king fahd university of petroleum & minerals college of computer science & engineering...

King Fahd University of Petroleum & MineralsCollege of Computer Science & Engineering

Information & Computer Science Department

Lecture 1 : Introduction

ICS102 - Introduction to computing

Components of a Personal Computer

Questions :

- what are the input devices ?

- what are the output devices ?

http://upload.wikimedia.org/wikipedia/commons/9/9d/Acer_E360_Socket_939_motherboard_by_Foxconn.svg

A motherboard, provides the electrical connections by which the other components of the system communicate and hosts the central processing unit as well as other subsystems and devices.

Motherboard

Motherboard example : Acer E360

http://upload.wikimedia.org/wikipedia/en/3/39/Java_logo.svg

CPU (Central Processing Unit)

The Central Processing Unit (CPU) or processor is the portion of a computer system that carries out the instructions of a computer program and is the primary element carrying out the computer's functions.

In an addition operation, the arithmetic logic unit (ALU) will be connected to a set of inputs and a set of outputs. The inputs provide the numbers to be added, and the outputs will contain the final sum.

Example: (3 + 2) = 5

3

2

5

CPU memory

Registers

Cache

Main memory (RAM):

Data has to be in main memory so that CPU can access it

Volatile: lost when program exits; computer shuts off

Hard Disk, CD, etc.

Persistent

This is where you keep the data for long-term storage

Memory

Memory refers to computer components, devices, and recording media that hold digital data used for computing for some interval of time.

There are mainly three types of memory :

Main Memory …

10021003

1004

1005

CellAddress

Memory is divided intoMany memory locations (cells)

Each memory cell has a numeric address, which uniquely identifies it

Each cell contains a datavalue, e.g. 22

Main Memory …

The word “Hello.” stored in 6 memory cells

Main Memory …

Memory units : Bits and Bytes

Bit -- most basic unit of memory

1 or 0, on or off

1 Byte = 8 bits

In a computer, data values are stored as a sequence of bits

2

7

1004

1005

00000010

00000111

1004

1005

Program / CPU / Memory Interaction

Example: Input read a number from keyboard Add 1 to it Output it on screen

KeyboardRAM

11 CPU

RAM

2 Monitor1 2 2

What is programming?

What is a program?

A (software) program is a list of instructions intended to a computer

The list must be ordered correctly

A program has inputs and outputs

Each instruction tells the computer to do something (an action, a calculation, a comparison)

Program Execution

A program tells the CPU how to manipulate and/or move information

Programming is about processing information

Take some input, manipulate it in some way, and produce a particular output

ManipulationInputs Outputs

Program

Example 1 : Recipe for Scrambled Eggs

Ingredients (Inputs) : two eggs, tablespoon of oil, salt

Instructions (program):

Add oil to pan

Heat pan on stove

Crack eggs into pan

Add salt

Mix until light and flakey

Output: scrambled eggs

Example 2 : Currency Exchange

Input: Amount Source Currency Desired Currency

Instructions Look up in table current exchange rate for the selected

currencies Calculate result as amount * exchange rate

• Output: result

Task : convert an amount of money in some currency (e.g. US Dollars) to another one (e.g. Saudi Riyals).

Programming language

A programming language is the language used to write programs

A programming language employs a set of rules that dictate how the words and symbols can be put together to form valid program statements

A programming language has a syntax and semantics

There are several types of languages (functional, Object-Oriented, etc.)

In this course we focus on Java programming language.

Java

• Java is a programming language originally developed by James Gosling at Sun Microsystems

• It was first released in 1995.

• The language derives much of its syntax from C and C++.

• But has a simpler object model and fewer low-level facilities than C and C++.

http://en.wikipedia.org/wiki/File:James_Gosling_2005.jpg

Why Java ?

Currently, Java is the most popular language in the world !

Why Java ?

1. Simple

• Get started quickly

2. Concise

• Write less code

3. Object-oriented

• Better quality code

4. Portable

• Architecture neutral (write once run anywhere)

5. Secure

• More appropriate for Internet

Portability of Java

Classical model:

Java model:

Portability of Java

Write once, run anywhere: Because applications written in the Java programming language are compiled into machine-independent bytecodes, they run consistently on any Java platform.

A Java Program

A Java program consists of one or more classes

A Java class consists of one or more methods

A Java method consists of one or more statements

A Java Program

Javaclasses

JavaMethods

A Java Program

A Java program resides in one or more files.

The file name of a Java program has extension .java.

One of the classes of a Java program is called the driver class.

The name of the driver class must be the same as the name of its Java file. (Java is case sensitive. So EX1 is different from ex1.)

The diver class must contain a method called main. The execution of Java program starts from the main method of the driver class.

Example of a Java Program


Class nameMain method

Instruction

Class body


Also notice:

Curly braces } {


Also notice:

Parentheses ( )

Curly braces } {


Also notice:

Parentheses ( )

Curly braces } { Square brackets] [


Also notice:

A pair of braces } { define a block

Compiling and running a program

Type in your program Save the program

Store all your files in one directory for now Give the program the same name as the class

Compile the program this produces a .class file Translates the program into something the computer can

understand and execute (Java bytecode) Run the program Observe the result and adjust the program if necessary

Edit

Text Editor

Public class/*

Adventure.java

Public class Adven/* This program is an Arit*/

public static void main /* Program statements g System.out.print(“Wel }}

Compile - With Errors

Adventure.java


public static void main /* Program statements g System.print(“Welcome }}

Compiler

Errors and Warnings-------------------Error : The method print(String) is undefined forType System

Compile - Success

Adventure.java


public static void main /* Program statements g System.out.print(“Wel }}

Compiler

Adventure.class

00101101000101110110110100010111011101010001011101110101101000101110111010110100010111011101011010001011101110101101000101110111000101110111010110100010111011100101110111010110100010111011100101110111010110100010111011100101110111010110100010111011100101110111010110100010111011100101110

Run Program

Welcome to the ArithmThe date is Monday SepWhat is your name?FredWell Fred, after a dayThe cube appears to beYou find a Green door,The door closes behindThere is a feel of mat

Adventure

Fred

1

ICS102

Lecture 1 : Expressions and Assignment



April 19, 2023 ICS102: Expressions & Assignment 38

Outline

Declaration Statements Primitive Data Types Identifiers Assignment Statements Arithmetic Operators and Expressions Precedence Rules Round-Off Errors in Floating-Point Numbers Integer and Floating-Point Division The % Operator Type Casting Increment and Decrement Operators


Variable

In mathematics:

x + y = 17

In Computer Science: a variable is an identifier (usually a letter, word, or phrase) that is linked to a value stored in the computer's memory

x = 5;

variable

variable

variable

Variable Declaration

Every variable in a Java program must be declared before it is used

int numberOfBeans;numberOfBeans = 10;

A variable declaration tells the compiler what kind of data (type) will be stored in the variable

Basic types in Java are called primitive types


- Primitive Data Types


Names of variables (Identifiers)

int variable = 3;

The name of a variable must not start with a digit, and all the characters must be letters, digits, or the underscore symbol

Java is a case-sensitive language: Rate, rate, and RATE are the names of three different variables

It is not allowed to use reserved words as variable names:

public class void static


Assignment Statements

In Java, the assignment statement is used to change the value of a variable

An assignment statement consists of a variable on the left side of the operator, and an expression on the right side of the operator

Variable = Expression;

Example

temperature = 98.6; count = numberOfBeans;


Assignment Statements With Primitive Types

When an assignment statement is executed, the expression is first evaluated, and then the variable on the left-hand side of the equal sign is set equal to the value of the expression

distance = rate * time;

Note that a variable can occur on both sides of the assignment operator

count = count + 2;

The assignment operator is automatically executed from right-to-left, so assignment statements can be chained

number2 = number1 = 3;


Initialize Variables …

A variable that has been declared but that has not yet been given a value by some means is said to be uninitialized

In certain cases an uninitialized variable is given a default value

It is best not to rely on this and always initialize your variables

int initialCount = 50; int finalCount;

What is the value of initialCount?

What is the value of finalCount?


