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c preprocessor undefine

C #undef:

A macro value in C replaces a part of a program with a value already defined by #define directive. The #undef is a preprocessor directive which is used to undefine the value defined by #define.


#undef macro_Name


#define e 2.718
#undef e
void main()
printf("e :%f\n",e );    


error: 'e' undeclared
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