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Friday, October 27, 2006

The C Program

About C
As a programming language, C is rather like Pascal or Fortran. Values are stored in variables. Programs are structured by defining and calling functions. Program flow is controlled using loops, if statements and function calls. Input and output can be directed to the terminal or to files. Related data can be stored together in arrays or structures.

Of the three languages, C allows the most precise control of input and output. C is also rather more terse than Fortran or Pascal. This can result in short efficient programs, where the programmer has made wise use of C's range of powerful operators. It also allows the programmer to produce programs which are impossible to understand.

Programmers who are familiar with the use of pointers (or indirect addressing, to use the correct term) will welcome the ease of use compared with some other languages. Undisciplined use of pointers can lead to errors which are very hard to trace. This course only deals with the simplest applications of pointers.

It is hoped that newcomers will find C a useful and friendly language. Care must be taken in using C. Many of the extra facilities which it offers can lead to extra types of programming error. You will have to learn to deal with these to successfully make the transition to being a C programmer.

This course teaches C under the UNIX operating system. C programs will look similar under any other system (such as VMS or DOS), some other features will differ from system to system. In particular the method of compiling a program to produce a file of runnable code will be different on each system.

The UNIX system is itself written in C. In fact C was invented specifically to implement UNIX. All of the UNIX commands which you type, plus the other system facilities such as password checking, lineprinter queues or magnetic tape controllers are written in C.

In the course of the development of UNIX, hundreds of functions were written to give access to various facets of the system. These functions are available to the programmer in libraries. By writing in C and using the UNIX system libraries, very powerful system programs can be created. These libraries are less easy to access using other programming languages. C is therefore the natural language for writing UNIX system programs.

A Very Simple Program
This program which will print out the message This is a C program

#include

main()
{
printf("This is a C program\n");
}
Though the program is very simple, a few points are worthy of note.

Every C program contains a function called main. This is the start point of the program.

#include allows the program to interact with the screen, keyboard and filesystem of your computer. You will find it at the beginning of almost every C program.

main() declares the start of the function, while the two curly brackets show the start and finish of the function. Curly brackets in C are used to group statements together as in a function, or in the body of a loop. Such a grouping is known as a compound statement or a block.

printf("This is a C program\n");
prints the words on the screen. The text to be printed is enclosed in double quotes. The \n at the end of the text tells the program to print a newline as part of the output.

Most C programs are in lower case letters. You will usually find upper case letters used in preprocessor definitions (which will be discussed later) or inside quotes as parts of character strings. C is case sensitive, that is, it recognises a lower case letter and it's upper case equivalent as being different.

While useful for teaching, such a simple program has few practical uses. Let us consider something rather more practical. The following program will print a conversion table for weight in pounds (U.S.A. Measurement) to pounds and stones (Imperial Measurement) or Kilograms (International).

Posted by:
Kimberly S. Jimenez