If I have
Code:
#define PI 3.14159
To re-define PI, do I need to #undef it or can I just put a new #define in?

Someone on this board said that this was permissible:
Code:
#define PI 3.14159
/*#undef PI*/
#define PI 3.14159265358979
But my compiler complains.

Is the #undef required?