Chapter 1
How a Computer Works

1.1 Introduction

In today's society, almost every task requires the use of a computer. In schools, students use computers to search the Internet and to send emails. At work, people use them to make presentations, to analyze data, and to communicate with customers. At home, people use computers to play games, to connect to social networks and to chat with other people all over the world. Of course, don't forget smartphones such as iPhones. They are computers as well!

Computers can perform so many different tasks because of their ability to be programmed. In other words, a computer can perform any job that a program tells it to. A program is a set of statements (often called instructions or commands) that a computer follows in order to perform a specific task.

Programs are essential to a computer, because without them a computer is a dummy machine that can do nothing at all. It is the program that actually tells the computer what to do and when to do it. On the other hand, the programmer or the software developer is the person who designs, creates, and often tests computer programs.

This book introduces you to the basic concepts of computer programming using the C# language.

1.2 What is Hardware?

The term hardware refers to all devices or components that make up a computer. If you have ever opened the case of a computer or a laptop you have probably seen many of its components, such as the microprocessor (CPU), the memory, and the hard disk. A computer is not a device but a system of devices that all work together. The basic components of a typical computer system are discussed here.

This is the part of a computer that actually performs all the tasks defined in a program (basic arithmetic, logical, and input/output operations).

This is the area where the computer holds the program (while it is being executed/run) as well as the data that the program is working with. All programs and data stored in this type of memory are lost when you shut down your computer or you unplug it from the wall outlet.

ROM or Read Only Memory is a special type of memory which can only be read by the computer (but cannot be changed). All programs and data stored in this type of memory are not lost when the computer is switched off. ROM usually contains manufacturer's instructions as well as a program called the bootstrap loader whose function is to start the operation of computer system once the power is turned on.

This is usually the hard disk or the SSD (Solid State Drive), and sometimes (but more rarely) the CD/DVD drive. In contrast to main memory (RAM), this type of memory can hold data for a longer period of time, even if there is no power to the computer. However, programs stored in this memory cannot be directly executed. They must be transferred to a much faster memory; that is, the main memory.

Input devices are all those devices that collect data from the outside world and enter them into the computer for further processing. Keyboards, mice, and microphones are all input devices.

Output devices are all those devices that output data to the outside world. Monitors (screens) and printers are output devices.

1.3 What is Software?

Everything that a computer does is controlled by software. There are two categories of software: system software and application software.

1.4 How a Computer Executes (Runs) a Program

When you turn on your computer, the main memory (RAM) is completely empty. The first thing the computer needs to do is to transfer the operating system from the hard disk to the main memory.

After the operating system is loaded to main memory, you can execute (run) any program (application software) you like. This is usually done by clicking, double clicking, or tapping the program's corresponding icon. For example, let's say you click on the icon of your favorite word processor. This action orders your computer to copy (or load) the word processing program from your hard disk to the main memory (RAM) so the CPU can execute it.

1.5 Compilers and Interpreters

Computers can execute programs that are written in a strictly defined computer language. You cannot write a program using a natural language such as English or Greek, because your computer won't understand you!

But what does a computer actually understand? A computer can understand a specific low-level language called the machine language. In a machine language all statements (or commands) are made up of zeros and ones. The following is an example of a program written in a machine language, that calculates the sum of two numbers.

Shocked? Don't worry, you are not going to write programs this way. Hopefully, no one writes computer programs this way anymore. Nowadays, all programmers write their programs in a high-level language and then they use a special program to translate them into a machine language.

There are two types of programs that programmers use to perform translation: compilers and interpreters.

A compiler is a program that translates statements written in a high-level language into a separate machine language program. You can then execute the machine language program any time you wish. After the translation, there is no need to run the compiler again unless you make changes in the high-level language program.

An interpreter is a program that simultaneously translates and executes the statements written in a high-level language. As the interpreter reads each individual statement in the high-level language program, it translates it into a machine language code and then directly executes it. This process is repeated for every statement in the program.

