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WRITING THE FIRST C PROGRAM
To write a C program, we first required to write the code. For that, open a text editor. If you are a Windows user, you may use Notepad and if you prefer working on UNIX/Linux, you can use emac or vi. Once the text editor is opened on your screen, type the following statements:
#include <stdio.h>
int main()
{
printf("\n Welcome to the world of C ");// prints the message on the screen
return 0;// returns a value 0 to the operating system
}
After writing the code, select the directory of your choice and save the file as first.c.
#include <stdio.h> This is the first statement in our code that includes a file called stdio.h.
This file has some in-built functions. By simply including this file in our code, we can use these functions directly. stdio basically stands for Standard / Input /Output, which means it has functions
for input and output of data like reading values from the keyboard and printing the results on the screen.
int main() Every C program contains a main() function which is the starting point of the program. int is the return value of the main function. After all the statements in the program have been executed, the last statement of the program will return an integer value to the operating system. The concepts will be clear to us when we read this chapter in toto. So even if you do not understand certain things, do not worry.
{ } The two curly brackets are used to group all the related statements of the main function.
printf("\n Welcome to the world of C "); The printf function is defined in the stdio.h file and is used to print text on the screen. The message that has to be displayed on the screen is enclosed within double quotes and put inside brackets.
\n is an escape sequence and represents a newline character. It is used to print the message on a new line on the screen.
Other escape sequences supported by C language are shown below
Table: Escape sequences
Escape Sequence Purpose
\a Audible signal
\b Backspace
\t Tab
\n New line
\v Vertical tab
\f New page\
Clear screen
\r Carriage return
return 0; This is a return command that is used to return value 0 to the operating system to give an indication that there were no errors during the execution of the program.
first.c. If you are a Windows user, then open the command prompt by clicking Start -> Run and typing “command” and clicking Ok. Using the command prompt, change to the directory in which you saved your file and then type:
C:\>tc first.c
In case you are working on UNIX/Linux operating system, then exit the text editor and type
$cc first.c –ofirst
The –o is for the output file name. If you leave out the –o, then the file name a.out is used.
This command is used to compile your C program. If there are any mistakes in the program, then the compiler will tell you what mistake(s) you have made and on which line the error has occurred. In case of errors, you need to re-open your .c file and correct the mistakes. However,
if everything is right, then no error(s) will be reported and the compiler will create an .exe file for your program. This .exe file can be directly run by typing
"first.exe" for Windows and "./first" for UNIX/Linux operating system
When you run the .exe file, the output of the program will be displayed on screen. That is,
Welcome to the world of C
Using Comments
Comments are a way of explaining what a program does. C supports two types of comments.
• // is used to comment a single statement.
• /* is used to comment multiple statements. A /* is ended with */ and all statements that lie between these characters are commented.
Note that comment statements are not executed by the compiler. Rather, they are ignored by the compiler as they are simply added in programs to make the code understandable by programmers as well as other users. It is a good habit to always put a comment at the top of a program that tells you what the program does. This helps in defining the usage of the program the moment you open it.
Standard Header Files
Till now we have used printf() function, which is defined in the stdio.h header file. Even in other programs that we will be writing, we will use many functions that are not written by us. For example, to use the strcmp() function that compares two strings, we will pass string arguments
and retrieve the result. We do not know the details of how these functions work. Such functions that are provided by all C compilers are included in standard header files. Examples of these standard header files include:
* string.h : for string handling functions
* stdlib.h : for some miscellaneous functions
* stdio.h : for standardized input and output functions
* math.h : for mathematical functions
* alloc.h : for dynamic memory allocation
* conio.h : for clearing the screen
All the header files are referenced at the start of the source code file that uses one or more functions from these files.