Shorthand Assignment Statements …

e.g. variable += 3;

Shorthand assignment notation combines the assignment operator (=) and an arithmetic operator

The general form is

Variable Op = Expression

Example: Equivalent To:

count += 2; count = count + 2;

sum -= discount; sum = sum – discount;

bonus *= 2; bonus = bonus * 2;

time /= rushFactor; time = time / rushFactor;

change %= 100; change = change % 100;

amount *= count1 + count2; amount = amount * (count1 + count2);


Arithmetic Operators and Expressions

These are : + (addition), - (subtraction), * (multiplication), / (division), and % (modulo, remainder)

If an arithmetic operator is combined with int operands, then the resulting type is int

If an arithmetic operator is combined with one or two double operands, then the resulting type is double

If different types are combined in an expression, then the resulting type is the right-most type on the following list that is found within the expression

byteshortintlongfloatdoubleChar


Assignment Compatibility

Is this legal ?

int x = 2.99;

And this ?

double variable = 2;


More generally, a value of any type in the following list can be assigned to a variable of any type that appears to the right of itbyteshortintlongfloatdouble

Note that as your move down the list from left to right, the range of allowed values for the types becomes larger

int x = 5.8; errordouble y = 6; okshort i = 14.3; errorfloat s = 9.2; ok

Assignment Compatibility

For these, you need type cast. next

slide


Type Casting

A type cast takes a value of one type and produces a value of another type with an "equivalent" value

int x = (int) 2.9;

When type casting from a floating-point to an integer type, the number is truncated, not rounded:

(int) 2.9 evaluates to 2, not 3


Integer and Floating-Point Division

When one or both operands are a floating-point type, division results in a floating-point type

15.0/2 evaluates to 7.5

When both operands are integer types, division results in an integer type

Any fractional part is discarded

The number is not rounded

15/2 evaluates to 7

Be careful to make at least one of the operands a floating-point type if the fractional portion is needed

April 19, 2023

Precedence Rules

e.g. int x = y = -2 + 5 * 7 – 7 / 2 % 5;

Low precedence

High precedence

First: The unray operators: +, -, ++, --, and !

Second: The binary arithmetic operators: *, /. And %

Third: The binary arithmetic operators: + and -

If some or all of the parentheses in an expression are omitted, Java will follow precedence rules to determine, in effect, where to place them.


Precedence Rules

e.g. int x = y = -2 + 5 * 7 – 7 / 2 % 5;This will be evaluated as..

int x = (y = ((-2 + (5 * 7)) – ((7 / 2) % 5)));

Low precedence

High precedence

First: The unray operators: +, -, ++, --, and !

Second: The binary arithmetic operators: *, /. And %

Third: The binary arithmetic operators: + and -

35 3

33 3

30

If some or all of the parentheses in an expression are omitted, Java will follow precedence rules to determine, in effect, where to place them.


The % (modulo) Operator

The % operator is used with operands of type int to recover the information lost after performing integer division

15/2 evaluates to the quotient 7

15%2 evaluates to the remainder 1

The % operator can be used to count by 2's, 3's, or any other number

To count by twos, perform the operation number % 2, and when the result is 0, number is even


Increment and Decrement Operators

The increment operator (++) adds one to the value of a variable

If n is equal to 2, then n++ or ++n will change the value of n to 3

The decrement operator (--) subtracts one from the value of a variable

If n is equal to 4, then n-- or --n will change the value of n to 3


Increment and Decrement Operators

When either operator precedes its variable, and is part of an expression, then the expression is evaluated using the changed value of the variable

If n is equal to 2, then 2*(++n) evaluates to 6

When either operator follows its variable, and is part of an expression, then the expression is evaluated using the original value of the variable, and only then is the variable value changed

If n is equal to 2, then 2*(n++) evaluates to 4

The end

Important to do at home :

- read section 1.2 of the textbook

ICS102

Lecture 3 : Math



What is a library?

Most programming languages come with a library.

A library is a collection of classes for common system services such as : math operations, input/output routines, etc.

The Java library is called : API (Application Programming Interface).

Java API is organized into packages.

Today we will see a class of this library : Math.

The Math Class

The Math class provides a number of standard mathematical methods

The Math class has two predefined constants, E (e, the base of the natural logarithm system) and PI (, 3.1415 . . .)

area = Math.PI * radius * radius;

Some Methods in the Class Math …

Some Methods in the Class Math

The end

Exercises

ICS102

Lecture 4 : String Class



April 19, 2023 ICS102: The class String 70

Outline

The String Class

Explicit String Objects

String variables are References

String Methods

Escape Sequences

April 19, 2023 ICS102: The class String

71

The String Class

There is no primitive type for strings in Java

The class (type) String is a predefined class in Java that is used to store and process strings

A String object is a sequence of characters that is treated as a single value.

When any “ ” (quote) appears, a String object is created automatically.

A variable of type String can be given the value of a String object

Explicit String Objects

A declaration and object creation are needed for instances of the String class. For example,

We normally use shorthand notation(only for Strings):

String name1;

name1 = new String("Salam");

String name1;

name1 = "Salam";

These two statements are equivalent.

These two statements are equivalent.

April 19, 2023 72ICS102: The class String

String variables are References…

Code

State of Memory

String word1, word2;

word1 = new String("Java");

word2 = word1;

AABoth word1 and word2 are allocated memory (to store references), but the objects themselves are not yet created, so they both contain null.

Both word1 and word2 are allocated memory (to store references), but the objects themselves are not yet created, so they both contain null.

word1

word2

After is executedAA

word1

word2

String variables are References…


Code String word1, word2;


word2 = word1;

BBOne String object is created and assigned to word1, so word1 contains the address of this object.

One String object is created and assigned to word1, so word1 contains the address of this object.

word1

word2

After is executedBB

String

Java

State of Memory

word1

word2

String

Java

String variables are References.


Code String word1, word2;


word2 = word1; CC

Content of word1, which is an address, is assigned to word2, making word2 refer to the same object.

Content of word1, which is an address, is assigned to word2, making word2 refer to the same object.

word1

word2

String

Java

After is executedCC

State of Memory


String Methods

String Concatenation

String Length

String Starting Position

Other Useful String Operators


String Concatenation

Concatenation: Using the + operator on two (or more) strings in order to connect them to form one longer string

A new String object is created operands are not affected. When a string is combined with almost any other type of

item, the result is a string

String greeting = "Hello ";String course = "ICS102";System.out.println(greeting + course); Hello ICS102

int x = 6, y = 10; String s = ".Hi.";System.out.println(s + x);System.out.println(x + y + s); System.out.println(s + (x + y));System.out.println(s + x + y);

.Hi.616.Hi. .Hi.16.Hi.610

String Length

We determine the number of characters in a String with the length method.

String name = "Espresso!";String str2 = "";String str3;

Error because no object is created for str3, so it is a null.

Error because no object is created for str3, so it is a null.

str2.length();

str3.length();

9

0

Error!

Individual characters in a String can be accessed with the charAt method.

Position of a character or String can be found with indexOf method.

String Starting Position

text

This variable refers to the whole string.

This variable refers to the whole string.

text.charAt(8)

The method returns the character at position # 8.

The method returns the character at position # 8.

String text = "Java is fun.";

text.indexOf("is")

The method returns the position of the string “is”.

The method returns the position of the string “is”.

Other Useful String Operators

Method Meaning

equals Checks if two strings are equal. (Use equalsIgnoreCase for case insensitive) str1.equals( str2 )

compareTo Compares the two strings. str1.compareTo( str2 )

substringExtracts the a substring from a string.

str1.substring( 1, 4 ) str1.substring( 5 )

trim Removes the leading and trailing spaces. str1.trim( )

toUpperCaseConverts a string to all caps string. (Use toLowerCase for all small)

str1.toUpperCase( )


Escape Sequences …

A backslash (\) immediately preceding a character (i.e., without any space) denotes an escape sequence or an escape character

The character following the backslash does not have its usual meaning

Although it is formed using two symbols, it is regarded as a single character


Escape Sequences

The end


- read section 1.3 (pages 65-77)

Exercises

For each of these expressions determine its result

String text = "Java Programming";

text.substring(0, 4) text.length( ) text.substring(8, 12) text.substring(0, 1) + text.substring(7, 9) text.substring(5, 6) + text.substring(text.length() – 3,

text.length())

Exercises

Write a program that initializes a String object to “Hello the World”, prints its length, then erases the word “the” from that String.

ICS102

Lecture 5 : Screen Output and Console Input



System.out.println

To write of the screen:

System.out.println(“Hello World");

It is possible to print more than one item:

A plus sign is used to connect more than one item

System.out.println("The answer is " + 42);

Every invocation of println generates a new line after it finishes

System.out.println(“Hello World”);System.out.println(“Hello World”);

Package name

Class name

Method name

The item to be printed on the screen

Hello World

Hello World

println Versus print

To stay on the same line, use print

System.out.print(“Hello World ”);System.out.print(“Hello World ”);

The print method is like println, except that it does not generate a new line

With println, the next output goes on a new line

With print, the next output goes on the same line

Hello World Hello World

Console Input

What if we want to let the user choose the values he wants to use ?

We can prompt the user by console (keyboard) input

Console Input

Import instruction

Create Scanner object

Read a first integer and assign it to variable a

Read a second integer and assign it to variable

b

Console Input

Details ..

Java includes a class for doing simple keyboard input named the Scanner class

In order to use the Scanner class, a program must include the following line near the start of the file:

import java.util.Scanner

This statement tells Java to

Make the Scanner class available to the program

Find the Scanner class in a library of classes (i.e., Java package) named java.util

The following line creates an object of the class Scanner and names the object keyboard :

Scanner keyboard = new Scanner(System.in); Although a name like keyboard is often used, a Scanner

object can be given any name For example, in the following code the Scanner object is

named scannerObject

Scanner scannerObject = new Scanner(System.in); Once a Scanner object has been created, a program can

then use that object to perform keyboard input using methods of the Scanner class

Details ..

