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Tirgul no. 2Tirgul no. 2

Topics covered:

Reading Input from the user. Printing Output to the user. if - else statement and switch. Boolean Operators. Checking your input. Logical Operators.

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Reading Input from the userReading Input from the user We leave the discussion of Input for later chapters For now we will supply you with classes for getting input in

all exercises and examples. The class called EasyInput allows you to read input from the

user.

In order to read an integer/double we use EasyInput as follows:

int numberOfItems = EasyInput.readInt(); double width = EasyInput.readDouble();

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Reading Input (contd.)Reading Input (contd.)

Please note that EasyInput is not a core Java class. We have supplied it for simplicity.

The EasyInput api is supplied on the course site.

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Example - Multiplying two numbersExample - Multiplying two numbers// Requests two integers and prints their// multiplication.class MultiplicationExample { public static void main(String[] args) { int a,b; int mul; System.out.print(“The first number: “); a = EasyInput.readInt(); System.out.print(“The second number: “); b = EasyInput.readInt(); mul = a * b; System.out.println(“The multiplication is”

+ mul); }}

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Example: Circle area and Example: Circle area and circumferencecircumference

// Reads the radius of a circle and prints// its circumference and area to an output windowclass CircleExample { static final double PI = 3.1415927; public static void main(String[] args) { double r, circumference, area;

System.out.print(“Enter radius: “); r = EasyInput.readDouble(); circumference = 2*PI*r; area = PI*r*r; System.out.print(“Circumference: “); System.out.println(circumference); System.out.print(“Area: “);

System.out.println(area); }}

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The if StatementThe if Statement

The Java if statement has the following syntax:

if (boolean-condition)

statement;

If the Boolean condition is true, the statement is executed; if it is false, the statement is skipped

This provides basic decision making capabilities

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TemperatureTemperature

class Temperature {

static final int THRESHOLD = 65;

public static void main(String[] args) {

System.out.print(“Enter the temperature:”);

int temperature = EasyInput.readInt();

System.out.println(“Current temperature “+

temperature);

if (temperature < THRESHOLD)

System.out.println(“It’s cold in here!”);

}

}

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Boolean ExpressionsBoolean Expressions

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

Java has several equality and relational operators:

Operator

==!=<<=>>=

Meaning

equal tonot equal to

less thanless than or equal to

greater thangreater than or equal to

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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 See Temperature2.java

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Example - Temperature2Example - Temperature2class Temperature2 { static final int THRESHOLD = 65;

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

}}

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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

See Temperature3.java.

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Example - Temperature3Example - Temperature3class Temperature3 {

static final int FREEZING_POINT = 32;

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

if (temperature <= FREEZING_POINT)

System.out.println(“It’s freezing!”);

else

System.out.println(“Above freezing.”);

}

}

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Checking your InputChecking your Input When requesting input from the user, you keep in

mind that the input may be invalid. One way to handle this is to use if – else control statement:

//number of items should be positive

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

System.out.println(price);}

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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

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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

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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

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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

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Logical OperatorsLogical Operators

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

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Logical OperatorsLogical Operators

Full expressions can be evaluated using truth tables

a < 1

falsefalsetruetrue

a%2!=0

falsetruefalsetrue

a<1 || a%2!=0

falsetruetruetrue

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Nested IfNested If// Receives 2 integers and compares themclass CompareExample { public static void main(String[] args) { System.out.print(“First number: ); int a = EasyInput.readInt(); System.out.print(“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”); }}

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Switch ConstructSwitch ConstructSwitch( integral expression) { case integral-value1: statements; break; case integral-value2: statements; break; : : case integral-valueN: statements; break; default: statements; break;}

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Switch Construct (cont.)Switch Construct (cont.)Switch( integral expression) { case integral-value1: case integral-value2: statements; break; : : case integral-valueN: statements; break; default: statements; break;}

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public class Schedule { public final int SUNDAY = 1; public final int MONDAY = 2; public final int TUESDAY = 3; public void printSchedule(int day) { System.out.println(“Schedule for today is:”); switch(day) { case SUNDAY: System.out.println(“10:00 - 12:00 : Infi”); System.out.println(“12:00 - 14:00 : Alg”); break; case MONDAY: case TUESDAY: System.out.println(“10:00 - 13:00 discrete math”); break; default: System.out.println(“Error: No such day”); break; } }}