How to use #undef to remove a value definition

Remove a value definition

To undefine the value defined by '#define' and specify a new value.

#include <stdio.h>
/*  w w  w .  j  av a  2 s .  c o  m*/
#define VAL 40;     
#undef  VAL          
#define VAL 50      

main()
{
    printf ("%d\n", VAL);  
}

The code above generates the following result.