The method nextInt reads one int value typed in at the keyboard and assigns it to a variable:

int numberOfPods = keyboard.nextInt();

The method nextDouble reads one double value typed in at the keyboard and assigns it to a variable:

double d1 = keyboard.nextDouble();

Multiple inputs must be separated by whitespace and read by multiple invocations of the appropriate method

Whitespace is any string of characters, such as blank spaces, tabs, and line breaks that print out as white space on paper

Details ..

The method next reads one string of non-whitespace characters delimited by whitespace characters such as blanks or the beginning or end of a line

Given the code

String word1 = keyboard.next();String word2 = keyboard.next();

and the input line

jelly beans

The value of word1 would be jelly, and the value of word2 would be beans

Details ..

The method nextLine reads an entire line of keyboard input The code,

String line = keyboard.nextLine();

reads in an entire line and places the string that is read into the variable line

The end of an input line is indicated by the escape sequence '\n‘

This is the character input when the Enter key is pressed

On the screen it is indicated by the ending of one line and the beginning of the next line

When nextLine reads a line of text, it reads the '\n' character, so the next reading of input begins on the next line

However, the '\n' does not become part of the string value returned (e.g., the string named by the variable line above does not end with the '\n' character)

Details ..

Dealing with the Line Terminator, '\n‘ …

The method nextLine of the class Scanner reads the remainder of a line of text starting wherever the last keyboard reading left off

This can cause problems when combining it with different methods for reading from the keyboard such as nextInt

Given the code,

Scanner keyboard = new Scanner(System.in);int n = keyboard.nextInt();String s1 = keyboard.nextLine();String s2 = keyboard.nextLine();

and the input,

2Heads are better than1 head.

what are the values of n, s1, and s2?

Dealing with the Line Terminator, '\n'

Given the code and input on the previous slide

n will be equal to 2,s1 will be equal to "", ands2 will be equal to "heads are better than“

If the following results were desired instead

n equal to 2, s1 equal to "heads are better than", ands2 equal to "1 head“

then an extra invocation of nextLine would be needed to get rid of the end of line character ('\n')

Dealing with the Line Terminator, '\n'

Given the codeScanner keyboard = new Scanner(System.in);int n = keyboard.nextInt();keyboard.nextLine();String s1 = keyboard.nextLine();String s2 = keyboard.nextLine();

and the input,

2Heads are better than1 head.

We get the desired result:

n equal to 2, s1 equal to "heads are better than", ands2 equal to "1 head“

Programming Tip …

Prompt for Input

A program should always prompt the user when he or she needs to input some data:

System.out.println( "Enter the number of pods followed by");

System.out.println("the number of peas in a pod:");

The end


- read chapter 2 (pages 90-118)

Methods in the Class Scanner …

Methods in the Class Scanner ….

Methods in the Class Scanner

ICS102

Lecture 6 : File Input Output



Outline

Text Files

Stream

Reading from a Text File using Scanner

Writing to a File

IOException

Pitfalls

Text Files

Files that are designed to be read by human beings, and that can be read or written with an editor are called text files

Text files can also be called ASCII files because the data they contain uses an ASCII encoding scheme

An advantage of text files is that they are usually the same on all computers, so that they can move from one computer to another

ASCII Encoding

Streams

A stream is an object that enables the flow of data between a program and some I/O device or file

If the data flows into a program, then the stream is called an input stream

If the data flows out of a program, then the stream is called an output stream

file

program

Output stream

Input stream

Streams

Input streams can flow from the keyboard or from a file

System.in is an input stream that connects to the keyboard

Scanner keyboard = new Scanner(System.in);

Output streams can flow to a screen or to a file

System.out is an output stream that connects to the screen

System.out.println("Output stream");

Reading From a Text File Using Scanner

The class Scanner can be used for reading from the keyboard as well as reading from a text file

Simply replace the argument System.in (to the Scanner constructor) with a suitable stream that is connected to the text file

Scanner StreamObject =new Scanner(new FileInputStream(FileName));

Methods of the Scanner class for reading input behave the same whether reading from the keyboard or reading from a text file

For example, the nextInt and nextLine methods

Reading Input from a Text File Using Scanner

Writing to a Text File

The class PrintWriter is a stream class that can be used to write to a text file

An object of the class PrintWriter has the methods print and println

These are similar to the System.out methods of the same names, but are used for text file output, not screen output


All the file I/O classes that follow are in the package java.io, so a program that uses PrintWriter will start with a set of import statements:

import java.io.PrintWriter;import java.io.FileOutputStream;import java.io.FileNotFoundException;

The class PrintWriter has no constructor that takes a file name as its argument

It uses another class, FileOutputStream, to convert a file name to an object that can be used as the argument to its (the PrintWriter) constructor


A stream of the class PrintWriter is created and connected to a text file for writing as follows:

PrintWriter outputStreamName;

outputStreamName = new PrintWriter(new

FileOutputStream(FileName));

The class FileOutputStream takes a string representing the file name as its argument

The class PrintWriter takes the anonymous FileOutputStream object as its argument


This produces an object of the class PrintWriter that is connected to the file FileName

The process of connecting a stream to a file is called opening the file

If the file already exists, then doing this causes the old contents to be lost

If the file does not exist, then a new, empty file named FileName is created

After doing this, the methods print and println can be used to write to the file

Path Names

When a file name is not in the same directory, the path name must be given

First way:

Scanner in = new Scanner(new FileInputStream(“c:/user/data.txt"));

Second way:

Scanner in = new Scanner(new FileInputStream(“c:\\user\\data.txt"));

The end


- read chapter 10 from 602 to 615

ICS102

Lecture 7 : Selection Structures



April 19, 2023 ICS102: The course 124

Outline

Block statement

Branching Statements

Simple if statement

if-else statement

if-else-elseif statement

switch statement

Nested if statements

Introduction to selection structures

Write a program that accepts an integer value from the user and then:

prints “positive” if the integer is larger or equal to zero

Prints “negative” if the integer is less than zero


Block Statement

A block statement consists of one or more Java statements enclosed in braces.

Example of a block statement:

{statement 1;statement 2; …statement n;

}

Blocks can be nested.

A block statement can be used anywhere that a single statement can be used.


Branching Statement

A branching statement consists of one or more block statements

The execution of a block statement in a branching statement is controlled by a boolean expression which we call a condition.

There are mainly the following four types of branching statement.

Simple if

If-else

If-elseif-else

switch


Simple if Statement …

Simple if statement has the following structure:

if ( <boolean_expression> ) {

<then_block>}

The boolean_expression must be enclosed in parentheses If the boolean_expression is true, then the then_block is

executed. Otherwise it will NOT execute.

if ( testScore >= 95 ) {

System.out.println("You are a good student");

}Then BlockThen Block

Boolean ExpressionBoolean Expression

April 19, 2023 ICS102: The course Chapter 6 - 129

-- Simple if Statement …

testScore >= 95?

testScore >= 95?

falseSystem.out.println("You are a good student");

System.out.println("You are a good student");

trueControl Flow of if:


… -- Simple if Statement

Example: Design and write a Java program prints the absolute value of a number.

import java.util.Scanner;class absolute {

public static void main(String [] args) { Scanner kb = new Scanner(System.in); System.out.print(“Enter a number: “); double x = kb.nextDouble(); double y = x; if( y < 0)

{ y = -y; } System.out.print(“The absolute value of “ + x + “ is “ +

y); }}


-- if-else Statement …

An if-else statement chooses between two alternative statements based on the value of a Boolean expression

If the boolean_expression is true, then the then_block is executed, otherwise the else_block is executed.

if (<boolean_expression>) { <then_block>} else { <else_block>}

if (testScore < 50) {

System.out.println("You did not pass");

} else {

System.out.println("You did pass");

}

Then BlockThen Block

Else BlockElse Block





falsetestScore < 50 ?testScore < 50 ?



true

-- if-else Statement …


Compound Statements

You have to use braces if the <then> or <else> block has multiple statements.

if only one statement is there braces are optional but it is advisable to always use them to enhance readability

if (testScore < 70)

{


System.out.println("Try harder next time");

}

else

{


System.out.println("Keep up the good work");

}

Then BlockThen Block

Else BlockElse Block


… -- if-else Statement

Design and write a Java program which prints the difference of two numbers.

