undef

Introduction

If you want to nullify the effect of the define directive and specify a new value, then you can use the undef directive.

Program

 #include  #define VAL 40;      //A #undef VAL           //B #define VAL 40       //C main() {     printf ("%d\n", VAL);   //D } 

Explanation

1. Statement A defines VAL as 40, that is, an erroneous definition.

2. Statement B indicates that the afore mentioned definition no longer exists.

3. Statement C allows a new definition.

4. Statement D uses new definition of 40.

Point to Remember

The undef directive nullifies the effect of an earlier definition.