Chapter 1. Introduction to Computers, the Internet and the Web
1.1 Introduction
Welcome to C and C++. C is a concise yet powerful computer programming language that’s appropriate for technically oriented people with little or no programming experience and for experienced programmers to use in building substantial software systems.
1.2 Hardware and Software
Computers can perform calculations and make logical decisions phenomenally faster than human beings can. Many of today’s personal computers can perform billions of calculations in one second–more than a human can perform in a lifetime.
Computers process data under the control of sequences of instructions called computer programs. These software programs guide the computer through ordered actions specified by people called computer programmers.
A computer consists of various devices referred to as hardware (e.g., the keyboard, screen mouse, hard disks, memory, DVD drives and processing units). Computing costs are dropping dramatically, owing to rapid developments in hardware and software technologies.
1.2.2 Computer Organization
Regardless of differences in physical appearance, computers can be envisioned as divided into various logical units or sections.
logical unit | Description
Input unit | This “receiving” section obtains information (data and computer programs) form input devices and places it at the disposal of the other units for processing. Most user input is entered into computers through keyboards, touch screens and mouse devices. Other forms of input include receiving voice commands, scanning images and barcodes, reading from secondary storage devices (such as hard drives, DVD drives, Blu-ray Disc drives and USB flash drives–also called “thumb drives” or “memory sticks”), receiving video from a webcam and having your computer receive information from the Internet (such as when you stream videos from YouTube or download e-books form Amazon). Newer forms or input include position data form a GPS device, and motion and orientation information from an accelerometer (a device that responds to up/down, left/right and forward/backward acceleration) in a smartphone or game controller.
Output unit | This “shipping” section takes information the computer has processed and places it on various output devices to make it available for use outside the computer. Most information that’s output from computers today is displayed on screens (including touch screens), printed on paper (“going green” discourages this), played as audio or video on PCs and media players (such as Apples iPods) and giant screens in ports stadiums, transmitted over the Internet or used to control other devices, such as robots and “intelligent” appliances. Information is also commonly output to secondary storage devices, such as hard drives, DVD drives and USB flash drives. Popular recent forms of output are smartphone and game controller vibration, and virtual reality devices like Oculus Rift.
Memory unit | This rapid-access, relatively low-capacity “warehouse” section retains information that has been entered through the input unit, making it immediately available for processing when needed. The memory unit also retains processed information until it can be replaced in output devices by the output input. Information in the memory unit is volatile–it’s typically lost when the computer’s power is turned off. The memory unit is often called either memory, primary memory or RAM (Random Access Memory). Main memories on desktop and notebook computers contain as much as 128 GB of RAM, though 2 to 16GB is most common. GB stand for gigabytes; a gigabyte is approximately one billion bytes. A byte is eight bits. A bit is either a 0 or a 1.
Arithmetic and logic unit (ALU) | This “manufacturing” section performs calculations, such as addition, subtraction, multiplication and division. It also contains the decision mechanisms that allow the computer, for example, to compare two items from the memory unit to determine whether they’ re equal. In today’s systems, the ALU is implemented as part of the next logical unit, the CPU.
Central processing unit (CPU) | This “administrative” section coordinates and supervises the operation of the other sections. The CPU tells the input unit when information from the memory unit should be used in calculations and tells the output unit when to send information from the memory unit to certain output devices. Many of today’s computers have multiple CPUs and, hence, can perform many operations simultaneously. A multi-core processor implements multiple processors on a single integrated-circuit chip–a dual-core processor has two CPUs and a quad-core processor has four CPUs. Today’s desktop computers have processors that can execute billions of instructions per second.
Secondary storage unit | This is the long-term, high-capacity “warehousing” section. Programs or data not actively being used by the other units normally are placed on secondary storage devices (e.g., your har drive) until they’re again needed, possibly hours, days, months or even years later. Information on secondary storage devices is persistent–it’s preserved even when the computer’s power is turned off. Secondary storage information takes much longer to access than information in primary memory, but its cons per unit is much less. Examples of secondary storage devices include hard drives, DVD drives and USB flash drives, some of which can hold over 2 TB. Typical hard drives on desktop and notebook computers hold up to 2 TB, and some desktop hard drives can hold up to 6 TB.