Scanner kb = new Scanner(System.in); System.out.println(“Enter the value of the first number: “); double first = kb.nextDouble(); System.out.println(“Enter the value of the second number: “); double second = kb.nextDouble(); if ( first > second) { double diff = first – second; System.out.println(diff); } else { double diff = second – first; System.out.println(diff); }


-- if-elseif-else Statement …

The multiway if-else statement is simply a normal if-else statement that nests another if-else statement at every else branch

It is indented differently from other nested statements

All of the Boolean_Expressions are aligned with one another, and their corresponding actions are also aligned with one another

The Boolean_Expressions are evaluated in order until one that evaluates to true is found

The final else is optional


if - elseif

if (score >= 85) {

System.out.println(”Grade is A");

} else if (score >= 75) {

System.out.println(”Grade is B");


System.out.println(”Grade is C");


System.out.println(”Grade is D");

} else {

System.out.println(”Grade is N");

}

Test Score Grade85 score A75 score 85 B65 score 75 C50 score 65 D score 50 N


- The switch Statement …

The switch statement is the only other kind of Java statement that implements multiway branching

When a switch statement is evaluated, one of a number of different branches is executed

The choice of which branch to execute is determined by a controlling expression enclosed in parentheses after the keyword switch

The controlling expression must evaluate to a char, int, short, or byte


Syntax for the switch Statement

switch ( fanSpeed ) {

case 1:

System.out.println("That's low");

break;

case 2:

System.out.println("That's medium");

break;

case 3:

System.out.println("That's high");

break;

}

switch ( <arithmetic expression> ) {

<case label 1> : <case body 1>

…

<case label n> : <case body n>

}

Case Body

Case Body

Arithmetic ExpressionArithmetic Expression

Case Label

Case Label


switch With break Statements

switch ( N ) {

case 1: x = 10;

break;

case 2: x = 20;

break;

case 3: x = 30;

break;

}

x = 10;x = 10;

false

trueN == 1 ?

N == 1 ?

x = 20;x = 20;

x = 30;x = 30;

N == 2 ?

N == 2 ?

N == 3 ?

N == 3 ?

false

false

true

true

break;break;

break;break;

break;break;


The switch Statement with default

switch ( binaryDigit ) {

case 0:

System.out.println("zero"); break;

case 1:

System.out.println("one"); break;

default:

System.out.println("That's not a binary digit"); break;

}

switch ( <arithmetic expression> ) {<case label 1> : <case body 1>…<case label n> : <case body n>default: <default body>

}


switch With No break Statements

switch ( N ) {

case 1: x = 10;

case 2: x = 20;

case 3: x = 30;

}

x = 10;x = 10;

false

trueN == 1 ?

N == 1 ?

x = 20;x = 20;

x = 30;x = 30;

N == 2 ?

N == 2 ?

N == 3 ?

N == 3 ?

false

false

true

true


… -- The switch Statement

double y = 30;double z = 20;Scanner kb = new Scanner(System.in);System.out.println("1. add ");System.out.println("2. Subtract ");System.out.println("3. Multiply ");System.out.println("Enter a value: between 1 and 3 ");int x = kb.nextInt();switch (x) {

case 1: System.out.println(z + y);break;

case 2: System.out.println(z - y); break;

case 3: System.out.println(z * y);break;

default: System.out.println("Wrong Choice.");break;

}


- Nested if Statements …

One of the block statements of a branching statement can be another if statement.

The inner if statement is executed when the enclosing block statement is executed

If statements can be nested to many levels

If(<boolean_expresion_1>){ Statement_1; <block_statement_2>; Statement_3;}

if(<boolean_expression_2>) { Statement_2A; Statement_2B: }

Statement_2A and 2B are only executed if boolean_expresions 1 and2 are true.



if (testScore >= 50) {

if (studentAge < 10) {

System.out.println("You did a great job");

} else {


}

} else { //test score < 50


}

Nested ifNested if





falseinner if



false

testScore >= 50 ?testScore >= 50 ?true

studentAge < 10 ?

studentAge < 10 ?

System.out.println("You did a great

job");

System.out.println("You did a great

job");

true


Nested simple-if

if (x < y) {

if (x < z)

System.out.println("Hello");

} else {

System.out.println("Good bye");

}

if (x < y) {

if (x < z) {


}

} else {


}

means


Nested if-else

if (x < y)

if (x < z)


else


if (x < y) {

if (x < z) {


} else {


}

}

really means

The end



Exercises

Write a Java program that reads an integer using Scanner then tests if it is odd or even. The format of the input and output should be as follows: Please enter a number: 1010 is even.

Write a program that reads three integers and prints them in increasing order. Example:Please enter three integers: 9, 5, 21The integers in increasing order are: 5 9 21

Exercises

Write a program that reads an integer between 0 and 10 and then prints the word corresponding to the number. Example:Please enter a number between 0 and 10: 7You entered: seven

ICS102

Lecture 8 : Boolean Expressions




Outline

Introduction Java Comparison Operators Evaluating Boolean Expressions Pitfall: Using == with Strings Lexicographic and Alphabetical Order Building Boolean Expressions Truth Tables Short-Circuit and Complete Evaluation Precedence and Associativity Rules Evaluating Expressions Rules for Evaluating Expressions


- Introduction

A Boolean expression is an expression that is either true or false

The simplest Boolean expressions compare the value of two expressions

time < limityourScore == myScore

Note that Java uses two equal signs (==) to perform equality testing: A single equal sign (=) is used only for assignment


- Java Comparison Operators


- Evaluating Boolean Expressions

Even though Boolean expressions are used to control branch and loop statements, Boolean expressions can exist independently as well

A Boolean variable can be given the value of a Boolean expression by using an assignment statement

A Boolean expression can be evaluated in the same way that an arithmetic expression is evaluated

The only difference is that arithmetic expressions produce a number as a result, while Boolean expressions produce either true or false as their result

boolean madeIt = (time < limit) && (limit < max);


- Pitfall: Using == with Strings

The equality comparison operator (==) can correctly test two values of a primitive type

However, when applied to two objects such as objects of the String class, == tests to see if they are stored in the same memory location, not whether or not they have the same value

In order to test two strings to see if they have equal values, use the method equals, or equalsIgnoreCase

string1.equals(string2)string1.equalsIgnoreCase(string2) Confusion between =, ==, and

equals method is one of the

most frequent programming

errors


- Lexicographic and Alphabetical Order

Lexicographic ordering is the same as ASCII ordering, and includes letters, numbers, and other characters

All uppercase letters are in alphabetic order, and all lowercase letters are in alphabetic order, but all uppercase letters come before lowercase letters

If s1 and s2 are two variables of type String that have been given String values, then s1.compareTo(s2) returns:

A negative number if s1 is before s2 in lexicographic ordering zero if the two strings are equal. A positive number if s2 comes before s1

When performing an alphabetic comparison of strings (rather than a lexicographic comparison) that consist of a mix of lowercase and uppercase letters, use the compareToIgnoreCase method instead


- Building Boolean Expressions

When two Boolean expressions are combined using the "and" (&&) operator, the entire expression is true provided both expressions are true

Otherwise the expression is false

When two Boolean expressions are combined using the "or" (||) operator, the entire expression is true as long as one of the expressions is true

The expression is false only if both expressions are false

Any Boolean expression can be negated using the ! Operator

Place the expression in parentheses and place the ! operator in front of it

Unlike mathematical notation, strings of inequalities must be joined by &&

Use (min < result) && (result < max) rather than min < result < max


- Truth Tables


- Short-Circuit and Complete Evaluation

Consider x > y || x > z

The expression is evaluated left to right. If x > y is true, then there’s no need to evaluate x > z because the whole expression will be true whether x > z is true or not.

This also happens when we use && operator and the first expression is false.

To stop the evaluation once the result of the whole expression is known is called short-circuit evaluation or lazy evaluation


- Short-Circuit and Complete Evaluation

What would happen if the short-circuit evaluation is not done for the following expression?

kids != 0 && toys/kids >= 2

There are times when using short-circuit evaluation can prevent a runtime error

Sometimes it is preferable to always evaluate both expressions, i.e., request complete evaluation

In this case, use the & and | operators instead of && and ||


- Precedence and Associativity Rules …

Boolean and arithmetic expressions need not be fully parenthesized

If some or all of the parentheses are omitted, Java will follow precedence and associativity rules (summarized in the following table) to determine the order of operations

If one operator occurs higher in the table than another, it has higher precedence, and is grouped with its operands before the operator of lower precedence

If two operators have the same precedence, then associativity rules determine which is grouped first


Precedence and Associativity Rules

Instead of relying on precedence and associativity rules, it is best to include most parentheses, except where the intended meaning is obvious

165

The end


- read chapter 3 from 141 to 153

ICS102

Lecture 9 : While and Do-While loops



April 19, 2023 ICS102: while & do-while 167

Outline

Introduction

while Loop

do-while Loop


- Introduction

Loops in Java are similar to those in other high-level languages

Java has three types of loop statements: The while The do-while The for

The code that is repeated in a loop is called the body of the loop

Each repetition of the loop body is called an iteration of the loop


- while loop

A while statement is used to repeat a portion of code (i.e., the loop body) based on the evaluation of a Boolean expression

The Boolean expression is checked before the loop body is executed

When false, the loop body is not executed at all

Before the execution of each following iteration of the loop body, the Boolean expression is checked again

If true, the loop body is executed again

If false, the loop statement ends

The loop body can consist of a single statement, or multiple statements enclosed in a pair of braces ({ })


-- while Loop Syntax

while ( <boolean expression> )

<statement> //only one statement

OR

while ( <boolean expression> ) {

<statement> //many

}

while ( number <= 100 ) {

sum = sum + number;

number = number + 1;

}


Statement(loop body)


These statements are executed as long as number is less than or equal to 100.

