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Introduction [cont'd]
Input Output Functions [cont'd]
Structures and Unions [cont'd]
# Preprocessor [cont'd]
#ifndef in # Preprocessor of Programming in C
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#ifndef 
#ifndef is used to find constant is defined or not. If given constant defined already then returns 0 otherwise 1. It is indirect to #ifdef.
Syntax 
#ifndef (condition)
statement;
#endif
Program
	#include
	#ifndef PI
	#define PI 3.14
	#endif
	void main(){
	   printf("%f",PI);
	}
Output 
3.140000
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