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Ifdef

The ifdef directive makes substitutions based on whether a particular identifier is defined. #define USA 1 // #define EUP 1 #include <stdio.h> #ifdef USA #define currency_rate 46 #endif #ifdef EUP #define currency_rate 100 #endif main() {     int rs;     rs = 10 * currency_rate;     printf ("%d\n", rs); } 460