These statements are executed as long as number is less than or equal to 100.


-- while Loop Control flow

int sum = 0, number = 1int sum = 0, number = 1

number <= 100 ?number <= 100 ?

falsesum = sum + number;


sum = sum + number;


true


-do-while Loop

A do-while statement is used to execute a portion of code (i.e., the loop body), and then repeat it based on the evaluation of a Boolean expression

The loop body is executed at least once The Boolean expression is checked after the loop body is executed

The Boolean expression is checked after each iteration of the loop body

If true, the loop body is executed again If false, the loop statement ends Don't forget to put a semicolon after the Boolean expression

Like the while statement, the loop body can consist of a single statement, or multiple statements enclosed in a pair of braces ({ })


-- do-while Loop Syntax

do {

sum += number;

number++;

} while (sum <= 1000000) ;

do {

<statement>

} while (<boolean expression>);




These statements are executed as long as sum is less than or equal to 1,000,000.

These statements are executed as long as sum is less than or equal to 1,000,000.


-- do-while Loop Control Flow

int sum = 0, number = 1int sum = 0, number = 1

sum += number;

number++;

sum += number;

number++;

sum <= 1000000 ?sum <= 1000000 ?true

false


Examples


Questions

1. Write a Java program which computes the sum of all the odd numbers between 0 and 100.

2. Write a Java program which reads 20 numbers using a scanner and computes their average.

3. Write a Java program which reads unknown number of integers using a scanner and counts the number of odd numbers and the number of even numbers. Assume the input integers are all positive. Use a negative number as a sentinel.


Solution using while loop


Q1 Solution

int n =1;int sum = 0;while (n < 100) { sum += n; n = n + 2;}System.out.println(“The sum is “

+ sum);

Write a Java program which computes the sum of all the odd numbers between 0 and 100.


Q2 Solution

Scanner kb = new Scanner(System.in);int cnt = 0;double x;double sum = 0;While (cnt < 20) { x = kb.nextDouble(); sum += x; cnt++;}System.out.println(“The Average is “ +

sum/cnt);

Write a Java program which reads 20 numbers using a scanner and computes their average.


Q3 Solution

Scanner kb = new Scanner(System.in);int even_cnt = 0;int odd_cnt = 0;double x = kb.nextInt();while (x > 0) { if ( mod(x,2) == 0) even_cnt++; else odd_cnt++; x = kb.nextInt();}System.out.println(“Even numbers are = “ +

even_count);System.out.println(“Odd numbers are = “ +

odd_count);

Write a Java program which reads unknown number of integers using a scanner and counts the number of odd numbers and the count of even numbers. Assume the input integers are all positive. Use any negative number as a sentinel.


Solution using do-while loop


Q1 Solution

int n = 1;int sum = 0;do {

sum += n;n = n + 2;

} While ( n < 100)System.out.println(“The sum is “

+ sum);



Q2 Solution

Scanner kb = new Scanner(System.in);int cnt = 0;double x;double sum = 0;do { System.out.println(“Enter a number”); x = kb.nextDouble(); sum += x; cnt++;} while (cnt < 20);System.out.println(“The Average is “ +

sum/cnt);



Q3 Solution

Scanner kb = new Scanner(System.in);int even_cnt = 0;int odd_cnt = 0;double x = kb.nextInt();if (x > 0) { do { if ( mod(x,2) == 0) even_cnt++; else odd_cnt++; x = kb.nextInt();

} while ( x > 0)}System.out.println(“Even numbers are = “ +

even_count);System.out.println(“Odd numbers are = “ +

odd_count);



Additional Slides

April 19, 2023 ICS102: while & do-while Chapter 7 - 186

while Loop Pitfall - 1

Infinite Loops Both loops will not terminate because the boolean expressions will never become false.

Infinite Loops Both loops will not terminate because the boolean expressions will never become false.int count = 1;

while ( count != 10 ) {

count = count + 2;

}

22

int product = 0;

while ( product < 500000 ) {

product = product * 5;

}

11



Using Real Numbers Loop 2 terminates, but Loop 1 does not because only an approximation of a real number can be stored in a computer memory.

Using Real Numbers Loop 2 terminates, but Loop 1 does not because only an approximation of a real number can be stored in a computer memory.double count = 0.0;

while ( count <= 1.0 ) {

count = count + 1.0/3.0;

}

22

double count = 0.0;

while ( count != 1.0 ) {

count = count + 1.0/3.0;

}

11



Goal: Execute the loop body 10 times.

count = 1;

while (count < 10) {

. . .

count++;

}

11

count = 0;

while (count <= 10) {

. . .

count++;

}

33

count = 1;

while (count <= 10) {

. . .

count++;

}

22

count = 0;

while (count < 10) {

. . .

count++;

}

44

11 33and exhibit off-by-one error.


Checklist for Repetition Control

1. Watch out for the off-by-one error (OBOE).

2. Make sure the loop body contains a statement that will eventually cause the loop to terminate.

3. Make sure the loop repeats exactly the correct number of times.

The end



ICS102

Lecture 10 : The for-loop and nested loops



Outline

The for Statement Syntax Semantics of the for Statement Nested Loops continue, break, and exit Statements

- The for Statement Syntax

for (Initializing; Boolean_Expression; Update) Block

Note that the three control expressions are separated by two, not three, semicolons

Note that there is no semicolon after the closing parenthesis at the beginning of the loop

- The for Statement

The for statement is most commonly used to step through an integer variable in equal increments

It begins with the keyword for, followed by three expressions in parentheses that describe what to do with one or more controlling variables

The first expression tells how the control variable or variables are initialized or declared and initialized before the first iteration

The second expression determines when the loop should end, based on the evaluation of a Boolean expression before each iteration

The third expression tells how the control variable or variables are updated after each iteration of the loop body

Example

- Nested Loops

Loops can be nested, just like other Java structures When nested, the inner loop iterates from beginning to

end for each single iteration of the outer loop

for (Initializing; Boolean_Expression; Update) Block 1

Block 1 can contain other loop statements as follows

Block 1 for (Initializing; Boolean_Expression; Update)

Block 2

- Nested Loops

Loops can be nested, just like other Java structures When nested, the inner loop iterates from beginning to

end for each single iteration of the outer loop

int rowNum, columnNum;for (rowNum = 1; rowNum <=3; rowNum++){ for (columnNum = 1; columnNum <=2; columnNum++) System.out.print(" row " + rowNum + " column " +

columnNum); System.out.println();}

Continue, break, and exit Statements

Class test { public static void main( String [] args) { for (int i = 0; i < 10; i++) { statement 1;

statement 2;if( cond) continue;statement 3;statement 4;

} statement 5; statement 6; }}


Class test { public static void main( String [] args) { for (int I = 0; I < 10; i++) { statement 1;

statement 2;if( cond) break;statement 3;statement 4;



Class test { public static void main( String [] args) { for (int I = 0; I < 10; i++) { statement 1;

statement 2;if( cond) System.exit(0);statement 3;statement 4;


For-loop examples


Questions

1. Write a Java program which computes the sum of all the odd numbers between 0 and 100.

2. Write a Java program which reads 20 numbers using a scanner and computes their average.

3. Write a Java program which reads unknown number of integers using a scanner and counts the number of odd numbers and the number of even numbers. Assume the input integers are all positive. Use a negative number as a sentinel.


Q1 Solution

int sum = 0;for( int n = 1; n <= 100; n = n + 2) { sum += n;}System.out.println(“The sum is “ +

sum);



Q2 Solution

Scanner kb = new Scanner(System.in);double x;double sum = 0;While (int cnt = 0; cnt < 20; cnt++) { System.out.println(“Enter a number”);

x = kb.nextDouble(); sum += x;}System.out.println(“The Average is “ +

sum/cnt);



Q3 Solution

Scanner kb = new Scanner(System.in);int even_cnt = 0;int odd_cnt = 0;int n;For(;;) { n = kb.nextInt(); if (n < 0) break; else if ( mod(n,2) == 0) even_cnt++; else odd_cnt++; }System.out.println(“Even = “ + even_count + “ odd = “

odd_cnt);


Nested-loop examples

Questions

1. Write a java program which gives the following output122333444455555

2. Write a java program which prints all the prime numbers less than 1000.

Q1 Solution

Write a java program which gives the following output122333444455555

for(int k = 1; k <= 5; k++) {For ( int j = 1; j <=k; j++)

System.out.print(k);System.out.println();

}

Q2 solution

Write a java program which prints all the prime numbers less than 1000.

int n, j;

for(int k = 2; k < 100; k++) {

n = 0;

j = 2;

while(n == 0 && j < k/2) {

if (mod(k,j) == 0) n++;

j++;

}

if( n ==0) System.out.println(k);

}

The end



ICS102

Lecture 11 : Arrays



Arrays

An array is an ordered list of values

An array of size N is indexed from zero to N-1

This array holds 4 values that are indexed from 0 to 3

Entire array hasA single name

Each value hasA numeric index

scores [0] [1] [2] [3]

79 62 98 57

Processing Array Elements

Access individual elements of an array using: the name of the array a number (index or subscript) that tells which of the

element of the array

What value is stored in scores[2]?

scores [0] [1] [2] [3]

79 62 98 57

Arrays

For example, an array element can be assigned a value, printed, or used in a calculation:

scores[2] = 87;

scores[first] = scores[first] + 2;

Avg = (scores[0] + scores[1])/2;

System.out.println (“Max = " + scores[3]);

Arrays

The values held in an array are called array elements

Array elements are of the same type

The element type can be a primitive type or an object reference.

