1unit 3

Flow of controlFlow of control

We will talk about:

• conditional statements if-then-else logical and conditional operators switch

• repetition statements while do for

basic programming

concepts

object oriented programming

topics in computer science

syllabus

2unit 3

Flow of ControlFlow of Control

Unless indicated otherwise, the order of statement execution through a method is linear: one after the other in the order they are written

Some programming statements modify that order, allowing us to:• decide whether or not to execute a particular

statement, or• perform a statement over and over repetitively

The order of statement execution is called the flow of control

3unit 3

Conditional StatementsConditional Statements

A conditional statement lets us choose which statement will be executed next; therefore they are sometimes called selection statements

Conditional statements give us the power to make basic decisions

Java's conditional statements are the if statement, the if-else statement, and the switch statement

4unit 3

The if StatementThe if Statement

The if statement has the following syntax:

if ( condition ) statement;

ifif is a Java is a Javareserved wordreserved word

The condition must be a The condition must be a boolean expressionboolean expression..It must evaluate to either true or falseIt must evaluate to either true or false..

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

5unit 3

Logic of an if statementLogic of an if statement

conditionevaluated

falsefalse

statement

truetrue

SimpleIf

6unit 3

Example - class TemperatureExample - class Temperature

class Temperature { static final int THRESHOLD = 27;

public static void main(String[] args) {System.out.println(“Enter temperature:”);int temperature = EasyInput.readInt();

System.out.println(“Current temperature “+ temperature); if (temperature > THRESHOLD) System.out.println(“It’s hot in here!”);

}}

7unit 3

ConditionsConditions via Boolean Expressions via Boolean Expressions

The condition of an if statement must evaluate to a true or false result

Booleas values (true/false) are obtained by relational operators:

In relational operators the operands are numbers, the result is boolean

Operator

==!=<<=>>=

Meaning

equal tonot equal to

less thanless than or equal to

greater thangreater than or equal to

8unit 3

Block StatementsBlock Statements

Several statements can be grouped together into a block statement

Blocks are delimited by braces

A block statement can be used wherever a statement is called for in the Java syntax

9unit 3

Example – class Temperature2Example – class Temperature2

class Temperature2 { static final int THRESHOLD = 27;

public static void main(String[] args) { System.out.println(“Enter temperature:”); int temperature = EasyInput.readInt(); System.out.println(“Current temperature “+ temperature); if (temperature > THRESHOLD) { System.out.println(“It’s hot in here!”); System.out.println(“But we’ll survive.”); }

}}

10unit 3

Block StatementsBlock Statements

Q: assembly languages do not have blocks; what do they do instead?

11unit 3

If .. Else StatementIf .. Else Statement

An else clause can be added to an if statement to make it an if-else statement:if (condition)

statement1;

else

statement2;

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

12unit 3

Logic of an if-else statementLogic of an if-else statement

conditionevaluated

statement1

truetrue falsefalse

statement2

13unit 3

Example – class Temperature3Example – class Temperature3

class Temperature3 { static final int FREEZING_POINT = 0;

public static void main(String[] args) { System.out.println(“Enter temperature:”); int temperature = EasyInput.readInt(); if (temperature <= FREEZING_POINT) System.out.println(“It’s freezing!”); else System.out.println(“Above freezing.”); }}

14unit 3

Example – class RightTriangleExample – class RightTriangle

// Receives the length of the edges of a triangle// and determine if this is a right triangleclass RightTriangle { public static void main(String[] args) { double a = EasyInput.readDouble(“Edge1:”); double b = EasyInput.readDouble(“Edge2:”); double c = EasyInput.readDouble(“Hypotenuse:”); boolean test = a*a+b*b == c*c; if (test) System.out.println(“It’s a right triangle”); else System.out.println( “It’s not a right triangle.”); }}

15unit 3

Example: handling invalid inputExample: handling invalid input

int numberOfItems = EasyInput.readInt(“Enter number of items:”); if (numberOfItems < 0) { System.out.println( “Number of items must be positive!”);}else { double price = numberOfItems * ITEM_PRICE; System.out.println(“The total price is:“ +price);}

16unit 3

How to specify the conditionHow to specify the condition

simple if command:

if ( condition ) statement1;

else statement2;

