Questions about dynamic memory allocation

*  What does the following operation do?

	int *p;
	p = new int(4);

* What does the following operation do?

	delete pn;

* What does the following operation do?

	int *p;
	p = new int[20];


* What does the following operation do?

	delete [] p;

* is there a problem with this code?
  if yes, what is the name of this problem?

	delete p;
	*p=2;

* is there s problem with this code?
  if yes, what is the name of this problem?

	int *p=new int(3);
	p=new int(4);