Therefore, we can create an array of integers, or an array of characters, or an array of String objects, etc.

In Java, the array itself is an object

Therefore the name of the array is a object reference variable, and the array itself must be instantiated

Declaring an Array

Defining an array

Type [] name

Where: Type specifies the kind of values the array stores the brackets [ ] indicate this is an array name is the handle to access the array

•This declares the handle only

•Initialized to null

•Stores an address when arrays are created

•This declares the handle only

•Initialized to null

•Stores an address when arrays are created

Declaring an Array

The scores array could be declared as follows:

double [] scores = new double [4];

The above statement will: allocate block of memory to hold 4 doubles initialize block with zeros create a handle or a pointer or a reference called scores

store address of the block in scores scores [0] [1] [2] [3]

0.0 0.0 0.00.0

BlockHandle

Syntax

Forms

ElementType [] arrayName;

ElementType [] arrayName = new ElementType [size];

ElementType [] arrayName = array-literal;

Where: ElementType is any type arrayName is the handle for the array array-literal is a list of literals enclosed in curly braces { }

Declaring Arrays

Some examples of array declarations:

float[] prices = new float[500];

boolean[] flags;

flags = new boolean[20];

char[] codes = new char[1750];

Bounds Checking

Once an array is created, it has a fixed size

An index used in an array reference must specify a valid element

That is, the index value must be in bounds (0 to N-1)

The Java interpreter throws an ArrayIndexOutOfBoundsException if an array index is out of bounds

This is called automatic bounds checking

Bounds Checking

For example, if the array scores can hold 100 values, it can be indexed using only the numbers 0 to 99

If scores has the value 100, then the following reference will cause an exception to be thrown:

System.out.println (scores[count]);

Often for loop is used to process array. It’s common to introduce off-by-one errors when using arrays

for (int index=0; index <= 100; index++)

scores[index] = index*50 + epsilon;

problem

Bounds Checking

Each array object has a public constant called length that stores the size of the array

It is referenced using the array name:

scores.length

Note that length holds the number of elements, not the largest index

Initializer Lists

An initializer list can be used to instantiate and initialize an array in one step

The values are delimited by braces and separated by commas

Examples:

int[] scores = {98, 76, 54, 83, 87, 65, 99, 66};

char[] letterGrades = {'A', 'B', 'C', 'D', ’F'};

Initializer Lists: Example 1

Used when exact size and initial values of an array are known in advance

int [] scores = {98, 76, 54, 83}

Visualize the results of the above command

scores [0] [1] [2] [3]

98 76 54 83

Initializer Lists: Example 2

Consider the following 4 student names

Note results:

String [] STUDENTS = { “Aref", “Ali“, “Emad”, "Sami"};

STUDENTS [0] [1] [2] [3]

Aref Ali Emad Sami

Array elements are handles for String values

Initializer Lists

Note that when an initializer list is used:

the new operator is not used

no size value is specified

The size of the array is determined by the number of items in the initializer list

An initializer list can only be used only in the array declaration

The Assignment Operation

Java provides a few operations to use with arrays, including assignment

Consider:

int [] alist = { 1, 12, 15, 7};int [] blist;blist = alist;

Recall that alist and blist are handles

alist contains an address of where the numbers are blist now contains that same address blist does not have a copy of alist

Array Cloning

To actually create another array with its own values, Java provides the .clone() method

int [] alist = {1, 12, 15, 7};

int [] blist;

blist = alist.clone();

Now there are two separate lists of numbers, one with handle alist, the other with handle blist

Array Cloning with Reference Types

Recall previous declaration:

Consider:

String[] s_list = STUDENTS.clone():

This will create another list of handles, also pointing to the names

Sometimes called a "shallow copy"

operation

Sometimes called a "shallow copy"

operation

STUDENTS [0] [1] [2] [3]

Aref Ali Emad Sami

Array Cloning with Reference Types

We can write our own "deep copy"

String [] original = {“Aref”,“Ali”, “Emad”, “Sami”};

String [] result = new String(original.length);

for (i = 0; i < original.length; i++)

result[i] = original[i].clone();

Array Equality

Java has an equals() method for classes

if (a1.equals(a2)) …

If a1 and a2 are arrays, the equals() method just looks at the addresses the handles point to

They could be pointing to different addresses but the contents of the array still be equal

It is the contents that we really wish to compare

Array Equality

We must write our own method to compare the arrays they both must have the same length then use a for( ) loop to compare element by element

for equality

Boolean areEqual = true;if (list1.length != list2.length) areEqual = false;

else { for (int i = 0; i < list1.length; i++) if (list1[i] != list2[i]) areEqual = false;

}if(areEqual) System.out.println(“They are equal”);else System.out.println(“They are NOT equal”);

As soon as one pair of elements is found not equal, the arrays are not equalAs soon as one pair of elements is found not equal, the arrays are not equal

Array exercises

Write an application that inputs 10 numbers, each between 10 and 100. As each number is read, display it only if it is not a duplicate of a number already read.

Given an array of integers, write a java code that allows to check if the array is “palindromic”. A palindromic array is a symmetric one:

For example the arrays 1 6 4 6 1 and 258852 are both palindromic, but the array 3753 is not.

Array exercises

Write a program that reads a sequence of 10 integers into an array and then computes the alternating sum of all elements in the array.

For example if the array is : 1 4 9 16 9 7 4 9 11 then it computes : 1 – 4 + 9 – 16 + 9 – 7 + 4 – 9 + 11

= -2

The end



ICS102

Lecture 12 : 2-D Arrays



Two-Dimensional Arrays

Two-dimensional (2-D) Arrays.

2-D Array Declaration.

Initializer Lists.

Differing Numbers of Elements in each Row.

Implementation of a 2-D Array

Printing a 2-D Array.

2-D Arrays: Why?

Often data come in a 2-D form. For example: The layout of a printed page is 2-D Computer screen is 2-D The spread sheet (table) is 2-D

For these situations we need a 2-D array.

A 2-D array can be thought of as a collection of "slots" laid out in a 2-D grid.

each slot can hold a value two indexes are needed to specify a slot.

Example: Table of Student Grades

Imagine a class of 7 students that have a quiz grades for 5 weeks. These grades can be stored in a table form.

A particular cell of the table is identified by student number and week number. For example:

The grade for student 0 in week 1 is 42



StudentWeek

0 1 2 3 4

0 99 42 74 83 100

1 90 91 72 88 95

2 88 61 74 89 96

3 61 89 82 98 93

4 93 73 75 78 99

5 50 65 92 87 94

6 43 98 78 56 99

2-D Arrays in Java

In Java, a table may be implemented as a 2-D array.

As with 1-D arrays, every slot in a 2-D array is of the same type which can be a primitive type or an object reference type.

Each slot of the array is specified with a row and column number.

Suppose that gradeTable is a 2-D array then the syntax to specify a particular slot should be gradeTable[row][col]

For example

gradeTable[0][1] is 42



StudentWeek

0 1 2 3 4

0 99 42 74 83 100

1 90 91 72 88 95

2 88 61 74 89 96

3 61 89 82 98 93

4 93 73 75 78 99

5 50 65 92 87 94

6 43 98 78 56 99

2-D Arrays in Java

The subscripted variables of a 2-D array can be used in assignment statements and arithmetic expressions just like any variable:

gradeTable[ 0 ][ 1 ] = 33 ; // puts a 33 into row 0 column 1.

gradeTable[ 3 ][ 4 ]++; //increments the value at row 3 column 4.

int value = (gradeTable[ 6 ][ 2 ] + 2)/2;//puts 40 into value

Write a Java statement that puts a zero into row 5 column 3.

StudentWeek

0 1 2 3 4

0 99 33 74 83 100

1 90 91 72 88 95

2 88 61 74 89 96

3 61 89 82 98 944 93 73 75 78 99

5 50 65 92 87 94

6 43 98 78 56 99

gradeTable[ 5 ][ 3 ] = 0

2-D Array Declaration

In Java, a 2-D array is an object.

