C Tutorial/Preprocessor/ifndef

ifndef

#ifndef is used to check whether a particular symbol is defined.

<source lang="cpp">#define USA 1 //#define EUP 1

1. include <stdio.h>
2. ifndef USA

  #define currency_rate 100    

1. endif
2. ifndef EUP

  #define currency_rate 46

1. endif

main() {

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

}</source>

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