Topic 03Control Statements

Programming II/ACMC2522 / CIM2561

Bavy Li

Background Our problem-solving solutions so far have the

straight-line property

public class DisplayForecast // main(): application entry point

public static void main(String[] args) { System.out.print("I think there is a

world"); System.out.print(" market for maybe five

"); System.out.println("computers. “); System.out.print(" Thomas Watson, IBM, “); System.out.println("1943.“);

}}

Background For general problem solving we need more capabilities

The ability to control which statements are executed The ability to control how often a statement is executed

Java provides the if and switch conditional constructs to control whether a statement list is executed

Java provides the while and for iteration constructs to control whether a statement list is executed

Selection Statements Using if and if...else

Single-selection structure Double-selection structure

Nested if Statements Multiple-selection structure

Using switch Statements

Conditional Operator

Basic if statement Syntax

if (Expression) { Action;

}

If the Expression is true then execute Action

Action is either a single statement or a group of statements (in block) within braces

Expression

Action

true false

Example – Basic if statement

if (value < 0) { value = -value;}

Value < 0

Value = -Value

true false

Is our number negative?

If Value is not less than zero then our number is fine as is

If Value is less than zero then we need to

update its value to that of its additive inverse

Our number is now definitely

nonnegative

Self-Test QuestionSystem.out.print("Enter an integer number: ");int value1 = Integer.parseInt(stdin.readLine());System.out.print("Enter another integer number: ");int value2 = Integer.parseInt(stdin.readLine());

// rearrange numbers if necessaryif (value2 < value1) {

// values are not in sorted orderint rememberValue1 = value1;value1 = value2;value2 = rememberValue1;

}

// display valuesSystem.out.println("The numbers in sorted order are "

+ value1 + " and then " + value2);

What happens if the user enters 11 and 28?

What happens if the user enters 11 and 4?

The if-else statement Syntax

if (Expression) { Action1

}else {

Action2

}

If Expression is true then executeAction1 otherwise execute Action2

Expression

Action1 Action2

true false

Example – Finding maximum values

System.out.print("Enter an integer number: ");int value1 = Integer.parseInt(stdin.readLine());System.out.print("Enter another integer number: ");int value2 = Integer.parseInt(stdin.readLine());

int maximum;if (value1 < value2) { // is value2 larger? maximum = value2; // yes: value2 is larger}else { // (value1 >= value2) maximum = value1; // no: value2 is not larger}System.out.println("The maximum of " + value1

+ " and " + value2 + " is " + maximum);

Example – Finding maximum values

value1 < value2

maximum = value2 maximum = value1

true false

Is value2 larger than value1

Yes, it is . So value2 is larger than value1. In this case, maximum is set to

value2No, its not. So value1 is at least as large as value2. In this case, maximum is set

to value1

Either case, maximum is set correctly

Multiple Alternative if Statements

if (score >= 90) grade = ‘A’;else if (score >= 80) grade = ‘B’; else if (score >= 70) grade = ‘C’; else if (score >= 60) grade = ‘D’; else grade = ‘F’;

if (score >= 90) grade = ‘A’;else if (score >= 80) grade = ‘B’;else if (score >= 70) grade = ‘C’;else if (score >= 60) grade = ‘D’;else grade = ‘F’;

switch statement

switch ( SwitchExpression ) {

case CaseExpression1 :Action1 ;

case CaseExpression2 :Action2 ;

...

case CaseExpressionn :Actionn;

default :Actionn+1 ;

}

Constantintegral

expressionJ avastatements

Integral expression tobe matched with acase expression

Example – Checking vowels

switch (ch) {case 'a': case 'A':case 'e': case 'E':case 'i': case 'I':case 'o': case 'O':case 'u': case 'U':

System.out.println("vowel“);break;

default: System.out.println("not a vowel“);

}

The break causes an exiting of the switch

Handles all of the other cases

Example – Simple calculations

System.out.print("Enter a number: ");int n1 = Integer.parseInt(stdin.readLine());

System.out.print("Enter another number: ");int n2 = Integer.parseInt(stdin.readLine());

System.out.print("Enter desired operator: ");char operator = stdin.readLine().charAt(0);

switch (operator) {case '+' : System.out.println(n1 + n2); break;case '-' : System.out.println(n1 - n2); break;case '*' : System.out.println(n1 * n2); break;case '/' : System.out.println(n1 / n2); break;default: System.out.println(“Illegal request“);

}

switch Statement Rules

The switch-expression must yield a value of char, byte, short, or int type and must always be enclosed in parentheses

The value1, ..., and valueN must have the same data type as the value of the switch-expression

