Program Development Cycle The Program Development Cycle has its own steps. In the first step, you use an editor to create a disk file containing your source code. In the second step, you compile the source code to create an object file. In the third step, you link the compiled code to create an executable file. The fourth step is to run the program to see whether it works as originally planned. Creating Source Code: Source code is a series of statements or commands that are used to instruct the computer to perform your desired tasks. As mentioned, the first step in the Program Development Cycle is to enter source code into an editor. For example, here is a line of C source code: printf("Hello, Mom!"); This statement instructs the computer to display the message Hello, Mom! on-screen. (For now, don’t worry about how this statement works.) Using an Editor Most compilers come with a built-in editor that can be used to enter source code; however, some don’t. Consult your compiler manuals to see whether your compiler came with an editor. If it didn’t, many alternative editors are available. Most computer systems include a program that can be used as an editor. If you’re using a UNIX system, you can use such editors as ed, ex, edit, emacs, or vi. If you’re using Microsoft Windows, Notepad is available. If you’re using MS/DOS 5.0 or later, you can use Edit. If you’re using a version of DOS before 5.0, you can use Edlin. If you’re using PC/DOS 6.0 or later, you can use E. If you’re using OS/2, you can use the E and EPM editors. Most word processors use special codes to format their documents. These codes can’t be read correctly by other programs. The American Standard Code for Information Interchange (ASCII) has specified a standard text format that nearly any program, including C, can use. Many word processors, such as WordPerfect, AmiPro, Word, WordPad, and WordStar, are capable of saving source files in ASCII form (as a text file rather than a document file). When you want to save a word processor’s file as an ASCII file, select the ASCII or text option when saving. If none of these editors is what you want to use, you can always buy a different editor. There are packages, both commercial and shareware, that have been designed specifically for entering source code. NOTE: To find alternative editors, you can check your local computer store or computer mail-order catalogs. Another place to look is in the ads in computer programming magazines. When you save a source file, you must give it a name. The name should describe what the program does. In addition, when you save C program source files, give the file a .C extension. Although you could give your source file any name and extension, .C is recognized as the appropriate extension to use. Compiliting Source Code: Although you might be able to understand C source code (at least, after reading this book you will be able to), your computer can’t. A computer requires digital, or binary, instructions in what is called machine language. Before your C program can run on a computer, it must be translated from source code to machine language. This translation, the second step in program development, is performed by a program called a compiler. The compiler takes your source code file as input and produces a disk file containing the machine language instructions that correspond to your source code statements. The machine language instructions created by the compiler are called object code, and the disk file containing them is called an object file. NOTE: This book covers ANSI Standard C. This means that it doesn’t matter which C compiler you use, as long as it follows the ANSI Standard. Each compiler needs its own command to be used to create the object code. To compile, you typically use the command to run the compiler followed by the source filename. The following are examples of the commands issued to compile a source file called RADIUS.C using various DOS/Windows compilers:
Compiler
Command
Microsoft C
cl radius.c
Borland’s Turbo C
tcc radius.c
Borland C
bcc radius.c
Zortec C
ztc radius.c
If you’re using a graphical development environment, compiling is even simpler. In most graphical environments, you can compile a program listing by selecting the compile icon or selecting something from a menu. Once the code is compiled, selecting the run icon or selecting something from a menu will execute the program. You should check your compiler’s manuals for specifics on compiling and running a program. After you compile, you have an object file. If you look at a list of the files in the directory or folder in which you compiled, you should find a file that has the same name as your source file, but with an .OBJ (rather than a .C) extension. The .OBJ extension is recognized as an object file and is used by the linker. On UNIX systems, the compiler creates object files with an extension of .O instead of .OBJ. Linking To Creating An Executable File One more step is required before you can run your program. Part of the C language is a function library that contains object code (code that has already been compiled) for predefined functions. A predefined function contains C code that has already been written and is supplied in a ready-to-use form with your compiler package. The printf() function used in the previous example is a library function. These library functions perform frequently needed tasks, such as displaying information on-screen and reading data from disk files. If your program uses any of these functions (and hardly a program exists that doesn’t use at least one), the object file produced when your source code was compiled must be combined with object code from the function library to create the final executable program. (Executable means that the program can be run, or executed, on your computer.) This process is called linking, and it’s performed by a program called (you guessed it) a linker. Figure 1.1 shows the progression from source code to object code to executable program




Figure 1.1. The C source code that you write is converted to object code by the compiler and then to an executable file by the linker


Once your program is compiled and linked to create an executable file, you can run it by entering its name at the system prompt or just like you would run any other program. If you run the program and receive results different from what you thought you would, you need to go back to the first step. You must identify what caused the problem and correct it in the source code. When you make a change to the source code, you need to recompile and relink the program to create a corrected version of the executable file. You keep following this cycle until you get the program to execute exactly as you intended.