1.6 What is Source Code?

The statements (often called instructions or commands) that the programmer writes in a high-level language are called source code or simply code. The programmer first types the source code into a program known as a code editor, and then uses either a compiler to translate it into a machine language program, or an interpreter to translate and execute it at the same time.

1.7 Review Questions: True/False

Choose true or false for each of the following statements.

1.8 Review Questions: Multiple Choice

Select the correct answer for each of the following statements.

Chapter 2
C# and Integrated Development Environments

2.1 What is C#?

C# (pronounced as “C sharp”) is a widely used general-purpose, high-level computer programming language that allows programmers to create desktop or mobile applications, large-scale applications, embedded systems, client-server applications, web pages, and many other types of software. C# is an extension of its predecessor, the C programming language.

C# is designed to be a platform-independent language. It is intended to let programmers “write once, run anywhere (WORA)”, meaning that code is written once but can run on any combination of hardware and operating system without being re-compiled.

2.2 What is the Difference Between a Script and a Program?

Technically speaking, a script is interpreted whereas a program is compiled, but this is actually not their major difference. There is another more important difference between them!

The main purpose of a script written in a scripting language such as JavaScript, or VBA (Visual Basic for Applications) is to control another application. So you can say that, in some ways JavaScript controls the web browser, and VBA controls a Microsoft^® Office application such as MS Word or MS Excel.

On the other hand, a program written in a programming language such as C#, C++, or Java (to name a few) executes independently of any other application. A program is executed as stand-alone any time the user wishes without the need of a hosting application.

2.3 Why You Should Learn C#

C# is what is known as a “high-level” computer language. C#'s coding style is similar to C language. It is quite easy to understand and highly efficient on multiple platforms such as Windows, Android, Linux, and Unix. C# is a flexible and powerful language, making it well-suited for developing large-scale applications, embedded systems, drivers, client-server applications, art applications, music players, or even video games.

C# is everywhere! It is on desktop computers, laptops, mobile devices, and even in data centers costing millions of dollars. With a huge community of developers worldwide, C# enables efficient development of many exciting applications and services. This huge availability of C# programmers is a major reason why organizations choose C# for new development over any other programming language. This is also a very good reason why you should actually learn C#!

2.4 How C# Works

Computers do not understand natural languages such as English or Greek, so you need a computer language such as C# to communicate with them. C# is a very powerful high-level computer language. The C# compiler (or, actually, a combination of two compilers) converts C# language to a language that computers can actually understand, and that is known as the “machine language”.

In the past, computer languages made use of either an interpreter or a compiler. Nowadays however, many computer languages including C# use two compilers. In C#, the first compiler translates statements into an intermediate language called Common Intermediate Language (CIL), which is a language similar to Java's bytecode. The CIL code is stored on disk in an executable file called an assembly, typically with an extension of .exe. Later, when a user wants to execute the file, .NET (pronounced as "dot net") performs a Just In Time (JIT) compilation to convert the CIL code into low-level machine language code for direct execution on the hardware.

In Figure 2–1 you can see how statements written in C# are compiled into CIL code and how CIL code is then compiled and executed using the .NET.

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Figure 2–1 Executing C# statements using the .NET

Now come some reasonable questions: Why all this trouble? Why does C# and other languages translate twice? Why are C# statements not directly translated into low-level machine language code? The answer lies in the fact that C# is designed to be a platform-independent programming language. This means that a program is written once but it can be executed on any device, regardless of its operating system or its architecture, as long as the appropriate version of .NET is installed on it. In the past, programs had to be recompiled, or even rewritten, for each computer platform. One of the biggest advantages of C# is that you only have to write and compile a program once! In Figure 2–2 you can see how statements written in C# are compiled into CIL code and how CIL code can then be executed on any platform that has the corresponding .NET installed on it.