The condition is determined via logical operators

logical operators operate on boolean variables, rather than numbers

17unit 3

Logical OperatorsLogical Operators

There are three logical operators in Java:

They all take boolean operands and produce boolean results

Logical NOT is unary (one operand), but logical AND and OR are binary (two operands)

Operator

!&&||

Operation

Logical NOTLogical ANDLogical OR

18unit 3

Logical NOTLogical NOT

The logical NOT is also called logical negation or logical complement

If a is true, !a is false; if a is false, then !a is true

Logical expressions can be shown using truth tables

a

falsetrue

!a

truefalse

19unit 3

Logical ANDLogical AND

The expression a && b is true if both a and b are true, and false otherwise

Truth tables show all possible combinations of all terms

a

falsefalsetruetrue

b

falsetruefalsetrue

a && b

falsefalsefalsetrue

20unit 3

Logical ORLogical OR

The expression a || b is true if a or b or both are true, and false otherwise

a

falsefalsetruetrue

b

falsetruefalsetrue

a || b

falsetruetruetrue

21unit 3

A note about truth tablesA note about truth tables

Any truth table defines a logical function

a

falsefalsetruetrue

b

falsetruefalsetrue

f(a,b)

falsefalsetruetrue

22unit 3

Logical Operators: what forLogical Operators: what for??

Logical operators are used to form more complex logical expressions

if (a<1 || a%2!=0) {

System.out.println(

“The input should be a positive “+

“even number!”);

return;

}

Logical operators have precedence relationships between themselves and other operators

23unit 3

Logical OperatorsLogical Operators

Full expressions can be evaluated using truth tables (like any logical function):

a < 1

falsefalsetruetrue

a%2!=0

falsetruefalsetrue

a<1 || a%2!=0

falsetruetruetrue

24unit 3

statement

truetrue

condition2evaluated

Nested if statementsNested if statements

IfAnd

falsefalse

condition1evaluated

truetrue

25unit 3

Example – class CompareExampleExample – class CompareExample// Receives 2 integers and compare themclass CompareExample { public static void main(String[] args) {

System.out.println (“First number:”); int a = EasyInput.readInt(); System.out.println (“Second number:”); int b = EasyInput.readInt(); if (a != b) if (a > b) System.out.println(a+” is greater”); else System.out.println(b+” is greater”); else System.out.println(“the numbers are equal”); }}

26unit 3

A note about styleA note about style

The statements within an if or if-else statement should always be put in a block statement; the previous example was written in a poor style

Use of block statements eliminates the confusion that can arise in the previous example (think of a longer code and many if-else statements)

Use of blocks can also avoid a type of semantic mistakes

27unit 3

The switch StatementThe switch Statement

The switch statement provides another means to decide which statement to execute next

The switch statement evaluates an expression, then attempts to match the result to one of several possible cases

Each case contains a value and a list of statements

The flow of control transfers to statement list associated with the first value that matches

28unit 3

The switch StatementThe switch Statement

The general syntax of a switch statement is:

switch ( expression ){

case value1 : statement-list1

case value2 : statement-list2

case value3: statement-list3

default:...

}

switchswitchandandcasecasearearereservedreservedwordswords

If If expressionexpressionmatches matches value2value2,,

control jumpscontrol jumpsto hereto here

29unit 3

The switch StatementThe 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

Sometimes this can be helpful, but usually we only want to execute the statements associated with one case

30unit 3

The switch StatementThe switch Statement

A switch statement can have an optional default case

The default case has no associated value and simply uses the reserved word default

If the default case is present, control will transfer to it if no other case value matches

Though the default case can be positioned anywhere in the switch, it is usually placed at the end

If there is no default case, and no other value matches, control falls through to the statement after the switch

31unit 3

The switch StatementThe switch Statement

The expression of a switch statement must result in an integral data type, like an integer or character; it cannot be a floating point value

Note that the implicit boolean condition in a switch statement is equality - it tries to match the expression with a value

You cannot perform relational checks with a switch statement

32unit 3

The The switchswitch Statement Statement

// A client that enables you to connect to the

// bank server and make remote banking operations...

public class BankClient {

public static final int VIEW_BALANCE = 1;

public static final int VIEW_SAVINGS = 2;

public static final int CASH_TRANSFER = 3;

public static final int VIEW_LAST_OPERATIONS = 4;

// ...

