out.println(“ch=a”);

case ‘b’ : System.out.println(“ch=b”);

case ‘c’ : System.out.println(“ch=c”);

default: System.out.println(“default”);}

char ch = ‘b’;switch (ch){ case ‘a’ : System.out.println(“ch=a”);

case ‘b’ : System.out.println(“ch=b”);

case ‘c’ : System.out.println(“ch=c”);

default: System.out.println(“default”);}

Output: ch=b ch=c default