To declare a reference variable myArray to a 2-D array of int: int[][] myArray;

To create an array object of 3 rows and 5 columns, and put the reference in myArray, we write

int[][] myArray = new int[3][5];

All the elements of myArray are initialized to zero. We can create the above array by using the initializer list: int[][] myArray = {{0,0,0,0,0}, {0,0,0,0,0}, {0,0,0,0,0}};

Different Numbers of Elements per Row

In Java, each row of a 2-D array may have a different number of elements. In the following example, the array A has

3 elements in its first row, 2 in its second row, and 5 in its last row.

int[][] A = {{ 1, 9, 4 }, { 0, 2}, { 0, 1, 2, 3, 4 }};

Length of a 2-D Array

The length of a 2-D array is the number of rows it has.

So the row index (number) is from 0 to length-1.

As each row in a 2-D array is like a 1-D array, we can refer to an entire row by specifying just the row index, e.g., A[0] means row # 0

The length of a row is the number of columns in the row.

As each row of a 2-D array can have a different number of columns, so each row has its own length.

int[][] A = { { 1, 9, 4 }, { 0, 2}, { 0, 1, 2, 3, 4 } };

System.out.println("Length of array is: " + A.length );

System.out.println("Length of row[0] is: " +

A[0].length );


A[1].length );


A[2].length );

Implementation of 2-D Array

A 2-D array is implemented as an array of 1-D arrays.

To understand the idea, we construct the 2-D array step-by-step.

int[][] myArray; // 1

declares a variable myArray

myArray = new int[3][] ; // 2

Creates an array object. The array object has 3 slots. Each slot may refer (in the future) to an array of int (i.e., a row)

myArray[0] = new int[3] ; // 3

Creates a 1-D array object and puts its reference in slot 0 of myArray.

Implementation of 2-D Array

A previously constructed 1-D array can be assigned to a row:

int[] x = {0, 2}; int[] y = {0, 1, 2, 3, 4}; myArray[1] = x ;myArray[2] = y ; //4

The rows do not need to have the same number of elements

Printing a 2-D Array

Suppose we want to print every element of a 2-D array A

It can be done by using nested loops.

The expression A[row].length gives a different number for each row of the array.

int[][] A = {{ 1, 9, 4 }, { 0, 2}, { 0, 1, 2, 3, 4 }}; for ( int row=0; row < A.length; row++ ) {

System.out.print("Row " + row + ": ");

for ( int col=0; col < A[row].length; col++ )

System.out.print( A[row][col] + " ");

System.out.println();

}

The end



Exercises

Write a program that creates a two dimensional array, fills it using Scanner, and then prints the sum of every column.

Given a two-dimensional array, dataTable, of type double such that the rows can have different lengths. Write a code fragment that computes the average of each row and saves it in a single-dimension array of size corresponding to the rows of dataTable.

ICS102

Lecture 13 : Methods



A Java Program

A Java program consists of one or more classes

A Java class consists of one or more methods

A Java method consists of one or more statements

A Java Program

Javaclasses

JavaMethods

- What is a method …

A Java application program consists of one or more classes.

Each class has one or more methods

Each method consists of one or more statements

Each method has a name.

One of the methods must be called main

When a Java application program is run, the run-time system automatically invokes the method named main

All Java application programs start with the main method

… - What is a method …

public class class-name {

method1

method 2

method 3

…

…

method n

}


Class nameMain method

Instruction

Class body

Exercise

Write a program that computes the factorial (n!) of 6 then the factorial of 3, then the factorial of 10.

Exercise

Write a program that computes the factorial (n!) of 6 then the factorial of 3, then the factorial of 10. Method

invocation

Method definition

Return instruction

Method parameter

Return type

-- Method Structure

public or private<static> <void or typeReturned>myMethod(<parameters>)

{statementstatementstatement……

statement }

Method body

If method doesn’t return value

Variable list

Method name

Type of the return value

- Invoking a Methods

The statements inside a method body are executed when the corresponding method is called from another method.

Calling a method is also called invoking a method Each time the method is invoked, its corresponding

body is executed

- return Statements …

The body of a method that returns a value must also contain one or more return statements

A return statement specifies the value returned and ends the method invocation.

… - return Statements

A void method need not contain a return statement, unless there is a situation that requires the method to end before all its code is executed

Example : write a method that prints all the numbers between 10 and 30

- Local Variables

A variable declared within a method definition is called a local variable

All variables declared inside the method are local variables

All method parameters are local variables

- Local Variables

If two methods each have a local variable of the same name, they are still two entirely different variables. Example:

Local variable in main method

Local variable in factorial method

Local variable in addition method

- Method Parameters …

A parameter list provides a description of the data required by a method

It indicates the number and types of data pieces needed, the order in which they must be given, and the local name for these pieces as used in the method

public double myMethod(int p1, int p2, double p3)

Example: What is the parameter list of a method division that divides two integers and returns the result (double)?

Parameters list

… - Method Parameters …

When a method is invoked, the appropriate values must be passed to the method in the form of arguments

The number, order, and types of the arguments must exactly match that of the parameter list

Error: type mismatch

Is this program correct?


Yes it is! .. Details in the next slide..


If argument and parameter types do not match exactly, Java will attempt to make an automatic type conversion

In the preceding example, the int value of argument a would be cast to a double

A primitive argument can be automatically type cast from any of the following types, to any of the types that appear to its right:

byteshortintlongfloatdouble char


A parameters is often thought of as a blank or placeholder that is filled in by the value of its corresponding argument

However, a parameter is more than that: it is actually a local variable

When a method is invoked, the value of its argument is computed, and the corresponding parameter (i.e., local variable) is initialized to this value

Even if the value of a formal parameter is changed within a method (i.e., it is used as a local variable) the value of the argument cannot be changed

Call-by-Value Example

Prints 10

Prints 10

- Method Parameters: Array Parameters

Methods can have array parameters.

Example: Write a program that prints the values of an array of integers, then increments each value of the array by two and then prints the new values of the same array.

A parameter of type array

- Details

Array arguments are always passed by reference. This means any changes done to an array element by the invoked method, will also change the corresponding array element in the caller method.

Example:

Public static void m1() { int [] A = { 1, 2, 3}; m2(A); }

public static void m2(int [] A){ A[1] = 5};

Method m2 changes the value of a[1] to 5; as a result the value of a[1] in m1 will also change to 5.

- Method Overloading …

In java the same class can have methods with the same name. Such methods are called overloaded methods.

Overloaded methods must differ in the number or type of their arguments.

The name of a method together with the number, order, and types of its arguments are called the method signature. No two methods of the same class must have the same signature.

The compiler treats overloaded methods as completely different methods. It knows which one to call by using method signatures.

- Method Overloading …

These 3 methods have the same

name but different

signatures

The end


- read pages 206-212

Exercises

Write a method that takes as input an integer value and returns the sum of all integers less than that value. For example, if the input is 6, the output is 5+4+3+2+1 = 15. If the input is negative, the output should be -1.

Write a method that takes as input an array of integers and returns the average of the values in the array.

Exercises

Write a method public void printTriangleNumbers(int n) such that:

The call: printTriangleNumbers(5) prints the following on the screen: 1 2 3 4 5 1 2 3 4 1 2 3 1 2 1 The call: printTriangleNumbers(6) prints the following on the screen: 1 2 3 4 5 6 1 2 3 4 5 1 2 3 4 1 2 3 1 2 1 The call: printTriangleNumbers(3) prints the following on the screen: 1 2 3 1 2 1

ICS102

Lecture 14 : Classes



Outline

Introduction

Class Definitions

A Class Is a Type

The new Operator

Instance Variables and Methods

Information Hiding

public and private Modifiers

Accessor and Mutator Methods

Encapsulation

- Introduction

Classes are the most important language feature that make object-oriented programming (OOP) possible

Programming in Java consists of defining a number of classes

Every program is a class All helping software consists of classes All programmer-defined types are classes

Classes are central to Java

Example

Suppose you are asked to write a program for a commercial company:

The company has several: Employees Clients Products Stores

If you use an object-oriented language such as Java, you will have to define:

Class Employee : public class Employee{ ..}

Class Client: public class Client{..} Class Product: public class Product{..} Class Store: public class Store{..}

- Class Definitions

You already know how to use classes and the objects created from them, and how to invoke their methods

For example, you have already been using the predefined String and Scanner classes

Now you will learn how to define your own classes and their methods, and how to create your own objects from them …

next slide

Class Definition

A class definition is composed of two parts: Data members (Data part) Methods (Operations part)

Example: define a class Employee

- Details

A class definition specifies the data items and methods that all of its objects will have

These data items and methods are sometimes called members of the object

Data items are called fields or instance variables

Instance variable declarations and method definitions can be placed in any order within the class definition

Object Declaration

It is possible to declare several objects from a class:

Class Employeee2

e1

e3

e4

Note: classes Employee and TestClass should be saved in the same directory

Strings2

s1

s3

s4

The same principle as:

- A Class Is a Type

A class is a special kind of programmer-defined type, and variables can be declared of a class type

A value of a class type is called an object or an instance of the class

The following phrases are equivalent: “e1 is of type Employee," “e1 is an object of the class Employee," and “e1 is an instance of the class Employee"

Employeee2

e1

e3

e4

- Object Creation

The instruction:Employee e1;

only declares e1. But the object is still not created.

