C/Macro Preprocessor/ifndef

#ifndef Demo

<source lang="cpp">

//file1.h

1. define USD 1

//file2.h

1. define UKP 1

//file3

1. include <stdio.h>
2. include <file1.h>
3. ifndef USD

      #define currency_rate 100 

1. endif
2. ifndef UKP

  #define currency_rate 46 

1. endif

main() {

 int rs;
 rs = 10 * currency_rate; 
 printf ("%d\n", rs);

}

      </source>