1.3 Data Hierarchy
Data items processed by computers form a data hierarchy that becomes larger and more complex in structure as we progress from 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 simplest 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 /). Digits, letters and special symbols 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 is a group of characters or bytes that conveys meaning. For example, a field consisting of uppercase and lowercase letters can be used to represent a person’s name, and 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 stored 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! 2 According to an IDC study, the global data supply will reach 40 zettabytes (equal to 40 trillion gigabytes) annually by 2020. 3 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 Languages, 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 languages 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:
+1300042774
+1400593419
+1200274027
Assembly Languages and Assemblers
Programming in machine language was simply too slow and tedious for most programmers. Instead of using the strings of numbers that computers could directly understand, programmers began using English-like abbreviations to represent elementary operations. These abbreviations formed the basis of assembly languages. Translator programs called assemblers were developed to convert early assembly-language programs to machine language at computer speeds. The following section of an assembly-language payroll program also adds overtime pay to base pay and stores the result in gross pay:
load basepay
add overpay
store grosspay
Although such code is clearer to humans, it’s incomprehensible to computers until translated to machine language.
High-Level Languages and Compilers
With the advent of assembly languages, computer usage increased rapidly, but programmers still had to use numerous instructions to accomplish even the simplest tasks. To speed the programming process, high-level languages were developed in which single statements could be written to accomplish substantial tasks. Translator programs called compilers convert high-level language programs into machine language. High-level languages allow you to write instructions that look almost like everyday English and contain commonly used mathematical notations. A payroll program written in a high-level language might contain a single statement such as
grossPay = basePay + overTimePay
From the programmer’s standpoint, high-level languages are preferable to machine and assembly languages. C is one of the most widely used high-level programming languages.
Interpreters
Compiling a large high-level language program into machine language can take considerable computer time. Interpreter programs, developed to execute high-level language programs directly, avoid the delay of compilation, although they run slower than compiled programs.
1.5 The C Programming Language
C evolved from two previous languages, BCPL and B. BCPL was developed in 1967 by Martin Richards as a language for writing operating systems and compilers. Ken Thompson modeled many features in his B language after their counterparts in BCPL, and in 1970 he used B to create early versions of the UNIX operating system at Bell Laboratories.
The C language was evolved from B by Dennis Ritchie at Bell Laboratories and was originally implemented in 1972. C initially became widely known as the development language of the UNIX operating system. Many of today’s leading operating systems are written in C and/or C++. C is mostly hardware independent—with careful design, it’s possible to write C programs that are portable to most computers.
Built for Performance
C is widely used to develop systems that demand performance, such as operating systems, embedded systems, real-time systems and communications systems (Figure 1.4).
Operating systems
C’s portability and performance make it desirable for implementing operating systems, such as Linux and portions of Microsoft’s Windows and Google’s Android. Apple’s OS X is built in Objective-C, which was derived from C. We discuss some key popular desktop/notebook operating systems and mobile operating systems in Section 1.11.
Embedded systems
The vast majority of the microprocessors produced each year are embedded in devices other than general-purpose computers. These embedded systems include navigation systems, smart home appliances, home security systems, smartphones, tablets, robots, intelligent traffic intersections and more. C is one of the most popular programming languages for developing embedded systems, which typically need to run as fast as possible and conserve memory. For example, a car’s antilock brakes must respond immediately to slow or stop the car without skidding; game controllers used for video games should respond instantaneously to prevent any lag between the controller and the action in the game, and to ensure smooth animations.
Real-time systems
Real-time systems are often used for “mission-critical” applications that require nearly instantaneous and predictable response times. Real-time systems need to work continuously—for example, an air-traffic-control system must constantly monitor the positions and velocities of the planes and report that information to air-traffic controllers without delay so that they can alert the planes to change course if there’s a possibility of a collision.
Communications systems
Communications systems need to route massive amounts of data to their destinations quickly to ensure that things such as audio and video are delivered smoothly and without delay.
By the late 1970s, C had evolved into what’s now referred to as “traditional C.” The publication in 1978 of Kernighan and Ritchie’s book, The C Programming Language, drew wide attention to the language. This became one of the most successful computer science books of all time.
1.7 C++ and Other C-Based Languages
C++ was developed by Bjarne Stroustrup at Bell Laboratories. It has its roots in C, providing a number of features that “spruce up” the C language. More important, it provides capabilities for object-oriented programming. Objects are essentially reusable software components that model items in the real world. Using a modular, object-oriented design-and-implementation approach can make software-development groups more productive. Chapters 15–23 present a condensed treatment of C++ selected from our book C++ How to Program. Figure 1.5 introduces several other popular C-based programming languages.
Objective-C
Objective-C is an object-oriented language based on C. It was developed in the early 1980s and later acquired by NeXT, which in turn was acquired by Apple. It has become the key programming language for the OS X operating system and all iOS-powered devices (such as iPods, iPhones and iPads).
Java
Sun Microsystems in 1991 funded an internal corporate research project which resulted in the C++-based object-oriented programming language called Java. A key goal of Java is to enable the writing of programs that will run on a broad variety of computer systems and computer-controlled devices. This is sometimes called “write once, run anywhere.” Java is used to develop large-scale enterprise applications, to enhance the functionality of web servers (the computers that provide the content we see in our web browsers), to provide applications for consumer devices (smartphones, television set-top boxes and more) and for many other purposes. Java is also the language of Android app development.