The case statements are executed in sequential order

The keyword break is optional to use at the end of each case in order to terminate from the switch statement. But if the break statement is not present, the next case statement will be executed

Conditional Operator

(Boolean Expression) ? exp1 : exp2

Examples – Conditional Operator

if (x > 0) y = 1else y = -1;

is equivalent to

y = (x > 0) ? 1 : -1;

if (num % 2 == 0) System.out.println(num + “is

even”);else System.out.println(num + “is odd”);

is equivalent to

System.out.println( (num % 2 == 0)? num + “is even” : num + “is odd”);

Repetition Statements Looping: while, do-while and for

DO it if and only if condition is true DO it at least one before testing the condition DO it repeatedly until the counter is over

Nested loops

Using break and continue

while Loop Flow Chart

Expression

Action

true f alse

Expression isevaluated at the

start of eachiteration of the

loop

If Expression istrue, Action is

executed If Expression isfalse, program

executioncontinues withnext statement

Example – while Loop Flow Chart

int i = 0;

while (i < 100) { System.out.println( "Welcome to Java!"); i++;}

false

true

System.out.println("Welcoem to Java!"); i++;

Next Statement

(i < 100)

i = 0;

The do-while statement Syntax

do { Action

} while (Expression)

Semantics Execute Action

If Expression is true then execute Action again

Repeat this process until Expression evaluates to false

Action

true

false

Expression

The for Statement

Logical test expression that determines whether the action and update step areexecuted

for ( ForInit ; ForExpression ; ForUpdate ) Action

Update step is performed afterthe execution of the loop body

Initialization step prepares for thefirst evaluation of the test

expression

The body of the loop iterates wheneverthe test expression evaluates to true

The for Statement

int currentTerm = 1;

for (int i = 0; i < 5; ++i) {System.out.println(currentTerm);currentTerm *= 2;

}

The for Statement

int currentTerm = 1;

for (int i = 0; i < 5; ++i) {System.out.println(currentTerm);currentTerm *= 2;

}

Initialization stepis performed onlyonce -- just prior

to the firstevaluation of thetest expression

The for Statement

int currentTerm = 1;

for (int i = 0; i < 5; ++i) {System.out.println(currentTerm);currentTerm *= 2;

}

The body of the loop iterateswhile the test expression istrueInitialization step

is performed onlyonce -- just prior

to the firstevaluation of thetest expression

The for Statement

int currentTerm = 1;

for (int i = 0; i < 5; ++i) {System.out.println(currentTerm);currentTerm *= 2;

}

The body of the loop iterateswhile the test expression istrueInitialization step

is performed onlyonce -- just prior

to the firstevaluation of thetest expression

The body of the loop displays thecurrent term in the number series.It then determines what is to be thenew current number in the series

The for Statement

int currentTerm = 1;

for (int i = 0; i < 5; ++i) {System.out.println(currentTerm);currentTerm *= 2;

}

After each iteration of thebody of the loop, the updateexpression is reevaluated

The body of the loop iterateswhile the test expression istrueInitialization step

is performed onlyonce -- just prior

to the firstevaluation of thetest expression

The body of the loop displays thecurrent term in the number series.It then determines what is to be thenew current number in the series

For Expr

Act i on

true false

For I ni t

Post Expr

Evaluated onceat the beginning

of the forstatements's

executionThe ForExpr is

evaluated at thestart of each

iteration of theloop

If ForExpr is true,Action isexecuted

After the Actionhas completed,

thePostExpression

is evaluated

If ForExpr isfalse, program

executioncontinues withnext statement

After evaluating thePostExpression, the next

iteration of the loop starts

Nested loops

int m = 2;int n = 3;for (int i = 0; i < n; ++i) {

System.out.println("i is " + i);for (int j = 0; j < m; ++j) {

System.out.println(" j is " + j);}

}

Nested loops

int m = 2;int n = 3;for (int i = 0; i < n; ++i) {

System.out.println("i is " + i);for (int j = 0; j < m; ++j) {

System.out.println(" j is " + j);}

}i is 0 j is 0 j is 1i is 1 j is 0 j is 1i is 2 j is 0 j is 1

The break Keyword

The continue Keyword

break Statement vs. continue Statement

The break Statement It causes an immediate exit from the structures

of (while, for, do-while), then execution continues with the next statement

The continue Statement It skips the remaining statement in the body of

the structures of (while, for, do-while), and proceeds with the next iteration of the loop

Using break and continue

Discussion on: Example 3.5 : TestBreak.java

Testing a break Statement

Example 3.6 : TestContinue.java Testing a continue Statement

Summary

Control Statements

Selection Statements if, if…else, nested if switch conditional operator

Repetition Statements while, do…while, for nested loops break and continue