1.3 Data Hierarchy
Data items processed by computers form a data hierarchy that becomes larger and more complex in structure as we progress form the simplest data items (called “bits”) to richer ones, such as characters and fields. Figure 1.2 illustrates a portion of the data hierarchy.
Bits
The smallest data item in a computer can assume the value 0 or the value 1. It’s called a bit (short for “binary digit” –a digit that can assume one of two values). Remarkably, the impressive functions performed by computers involve only the manipulations of 0s and 1s –examining a bit’s value, setting a bit’s value and reversing a bit’s value (from 1 to 0 or from 0 to 1).
Characters
It’s tedious for people to work with data in the low-level form of bits. Instead, they prefer to work with decimal digits (0-9), letters (A–Z and a–z) and special symbols (e.g., $, @, %, &, *, (, ), –, +, “, :, ? and /) are known as characters. The computer’s character set is the set of all the characters used to write programs and represent data items. Computers process only 1s and 0s, so a computer’s character set represents every character as a pattern of 1s and 0s. C supports various character sets (including Unicode) that are composed of characters containing one, two or four bytes (8, 16 or 32 bits). Unicode contains characters for many of the world’s languages. See Appendix B for more information on the ASCII (American Standard Code for Information Interchange) character set–the popular subset of Unicode that represents uppercase and lowercase letters, digits and some common special characters.
Fields
Just as characters are composed of bits, fields are composed of characters or bytes. A field consisting of decimal digits could represent a person’s age.
Records
Several related fields can be used to compose a record. In a payroll system, for example, the record for an employee might consist of the following fields (possible types for these fields are shown in parentheses):
- Employee identification number (a whole number)
- Name (a string of characters)
- Address (a string of characters)
- Hourly pay rate (a number with a decimal point)
- Year-to-date earnings (a number with a decimal point)
- Amount of taxes withheld (a number with a decimal point)
Thus, a record is a group of related fields. In the preceding example, all the fields belong to the same employee. A company might have many employees and a payroll record for each.
Files
A file is a group of related records. [Note: More generally, a file contains arbitrary data in arbitrary formats. In some operating systems, a file is viewed simply as a sequence of bytes — any organization of the bytes in a file, such as organizing the data into records, is a view created by the application programmer.] It’s not unusual for an organization to have many files, some containing billions, or even trillions, of characters of information.
Database
A database is a collection of data organized for easy access and manipulation. The most popular model is the relational database, in which data is sorted in simple tables. A table includes records and fields. For example, a table of students might include first name, last name, major, year, student ID number and grade point average fields. The data for each student is a record, and the individual pieces of information in each record are the fields. You can search, sort and otherwise manipulate the data based on its relationship to multiple tables or databases. For example, a university might use data from the student database in combination with data from databases of courses, on-campus housing, meal plans, etc.
Big Data
The amount of data being produced worldwide is enormous and growing quickly. According to IBM, approximately 2.5 quintillion bytes (2.5 exabytes) of data are created daily and 90% of the world’s data was created in just the past two years! According to an IDC study, the global data supply will reach 40 zettabytes (equal to 40 trillion gigabytes) annually by 2020. Figure 1.3 shows some common byte measurements. Big data applications deal with massive amounts of data and this field is growing quickly, creating lots of opportunity for software developers. According to a study by Gartner Group, over 4 million IT jobs globally will support big data by 2015.
1.4 Machine Language, Assembly Languages and High-Level Languages
Programmers write instructions in various programming languages, some directly understandable by computers and others requiring intermediate translation steps. Hundreds of such languages are in use today. These may be divided into three general types:
- Machine languages
- Assembly languages
- High-level languages
Machine Languages
Any computer can directly understand only its own machine language, defined by its hardware design. Machine languages generally consist of strings of numbers (ultimately reduced to 1s and 0s) that instruct computers to perform their most elementary operations one at a time. Machine languages are machine dependent (a particular machine language can be used on only one type of computer). Such language are cumbersome for humans. For example, here’s a section of an early machine-language payroll program that adds overtime pay to base pay and stores the result in gross pay:
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