Why should we assign NULL to the elements (pointer) after freeing them? 

This is paranoia based on long experience. After a pointer has been freed, you can no longer use the pointed-to data. The pointer is said to dangle; it doesnt point at anything useful. If you NULL out or zero out a pointer immediately after freeing it, your program can no longer get in trouble by using that pointer. True, you might go indirect on the null pointer instead, but thats something your debugger might be able to help you with immediately. Also, there still might be copies of the pointer that refer to the memory that has been deallocated; thats the nature of C. Zeroing out pointers after freeing them wont solve all problems;