One final note on compiling and linking: Although compiling and linking are mentioned as two separate steps, many compilers, such as the DOS compilers mentioned earlier, do both as one step. Regardless of the method by which compiling and linking are accomplished, understand that these two processes, even when done with one command, are two separate actions.
The C Development Cycle

Step 1: Use an editor to write your source code. By tradition, C source code files have the extension .C (for example, MYPROG.C, DATABASE.C, and so on).
Step 2: Compile the program using a compiler. If the compiler doesn’t find any errors in the program, it produces an object file. The compiler produces object files with
an .OBJ extension and the same name as the source code file (for example, MYPROG.C compiles to MYPROG.OBJ). If the compiler finds errors, it reports them. You must return to step 1 to make corrections in your source code. Step 3: Link the program using a linker. If no errors occur, the linker produces an executable program located in a disk file with an .EXE extension and the same name as the object file (for example, MYPROG.OBJ is linked to create MYPROG.EXE). Step 4: Execute the program. You should test to determine whether it functions properly. If not, start again with step 1 and make modifications and additions to your source code. Figure 1.2 shows the program development steps. For all but the simplest programs, you might go through this sequence many times before finishing your program. Even the most experienced programmers can’t sit down and write a complete, error-free program in just one step! Because you’ll be running through the edit-compile-link-test cycle many times, it’s important to become familiar with your tools: the editor, compiler, and linker.

Getting The C Language

A Brief History Of The C Language

You might be wondering about the origin of the C language and where it got its name. C was created by Dennis Ritchie at the Bell Telephone Laboratories in 1972. The language wasn’t created for the fun of it, but for a specific purpose: to design the UNIX operating system (which is used on many computers). From the beginning, C was intended to be useful—to allow busy programmers to get things done.

Because C is such a powerful and flexible language, its use quickly spread beyond Bell Labs. Programmers everywhere began using it to write all sorts of programs. Soon, however, different organizations began utilizing their own versions of C, and subtle differences between implementations started to cause programmers headaches. In response to this problem, the American National Standards Institute (ANSI) formed a committee in 1983 to establish a standard definition of C, which became known as ANSI Standard C. With few exceptions, every modern C compiler has the ability to adhere to this standard.

Now, what about the name? The C language is so named because its predecessor was called B. The B language was developed by Ken Thompson of Bell Labs. You should be able to guess why it was called B.


Why Use C ?

In to unit’s world of computer programming, there are many high-level languages to choose from, such as C, Pascal, BASIC, and Java. These are all excellent languages suited for most programming tasks. Even so, there are several reasons why many computer professionals feel that C is at the top of the list:· C is a powerful and flexible language. What you can accomplish with C is limited only by your imagination. The language itself places no constraints on you. C is used for projects as diverse as operating systems, word processors, graphics, spreadsheets, and even compilers for other languages. · C is a popular language preferred by professional programmers. As a result, a wide variety of C compilers and helpful accessories are available. · C is a portable language. Portable means that a C program written for one computer system (an IBM PC, for example) can be compiled and run on another system (a DEC VAX system, perhaps) with little or no modification. Portability is enhanced by the ANSI standard for C, the set of rules for C compilers. · C is a language of few words, containing only a handful of terms, called keywords, which serve as the base on which the language’s functionality is built. You might think that a language with more keywords (sometimes called reserved words) would be more powerful. This isn’t true. As you program with C, you will find that it can be programmed to do any task.

· C is modular. C code can (and should) be written in routines called functions. These functions can be reused in other applications or programs. By passing pieces of information to the functions, you can create useful, reusable code.

As these features show, C is an excellent choice for your first programming language. What about C++? You might have heard about C++ and the programming technique called object-oriented programming. Perhaps you’re wondering what the differences are between C and C++ and whether you should be teaching yourself C++ instead of C.

Not to worry! C++ is a superset of C, which means that C++ contains everything C does, plus new additions for object-oriented programming. If you do go on to learn C++, almost everything you learn about C will still apply to the C++ superset. In learning C, you are not only learning one of to unit’s most powerful and popular programming languages, but you are also preparing yourself for object-oriented programming.

Another language that has gotten lots of attention is Java. Java, like C++, is based on C. If later you decide to learn Java, you will find that almost everything you learned about C can be applied.


Preparing To Program

You should take certain steps when you’re solving a problem. First, you must define the problem. If you don’t know what the problem is, you can’t find a solution! Once you know what the problem is, you can devise a plan to fix it. Once you have a plan, you can usually implement it. Once the plan is implemented, you must test the results to see whether the problem is solved. This same logic can be applied to many other areas, including programming.
When creating a program in C (or for that matter, a computer program in any language), you should follow a similar sequence of steps:
1. Determine the objective(s) of the program.
2. Determine the methods you want to use in writing the program.
3. Create the program to solve the problem.
4. Run the program to see the results.
An example of an objective (see step 1) might be to write a word processor or database program. A much simpler objective is to display your name on the screen. If you didn’t have an objective, you wouldn’t be writing a program, so you already have the first step done.
The second step is to determine the method you want to use to write the program. Do you need a computer program to solve the problem? What information needs to be tracked? What formulas will be used? During this step, you should try to determine what you need to know and in what order the solution should be implemented.
As an example, assume that someone asks you to write a program to determine the area inside a circle. Step 1 is complete, because you know your objective: determine the area inside a circle. Step 2 is to determine what you need to know to ascertain the area. In this example, assume that the user of the program will provide the radius of the circle. Knowing this, you can apply the formula pr2 to obtain the answer. Now you have the pieces you need, so you can continue to steps 3 and 4, which are called the Program Development Cycle.
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