#Ifndef

 It is a preprocessor keyword.

 Normally it is used to add or delete some parts of code based on the inexistence of a certain           definition. 

Example 1 :- 

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#include <stdio.h> #include <stdlib.h> #define log int main() { int x =1 ; printf("%d\n",x); #ifndef log printf("Hello world!\n"); #else printf("Hello Earth\n"); #endif // log return 0 ; }

Example 2 :- 

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#include <stdio.h> #include <stdlib.h> int main() { int x =1 ; printf("%d\n",x); #ifndef log printf("Hello world!\n"); #else printf("Hello Earth\n"); #endif // log return 0 ; }

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