Preprocessor directives are actually the instructions to the compiler itself.
They are operated directly by the compiler.
The most common preprocessor directives are:
- include directive
- define directive
include directive:
The include directive is used to include files.
Example :
#include <stdio.h>
Here, stdio.h is a header file and the preprocessor replace the above line with the contents of header file.
define directive :
It is used to assign names to different constants or statements which are to be used repeatedly in a program.
These defined values or statement can be used by main or in the user defined functions as well.
Preprocessing directive #define has two forms.
- #define identifier token_string
- Macros with argument
NOTE :
- Preprocessor statements begin with a hash symbol(#).
- Preprocessors are NOT terminated by a semicolon.
Define identifier token
token_string : is optional but, are used almost every time in program.
E.g.
#define x 2
Whenever x occurs, it is replaced by the token string 2.
Example :
#include <stdio.h>
#define PI 3.1415
int main(){
int radius;
float area;
printf("Enter the radius : ");
scanf("%d",&radius);
area=PI*radius*radius;
printf("\nArea=%.2f",area);
return 0;
}
Output :
Enter the radius : 2 Area=12.57
Explanation :
The PI is a define directive and is replaced by its value in every occurrence.
Macro with argument
#define can be used to write macro definitions with parameters.
Syntax :
#define identifier(identifier 1,.....identifier n) token_string
Token string: is optional but used in every case.
Example :
#include <stdio.h>
#define PI 3.1415
#define area(r) (PI*(r)*(r))
int main(){
int radius;
float area;
printf("Enter the radius : ");
scanf("%d",&radius);
area=area(radius);
printf("\nArea=%.2f",area);
return 0;
}
Output :
Enter the radius : Area=12.57
Explanation :
Every time the program encounters area(radius), it will be replace by (3.1415*(r)*(r)).