Why #define is used in C?

#define is preprocessor command used to define constants and aliases. Look at this example:

#include<stdio.h>  
#define PI 3.14 

#define ERROR_1 "File not found." 

#define QUOTE "Hello World!"  
main() 

{  

printf("Area of circle = %f * diameter", PI );  

printf("\nError : %s",ERROR_1); 

printf("\nQuote : %s\n",QUOTE); 

system("pause");

} 

Output results:

Area of circle = 3.140000 * diameter

Error : File not found.

Quote : Hello World!

Preprocessor step is performed before compilation and it will change the actual source code to below code. Compiler will see the program as below one:

#include<stdio.h>
 main()
 {  
  printf("Area of circle = %f * diameter", 3.14 );
  printf("\error : %s","File not found.");
  printf("\nQuote : %s","Hello World!\n");  
  system("pause");
 } 

Final Word

In brief #define allows us to define symbolic constants. We usually use uppercase names for #define variables. Pay attention that we do not use ';' after preprocessor statements.
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