Pointer Variables

1. Pointer Variables int i; • declares an int variable and sets aside a named memory location to store the int int * iptr; • declares a variable that holds the address of an int; memory is allocated for the address (pointer) but not the int

2. Pointing to an existing int int i = 25; • Allocates memory for an int and stores the valkue 25 int * iptr; • Allocates memory for a pointer to an int iptr = & i; • sets the value of iptr to the address of i • i and *iptr are the same • changing either one will change the other

3. Creating a new int int j = 15; • Allocates momory for an int and stores the value 15 int * jptr; = new int; • sets aside memory for an int and puts the address int j *j = j; • stores value of j in memory pointed to by jptr

4. Pointers and Dynamic arrays • A dynamic array is declared as a pointer int * iArray; • use new to allocate appropriate amount of memory iArray = new int[ desiredSize]; • use iArray just like you use any array iArray[index] = someValue;

5. Changing the size • Allocate a new block of memory with new int *temp = new int[newSize]; • Copy the elements in the current memory for (int j=0; j<currentSize; j++) temp[j] = iArray[i]; • delete the old memory • delete [] iArray; • reassign the pointer iarray = temp;

6. Pointers and the “new” Operator • Pointer Declarations • pointer variable of type “pointer to double” • can store the address of a double t in p double *p; • The new operator creates a variable of type double & puts the address of the variable in pointer p p = new double; • Dynamic allocation - memory is allocated while the program is running instead of before it starts

7. Pointers • Actual address has no meaning • Form: type *variable; • Example: double *p; P ?

8. new Operator • Actually allocates storage • Form: new type; // one memory location new type [n]; // n memory locations • Example: p = new double;

9. Pointer Variables • If they aren't used for arrays, you have to use them differently

10. Accessing Data with Pointers • * - indirection operator *p = 15.5; • Stores floating value 15.5 in memory location *p - the location pointed to by p p 15.5

11. Pointer Statements double *p; p = new doublet; *p = 15.5; cout << “The contents of the memory cell pointed to by p is “ << *p << endl; Output The contents of memory cell pointed to by p is 15.5

12. Pointer Operations • Pointers can only contain addresses • So the following are errors: p = 1000; p = 15.5; • You need to assign an address to p p = &varOfAppropriateType

13. Pointer Operations • Assignment of pointers if q & p are the same pointer type q = p; • p and q both refer to the same memeory location - the same variable • relational operations == and != compare addresses not the values stored at those addresses

14. Pointers to Objects class electric { public: string current; int volts; }; electric *p, *q; • p and q are pointers to objects of type electric

15. Pointers to objects p = new electric; • Allocates storage for struct of type electric and places address into pointer p p current volts ? ?

16. Assignments *p.current = “AC”; *p.volts = 115; • Statements above can also be written • p ->current = “AC”; • p ->volts = 115; p current volts AC 115

17. Member Access via Pointers • From: p ->m • Example: p ->volts • Example: • cout << p->current << p->volts << endl; • Output • AC115

18. Pointers to Objects q = new electric; • Allocates storage for object of type electric and places address into pointer q • Copy contents of p struct to q struct *q = *p; q q->current q->volts AC 115

19. Pointers to Objects q->volts = 220; q = p; q q->current q->volts AC 220 p->current p->volts p q->current q->volts 115 AC q AC 220

20. 4/24/02 Section 2 • Final: Wed May 15, 1:00-3:00 pm • one problem from each of last two exams • practice exams • Program 6 - due May 3 • demos - May 1 - May 8 • anyone who finishes early can make arrangements to come in sooner

21. Appointment Class • Want to write a program to make appointments for demos for program 6 • What should go into Appointment class? • need extra classes • Time • Date • How do we use the class?

22. Appointment.h class Appointment { public: private: } Appointment(); // constructors // set and get functions Time start, end; Date day; String names[][], notes, userID; int howMany;

23. 13.2 Manipulating the Heap • When new executes where is struct stored ? • Heap • C++ storage pool available to new operator • Effect of p = new node; • Figure 14.1 shows Heap before and after executing new operator

24. Effect on new on the Heap

25. Returning Cells to the Heap • Operation • delete p; • Returns cells back to heap for re-use • When finished with a pointer delete it • Watch dual assignments and initialization • Form: delete variable; • Example: delete p;

26. 13.3 Linked Lists • Arrange dynamically allocated structures into a new structure called a linked list • Thinkof a set of children’s pop beads • Connecting beads to make a chain • You can move things around and re-connect the chain • We use pointers to create the same effect

27. Children’s Beads

28. Declaring Nodes • If a pointer is included in a struct we can connect nodes struct node { string word; int count; node *link; }; node *p, *q, *r;

29. Declaring Nodes • Each var p, q and r can point to a struct of type node • word (string) • count (int) • link (pointer to a node address) Struct of type node word count link String Integer Address

30. Connecting Nodes • Allocate storage of 2 nodes p = new node; q = new node; • Assignment Statements p->word = “hat”; p->count = 2; q->word = “top”; q->count = 3;

31. Figure 13.3

32. Connecting Nodes • Link fields undefined until assignment p->link = q; • Address of q is stored in link field pointed to by p • Access elements as follows q->word or p->link->word • Null stored at last link field q->link = NULL; or p->link->link = NULL;

33. Connecting Nodes

34. Inserting a Node • Create and initialize node r = new node; r->word = “the”; r->count = 5; • Connect node pointed to by p to node pointed to by r p->link = r; • Connect node pointed to by r to node pointed to • by q r->link = q;

35. Inserting a New Node in a List

36. Insertion at Head of List • OldHead points to original list head oldHead = p; • Point p to a new node p = new node; • Connect new node to old list head p->link = oldHead;

37. Insertion at Head of List

38. Insertion at End of List • Typically less efficient (no pointer) • Attach new node to end of list last->link = new node; • Mark end with a NULL last->link->link = NULL;

39. Insertion at End of List

40. Deleting a Node • Adjust the link field to remove a node • Disconnect the node pointed to by r p->link = r->link; • Disconnect the node pointed to by r from its successor r->link = NULL; • Return node to Heap delete r;

41. Deleting a Node

42. Traversing a List • Often need to traverse a list • Start at head and move down a trail of pointers • Typically displaying the various nodes contents as the traversing continues • Advance node pointer head = head->link; • Watch use of reference parameters

43. PrintList.cpp // FILE: PrintList.cpp // DISPLAY THE LIST POINTED TO BY HEAD void printList (listNode *head) { while (head != NULL) { cout << head->word << " " << head ->count << endl; head = head->link; } }