To create the object the operator new must be used:

e1 = new Employee();

These can be combined as follows:

Employee e1 = new Employee();

- Object Creation

Declaration Creation

Question: What is the name of Employee e1?

How to change the name of Employee e1? … next slide ..

- Instance Variables and Methods …

In order to refer to a particular instance variable, preface it with its object name as follows:

objectName.instanceVar1objectName.instanceVar2

Example: e1.name e1.age e1.salary

To change the name of e1: e1.name = “Mohamed”;


In order to invoke a method of a class, you need an object of that class:

objectName.method1()objectName.method2(argument1)

Example: e1.outputDetails(); e1.outputDetails();


Modifying the name Accessing the name

Invoking a method

Calling object

- Information Hiding and Encapsulation

Information hiding is the practice of separating how to use a class from the details of its implementation

Abstraction is another term used to express the concept of discarding details in order to avoid information overload

Encapsulation means that the data and methods of a class are combined into a single unit (i.e., a class object), which hides the implementation details

Knowing the details is unnecessary because interaction with the object occurs via a well-defined and simple interface

In Java, hiding details is done by marking them private

- public and private Modifiers …

Instance variables and methods of a class can be declared public, or private

Example: public String name; private int age; public void outputDetails{..}

The modifier public means that there are no restrictions on where an instance variable or method can be used

The modifier private means that an instance variable or method cannot be accessed by name outside of the class

It is considered good programming practice to make all instance variables private

- public and private Modifiers …

Illegal because we try to access a private member (age) from outside the class Employee

… - public and private Modifiers

Within the definition of a class, private members of any object of the class can be accessed, not just public members of the calling object

Problem ..

It is considered good programming practice to make all instance variables private

Question: how to access and modify the instance variables of Employee objects e1, e2, e3 and e4? .. answer .. Use accessor and mutaor methods ….

next slide..

- Accessor and Mutator Methods …

Accessor methods allow the programmer to obtain the value of an object's instance variables

The data can be accessed but not changed The name of an accessor method typically starts with the

word get Example: public String getName()

{ return name;}

Mutator methods allow the programmer to change the value of an object's instance variables in a controlled manner

Incoming data is typically tested and/or filtered The name of a mutator method typically starts with the word

set Example: public void setName(String n)

{ name = n;}

- Accessor and Mutator Methods (Example)

Accessor method for instance variable name

Mutator method for instance variable name

Modifying the name of e1 using a mutator method

The end


- read sections 4.1 and 4.2 (pages 200-243)

Exercise

Define a class Date with three instance variable:

month of type String day of type integer year of type integer

Provide a method WriteOutput to print the details of the Date

Provide accessor and mutator methods for all instance variables.

Solution

ICS102

Lecture 15 : Classes II



Constructors

A constructor is a special kind of method that is designed to initialize the instance variables for an object:

public ClassName(ParametersList){…}

A constructor must have the same name as the class

A constructor has no type returned, not even void

Constructors are typically overloaded

Constructor Example

Constructor

How Constructors are called

A constructor is called when an object of the class is created using new

ClassName objectName = new ClassName(anyArgs);

The name of the constructor and its parenthesized list of arguments (if any) must follow the new operator

A constructor cannot be invoked like an ordinary method

Example:

Employee e1 = new Employee(“Mohamed”, 20, 5000);

Constructors

Constructor

Calling the constructor

Constructors

In the previous lecture we saw this example:

We did not define any constructor,

but

we created objects using new!!

Explanation .. Next slide ..

Include a No-Argument Constructor

If you do not include any constructors in your class, Java will automatically create a default or no-argument constructor that takes no arguments, performs no initializations, but allows the object to be created

If you include even one constructor in your class, Java will not provide this default constructor

If you include any constructors in your class, be sure to provide your own no-argument constructor as well

No-argument constructor

No-argument constructor

Copy Constructor

A copy constructor is a constructor with a single argument of the same type as the class.

It creates an object which is an exact copy of the argument object

Example:

How to invoke a copy constructor:

- The methods equals and toString

Java expects certain methods, such as equals and toString, to be in all, or almost all, classes

The purpose of equals, a boolean valued method, is to compare two objects of the class to see if they satisfy the notion of "being equal“

Note: You cannot use == to compare objects

public boolean equals(ClassName objectName)

The purpose of the toString method is to return a String value that represents the data in the object

public String toString()

equals example

equals example

Invoking equals method:

equals invocation

toString example

toString invocation

The end


- read sections 4.3 and 4.4 (pages 244-273)

Exercise: Temperature Class (1/2)

Write a Temperature class that has two instance variables: a temperature value (a floating-point number) and a character for the scale, either 'C' for Celsius or 'F' for Fahrenheit.

The class should have four constructor methods: one for each instance variable (assume zero degrees if no

value is specified and Celsius if no scale is specified), one with two parameters for the two instance variables, and a no-argument constructor (set to zero degrees Celsius).

Include two accessor methods to return the temperature: getTempCelsius: to return the degrees Celsius, getTempFahrenheit: to return the degrees Fahrenheit

IMPORTANT: use the following formulas : degreesC = 5(degreesF - 32)/9 degreesF = (9(degreesC)/5) + 32

Include three mutator methods, setValue to set the value, setScale to set the scale ('F' or 'C'), and setValueAndScale to set both;

Include a suitable toString method. Include a equals method

Then write a test class called TestTemperature that tests all the methods. Be sure to use each of the constructors.

Exercise: Temperature Class (2/2)

ICS102

Lecture 16 : Classes III



April 19, 2023 ICS102: Classes 3/5 320

Class variables Vs Primitive type variables

int i = 15;


i is a variable of type integer that contain the value 15.

e1 is a variable of type Employee that contains the address where the object is located.

The object named by the variable is stored in some other location in memory

i 15

e1

“Mohamed”20

5000


References

What happens if we do:


Employee e2 = e1;

e1 and e2 will refer to the same object! The assignment operator sets the reference (memory

address) of one class type variable equal to that of another

Any change to the object named by one of theses variables will produce a change to the object named by the other variable, since they are the same object !!

Example

What will be printed here?


- Class Parameters …

All parameters in Java are call-by-value parameters

A parameter is a local variable that is set equal to the value of its argument

Therefore, any change to the value of the parameter cannot change the value of its argument

Class type parameters appear to behave differently from primitive type parameters

They appear to behave in a way similar to parameters in languages that have the call-by-reference parameter passing mechanism

Call-by-Value

Call-by-Reference

What salary will printed here?


… - Class Parameters

The value plugged into a class type parameter is a reference (memory address)

Therefore, the parameter becomes another name for the argument

Any change made to the object named by the parameter (i.e., changes made to the values of its instance variables) will be made to the object named by the argument, because they are the same object


- Differences Between Primitive and Class-Type Parameters

A method cannot change the value of a variable of a primitive type that is an argument to the method

In contrast, a method can change the values of the instance variables of a class type that is an argument to the method


Pitfall: Use of == with Class type variables

The test for equality (==) also behaves differently for class type variables

The == operator only checks that two class type variables have the same memory address

Unlike the equals method, it does not check that their instance variables have the same values

Two objects in two different locations whose instance variables have exactly the same values would still test as being "not equal“

Use equals method instead:


- The Constant null

null is a special constant that may be assigned to a variable of any class type

YourClass yourObject = null;

It is used to indicate that the variable has no "real value“

The end


- read section 5.2

(pages 310-324)

Advanced Example

An advanced example of classes is the Class Person defined in Display 5.19 of Chapter 5 of the textbook

(Pages 296-304 of the 4th edition)

ICS102

Lecture 18 : Static Members



Static Methods

A static method is one that can be used without a calling object.

A static method still belongs to a class, and its definition is given inside the class definition.

When a static method is defined, the keyword static is placed in the method headerpublic static returnedType myMethod(parameters) { . . . }

Static methods are invoked using the class name in place of a calling objectreturnedValue = MyClass.myMethod(arguments);

Static Variables

Static variables can be declared and initialized at the same time

private static int myStaticVariable = 0;

If not explicitly initialized, a static variable will be automatically initialized to a default value

boolean static variables are initialized to false Other primitive types static variables are initialized to the

zero of their type Class type static variables are initialized to null

It is always preferable to explicitly initialize static variables rather than rely on the default initialization

Static Variables

A static variable should always be defined private, unless it is also a defined constant

The value of a static defined constant cannot be altered, therefore it is safe to make it public

In addition to static, the declaration for a static defined constant must include the modifier final, which indicates that its value cannot be changed

public static final int BIRTH_YEAR = 1954;

When referring to such a defined constant outside its class, use the name of its class in place of a calling object

int year = MyClass.BIRTH_YEAR;

Pitfall: Invoking a Nonstatic Method Within a Static Method

A static method cannot refer to an instance variable of the class, and it cannot invoke a nonstatic method of the class

A static method has no this, so it cannot use an instance variable or method that has an implicit or explicit this for a calling object

A static method can invoke another static method, however

Simple Example

static field

static method

Example (Part 1 of 4)

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