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Preprocessor in c language

The C preprocessor is exactly what its name implies. It is a program that processes our source program before it is passed to the compiler. Preprocessor commands (often known as directives) form what can almost be considered a language within C language.

The C Preprocessor is not a part of the compiler, but is a separate step in the compilation process. In simple terms, a C Preprocessor is just a text substitution tool and it instructs the compiler to do required pre-processing before the actual compilation. We'll refer to the C Preprocessor as CPP.

All Preprocessor commands beging with a hash symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in the first column.

Sr.No.	Directive	Description
1	#define	Substitutes a preprocessor macro.
2	#include	Inserts a particular header from another file.
3	#undef	Undefines a preprocessor macro.
4	#ifdef	Returns true if this macro is defined.
5	#ifndef	Returns true if this macro is not defined.
6	#if	Tests if a compile time condition is true.
7	#else	The alternative for #if.
8	#elif	#else and #if in one statement.
9	#endif	Ends preprocessor conditional.
10	#error	Prints error message on stderr.
11	#pragma	Issues special commands to the compiler, using a standardized method.

Example:-

#include <stdio.h>
#define PI 3.14
int main()
{
    int radius;
    float area;
    printf("Enter the radius=");
    scanf("%d",& radius);
    area = PI*radius*radius;
    printf("Area =%f",area);
    getch();
    return 0;
}

Output:

 Enter the rasius 5
 Area= 78.5

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