C#
Microsoft’s three primary object-oriented programming languages are Visual Basic (based on the original Basic), Visual C++ (based on C++) and Visual C# (based on C++ and Java, and developed for integrating the Internet and the web into computer applications). Non-Microsoft versions of C# are also available.
PHP
PHP, an object-oriented, open-source scripting language supported by a community of users and developers, is used by millions of websites. PHP is platform independent—implementations exist for all major UNIX, Linux, Mac and Windows operating systems. PHP also supports many databases, including the popular open-source MySQL.
Python
Python, another object-oriented scripting language, was released publicly in 1991. Developed by Guido van Rossum of the National Research Institute for Mathematics and Computer Science in Amsterdam (CWI), Python draws heavily from Modula-3—a systems programming language. Python is “extensible” it can be extended through classes and programming interfaces.
JavaScript
JavaScript is the most widely used scripting language. It’s primarily used to add dynamic behavior to web pages—for example, animations and improved interactivity with the user. It’s provided with all major web browsers.
Swift
Swift, Apple’s new programming language for developing iOS and Mac apps, was announced at the Apple World Wide Developer Conference (WWDC) in June 2014. Although apps can still be developed and maintained with Objective-C, Swift is Apple’s app-development language of the future. It’s a modern language that eliminates some of the complexity of Objective-C, making it easier for beginners and those transitioning from other high-level languages such as Java, C#, C++ and C. Swift emphasizes performance and security, and has full access to the iOS and Mac programming capabilities.
1.9 Typical C Program-Development Environment
C systems generally consist of several parts: a program-development environment, the language and the C Standard Library. The following discussion explains the typical C development environment shown in Fig. 1.6.
C programs typically go through six phases to be executed (Fig. 1.6). These are: edit, preprocess, compile, link, load and execute. Although C How to Program, 8/e, is a generic C textbook (written independently of the details of any particular operating system), we concentrate in this section on a typical Linux-based C system. [Note: The programs in this book will run with little or no modification on most current C systems, including Microsoft Windows-based systems.] If you’re not using a Linux system, refer to the documentation for your system or ask your instructor how to accomplish these tasks in your environment. Check out our C Resource Center at http://www.deitel.com/C to locate “getting started” tutorials for popular C compilers and development environments.
1.9.1 Phase 1: Creating a Program
Phase 1 consists of editing a file. This is accomplished with an editor program. Two editors widely used on Linux systems are vi and emacs. Software packages for the C/C++ integrated program development environments such as Eclipse and Microsoft Visual Studio have editors that are integrated into the programming environment. You type a C program with the editor, make corrections if necessary, then store the program on a secondary storage device such as a hard disk. C program filenames should end with the .c extension.
1.9.2 Phases 2 and 3: Preprocessing and Compiling a C Program
In Phase 2, you give the command to compile the program. The compiler translates the C program into machine-language code (also referred to as object code). In a C system, a preprocessor program executes automatically before the compiler’s translation phase begins. The C preprocessor obeys special commands called preprocessor directives, which indicate that certain manipulations are to be performed on the program before compilation. These manipulations usually consist of including other files in the file to be compiled and performing various text replacements. The most common preprocessor directives are discussed in the early chapters; a detailed discussion of preprocessor features appears in Chapter 13.
In Phase 3, the compiler translates the C program into machine-language code. A syntax error occurs when the compiler cannot recognize a statement because it violates the rules of the language. The compiler issues an error message to help you locate and fix the incorrect statement. The C Standard does not specify the wording for error messages issued by the compiler, so the error messages you see on your system may differ from those on other systems. Syntax errors are also called compile errors, or compile-time errors.
1.9.3 Phase 4: Linking
The next phase is called linking. C programs typically contain references to functions defined elsewhere, such as in the standard libraries or in the private libraries of groups of programmers working on a particular project. The object code produced by the C compiler typically contains “holes” due to these missing parts. A linker links the object code with the code for the missing functions to produce an executable image (with no missing pieces). On a typical Linux system, the command to compile and link a program is called gcc (the GNU C compiler). To compile and link a program named welcome.c, type
gcc welcome.c
at the Linux prompt and press the Enter key (or Return key). [Note: Linux commands are case sensitive; make sure that each c is lowercase and that the letters in the filename are in the appropriate case.] If the program compiles and links correctly, a file called a.out (by default) is produced. This is the executable image of our welcome.c program.
1.9.4 Phase 5: Loading
The next phase is called loading. Before a program can be executed, the program must first be placed in memory. This is done by the loader, which takes the executable image from disk and transfers it to memory. Additional components from shared libraries that support the program are also loaded.
1.9.5 Phase 6: Execution
Finally, the computer, under the control of its CPU, executes the program one instruction at a time. To load and execute the program on a Linux system, type ./a.out at the Linux prompt and press Enter.
1.9.6 Problems That May Occur at Execution Time
Programs do not always work on the first try. Each of the preceding phases can fail because of various errors that we’ll discuss. For example, an executing program might attempt to divide by zero (an illegal operation on computers just as in arithmetic). This would cause the computer to display an error message. You would then return to the edit phase, make the necessary corrections and proceed through the remaining phases again to determine that the corrections work properly.
GrayCS 