33unit 3

The The switchswitch Statement Statement

// Inside the main loop of the client:int option = EasyInput.readInt(“Enter your choice:”);

switch(option) {case VIEW_BALANCE: showBalance(); break;case VIEW_SAVINGS: showSavings(); break;default: System.out.println(“No such option!”);}

34unit 3

simple if command:

if ( condition ) statement1;

else statement2;

The conditional operator is similar; but it is intended to select between different values, rather than between different actions

35unit 3

The Conditional OperatorThe Conditional Operator

Java has a conditional operator that evaluates a boolean condition, determining which of two expressions is evaluated:

condition ? expression1 : expression2

If the condition is true, expression1 is evaluated; if it is false, expression2 is evaluated

expression1 and expression2 must return a value! The result of the chosen expression is the result of

the entire conditional operator

36unit 3

The Conditional OperatorThe Conditional Operator

It is similar to an if-else statement, except that it is an expression that returns a value; for example:

int max = (a > b) ? a : b;

If a is greater than b, then a is assigned to max; otherwise, b is assigned to max

The conditional operator is ternary, meaning it requires three operands

37unit 3

The Conditional Operator: exampleThe Conditional Operator: example

System.out.println ("Your change is "

+ count +

((count == 1) ? "Dime" : "Dimes”));

• input count is 1, program will print: Your change is 1 Dime

• input count is 10, program will print: Your change is 10 Dimes

38unit 3

Flow of controlFlow of control

We will talk about:

• conditional statements if-then-else logical and conditional operators switch

• repetition statements while do for

basic programming

concepts

object oriented programming

topics in computer science

syllabus

39unit 3

Repetition StatementsRepetition Statements

Repetition statements allow us to execute a statement multiple times repetitively; they are often simply referred to as loops

Like conditional statements, they are controlled by boolean expressions

Java has three kinds of repetition statements: the while loop, the do loop, and the for loop; the programmer must choose the right kind of loop for the situation

40unit 3

The while StatementThe while Statement

The while statement has the following syntax:

while ( condition ) statement;

whilewhile is a is areserved wordreserved word

If the condition is true, the statement is executedIf the condition is true, the statement is executed..Then the condition is evaluated againThen the condition is evaluated again..

The statement is executed repetitively untilThe statement is executed repetitively untilthe condition becomes falsethe condition becomes false..

41unit 3

Logic of a while loopLogic of a while loop

statement

truetrue

conditionevaluated

falsefalse

FlowNestedIf

42unit 3

The while statementThe while statement

If the condition of a while statement is false initially, the statement is never executed; therefore, we say that a while statement executes zero or more times

As in the case of if, always use a block statement for the body of the while statement, even if it consists of only a single statement

43unit 3

Example – class CounterExample – class Counter

// Counts from 1 to 5class Counter { static final int LIMIT = 5;

public static void main(String[] args) { int count = 1; while (count <= LIMIT) { System.out.println(count); count = count + 1; } System.out.println(“done”); }}

44unit 3

Examples – class FactorsExamples – class Factors

// Gets an integer and prints its factors

public static void main(String[] args) {

int a = EasyInput.readInt(“Enter a number:”);

int i = 1;

System.out.println(“The divisors of “+a+” are:”);

while(i <= a) {

if (a%i == 0) {

System.out.println(i);

}

i = i + 1;

}

}

45unit 3

Example – class PowersOfTwoExample – class PowersOfTwo

// Prints the first ten powers of twoclass PowersOfTwo { static final int LIMIT = 10; public static void main(String[] args) { int power = 1; int i = 1; while (i <= LIMIT) { power = power * 2; System.out.println(power); i = i + 1; } }}

46unit 3

Infinite LoopsInfinite Loops

The body of a while loop must eventually make the condition false; if not, it is an infinite loop, which will execute until the user interrupts the program

This is a common type of logical error -- always double check that your loops will terminate normally

47unit 3

Example – class ForeverExample – class Forever

// This program contains an infinite loop

class Forever {

static final int LIMIT = 25;

public static void main(String[] args) {

int count = 1;

while (count <=<= LIMIT) {

System.out.println(count);

count = count - 1;

}

}

}

48unit 3

The do StatementThe do Statement

The do statement has the following syntax:do{

statement;}while ( condition )

