ifdef
The ifdef directive makes substitutions based on whether a particular identifier is defined.
Example
#define USD 1 // first of all we have defined a macro USD
#ifdef USD // here preprocessor cheking, if USD is defined or not
#define currency_rate 50 // if it's defined, currency_rate is equals to 50
#endif // end of ifdef, these three lines are finished now
#include<stdio.h>
int main()
{
int rs;
// currency_rate is defined in preprocessor,
// no need define (int currency_rate)
rs = 10 * currency_rate;
printf ("%d\n", rs);
getchar();
return 0;
}