Figure 2–2 Executing C# statements on different platforms

2.5 Integrated Development Environments

An Integrated Development Environment, or IDE, is a type of software that includes all the basic tools programmers need to write and test programs. An IDE typically contains a source code editor and integrates tools such as a compiler or an interpreter, along with a debugger. Visual Studio Community and Visual Studio Code are two examples of IDEs that let programmers write, execute and debug their source code.

2.6 Microsoft Visual Studio

Microsoft Visual Studio is an Integrated Development Environment (IDE) that provides a great set of tools for many programming languages (via extensions installed separately) and lets you easily create applications for Android, iOS, macOS, Windows, and the cloud, as well as websites, web applications, and web services.

Visual Studio is much more than a text editor. It can indent lines, match words and brackets, and highlight source code that is written incorrectly. It also provides automatic code (IntelliSense^®), which means that as you type, it displays a list of possible completions. The IDE also provides hints to help you analyze your code and find any potential problems. It even suggests some simple solutions to fix those problems. You can use the Visual Studio not only to write but also to execute your programs directly from the IDE.

Visual Studio has a large community of users all around the world and this is why it comes in so many different flavors. Specifically, in Microsoft's download page you can download:

In the next chapter (Chapter 3), you will find links guiding you to instructions on how to install and configure whatever is necessary on your computer, such as Visual Studio Community or Visual Studio Code and .NET Core SDK, on either Windows or Linux. Then, in Chapter 9, you will discover guidance on using Visual Studio Community or Visual Studio Code to write, execute and debug C# programs. These instructions are available on my website. Additionally, you will find numerous tips and tricks there that will be valuable in your first steps as a budding programmer!

Chapter 3
Software Packages to Install

3.1 What to Install

For the purposes of this book, you need to install an Integrated Development Environment (IDE) on your computer. Visual Studio Community and Visual Studio Code are examples of such IDEs. It's up to you to choose which one you are going to use.

All the instructions you need regarding how to set up Visual Studio Community or Visual Studio Code and a C# compiler, on either Windows or Linux are maintained on my website at the following addresses. This gives me the flexibility to review them frequently and keep them up-to-date.

https://tinyurl.com/48nhbxd3

https://www.bouraspage.com/cs-setup-write-execute-debug

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If you find any inconsistencies, please let me know, and I will update the instructions as soon as possible. To report issues, visit one of the following addresses:

https://tinyurl.com/28nwh2nf

https://www.bouraspage.com/report-errata

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Review in “Introductory Knowledge”

Review Crossword Puzzles

Across

1) Statements or commands.

3) Windows is such a system.

4) A computer component.

6) A category of software.

8) An input device.

11) It's the person who designs computer programs.

14) An output device.

15) Antivirus is such a software.

16) In today's society, almost every task requires the use of this device.

17) A computer component.

18) All these devices make up a computer.

Down

2) These devices are also computers.

5) Computers can perform so many different tasks because of their ability to be _____________.

7) Special memory that can only be read.

9) A secondary storage device.

10) A browser is this type of software.

12) An input device.

13) An operating system.

Across

2) The CPU performs one of these basic operations.

7) In a machine language all statements are made up of zeros and _____.

9) A set of statements.

11) __________ is a language executed by the .NET

12) This software controls a device that is attached to your computer.

13) All data stored in this type of memory are lost when you shut down your computer.

14) .__________ converts the bytecode into low-level machine language code.

15) Run a program.

Down

1) A program that simultaneously translates and executes the statements written in a high-level language.

3) Visual Studio is such a software.

4) The statements that a programmer writes to solve a problem.

5) A scripting language.

6) A program that translates statements written in a high-level language into a separate machine language program.

8) A category of programming language.

10) A low-level language.

Across

3) An input device.

5) C# is suitable for developing ________ applications.

6) A scripting language that can control Microsoft Word.

Down

1) It performs logical operations.

2) A script requires a __________ application in order to execute.

3) It displays data to the user.

4) An intermediate language that C# uses.

5) Scripts written in VBA.

Review Questions

Answer the following questions.