Uses bothUses boththe the dodo and andwhilewhilereservedreservedwordswords

The statement is executed once initially, then the condition is evaluatedThe statement is executed once initially, then the condition is evaluated

The statement is repetitively executed until the condition becomes falseThe statement is repetitively executed until the condition becomes false

49unit 3

Logic of a do loopLogic of a do loop

truetrue

conditionevaluated

statement

falsefalse

50unit 3

The do StatementThe do Statement

A do loop is similar to a while loop, except that the condition is evaluated after the body of the loop is executed

Therefore the body of a do loop will execute at least one time

51unit 3

Comparing the while and do loopsComparing the while and do loops

statement

truetrue

conditionevaluated

falsefalse

while loopwhile loop

truetrue

conditionevaluated

statement

falsefalse

do loopdo loop

52unit 3

The for StatementThe for Statement

The for statement has the following syntax:

for ( initialization ; condition ; increment ) statement;

ReservedReservedwordword

The The initializationinitialization portion portionis executed onceis executed oncebefore the loop beginsbefore the loop begins

The statement isThe statement isexecuted until theexecuted until theconditioncondition becomes false becomes false

The The incrementincrement portion is executed at the end of each iteration portion is executed at the end of each iteration

53unit 3

The for StatementThe for Statement

is equivalent to the following while loop:

initialization;while ( condition ){

statement; increment;

}

for ( initialization ; condition ; increment ) statement;

54unit 3

Logic of a for loopLogic of a for loop

statement

truetrue

conditionevaluated

falsefalse

increment

initialization

55unit 3

initialization

Comparing the while and for loopsComparing the while and for loops

statement

truetrue

conditionevaluated

falsefalse

while loopwhile loop

statement

truetrue

conditionevaluated

falsefalse

for loopfor loop

increment

56unit 3

The for StatementThe for Statement

Like a while loop, the condition of a for statement is tested prior to executing the loop body

Therefore, the body of a for loop will execute zero or more times

It is well suited for executing a specific number of times that can be determined in advance

57unit 3

The The forfor Statement: examples Statement: examples

for (int count=1; count < 75; count++) {

System.out.println (count);

}

for (int num=5; num <= 10; num *= 2) {

sum += num;

System.out.println (sum);

}

58unit 3

The The forfor Statement Statement

Each expression in the header of a for loop is optional• If the initialization is left out, no initialization is

performed• If the condition is left out, it is always considered

to be true, and therefore makes an infinite loop• If the increment is left out, no increment operation

is performed

Regardless, both semi-colons are always requiredFlowIfElse

59unit 3

The The breakbreak and and continuecontinue statements statements

The break statement, which we used with switch statements, can also be used inside a loop: when the break statement is executed, control jumps to the statement after the loop (the condition is not evaluated again)

A similar construct, the continue statement, can also be executed in a loop: when the continue statement is executed, control jumps to the end of the loop and the condition is evaluated

60unit 3

Nested Loops - Multiplication TableNested Loops - Multiplication Table

int numRows = EasyInput.readInt(“Enter number of rows”);int numColumns = EasyInput.readInt(“Enter number of columns”);

for (int row = 1; row <= numRows; row++) {

for (int column = 1; column <= numColumns; column++) { System.out.print(row * column + “ “); }

System.out.println();}

61unit 3

Nested LoopsNested Loops

public class Mystery {

public static void main(String[] args) {int dimension =

EasyInput.readInt(“Enter dimension”);

for (int j = 0; j < dimension; j++) { for (int k = 1; k < dimension - j; k++) { System.out.print(" "); } for (int k = 0; k < j; k++) { System.out.print("*"); } System.out.println(); } } }

62unit 3

Why do We Need IndentationWhy do We Need Indentation??

public class Mystery {

public static void main(String[] args) {InputRequestor in = new InputRequestor();int dimension = EasyInput.readInt(“Enter dimension”);

for (int j = 0; j < dimension; j++) {for (int k = 1; k < dimension - j; k++) {System.out.print(" ");} for (int k = 0; k < j; k++) {System.out.print("*");} System.out.println();}} }

63unit 3

What you should be able to do nowWhat you should be able to do now......

write a program which can: decide online which piece of code to follow repeat a piece of code a number of times

exercise: compute the divisors of a number (class Factor) in 3 different ways