1. Computer Science: A Structured Programming Approach Using C1 Objectives ❏ To understand the concept and use of pointers ❏ To be able to declare, define, and initialize pointers ❏ To write programs that access data through pointers ❏ To use pointers as parameters and return types ❏ To understand pointer compatibility, especially regarding pointers to pointers ❏ To understand the role of quality in software engineering Chapter 9 Chapter 9 Pointers Pointers

2. Computer Science: A Structured Programming Approach Using C2 FIGURE 9-1 Derived Types

3. Computer Science: A Structured Programming Approach Using C3 9-1 Introduction A pointer is a constant or variable that contains an address that can be used to access data. Pointers are built on the basic concept of pointer constants. Pointer Constants Pointer Values Pointer Variables Accessing Variables Through Pointers Pointer Declaration and Definition Declaration versus Redirection Initialization of Pointer Variables Topics discussed in this section:

4. Computer Science: A Structured Programming Approach Using C4 FIGURE 9-2 Character Constants and Variables

5. Computer Science: A Structured Programming Approach Using C5 FIGURE 9-3 Pointer Constants

6. Computer Science: A Structured Programming Approach Using C6 Pointer constants, drawn from the set of addresses for a computer, exist by themselves. We cannot change them; we can only use them. Note

7. Computer Science: A Structured Programming Approach Using C7 An address expression, one of the expression types in the unary expression category, consists of an ampersand (&) and a variable name. Note

8. Computer Science: A Structured Programming Approach Using C8 FIGURE 9-4 Print Character Addresses

9. Computer Science: A Structured Programming Approach Using C9 A variable’s address is the first byte occupied by the variable. Note

10. Computer Science: A Structured Programming Approach Using C10 FIGURE 9-5 Integer Constants and Variables

11. Computer Science: A Structured Programming Approach Using C11 FIGURE 9-6 Pointer Variable

12. Computer Science: A Structured Programming Approach Using C12 FIGURE 9-7 Multiple Pointers to a Variable

13. Computer Science: A Structured Programming Approach Using C13 A pointer that points to no variable contains the special null-pointer constant, NULL. Note

14. Computer Science: A Structured Programming Approach Using C14 An indirect expression, one of the expression types in the unary expression category, is coded with an asterisk (*) and an identifier. Note

15. Computer Science: A Structured Programming Approach Using C15 FIGURE 9-8 Accessing Variables Through Pointers

16. Computer Science: A Structured Programming Approach Using C16 FIGURE 9-9 Address and Indirection Operators

17. Computer Science: A Structured Programming Approach Using C17 FIGURE 9-10 Pointer Variable Declaration

18. Computer Science: A Structured Programming Approach Using C18 FIGURE 9-11 Declaring Pointer Variables

19. Computer Science: A Structured Programming Approach Using C19 PROGRAM 9-1Demonstrate Use of Pointers

20. Computer Science: A Structured Programming Approach Using C20 PROGRAM 9-1Demonstrate Use of Pointers

21. Computer Science: A Structured Programming Approach Using C21 FIGURE 9-12 Uninitialized Pointers

22. Computer Science: A Structured Programming Approach Using C22 FIGURE 9-13 Initializing Pointer Variables

23. Computer Science: A Structured Programming Approach Using C23 PROGRAM 9-2Fun with Pointers

24. Computer Science: A Structured Programming Approach Using C24 PROGRAM 9-2Fun with Pointers

25. Computer Science: A Structured Programming Approach Using C25 PROGRAM 9-2Fun with Pointers

26. Computer Science: A Structured Programming Approach Using C26 FIGURE 9-14 Add Two Numbers Using Pointers

27. Computer Science: A Structured Programming Approach Using C27 PROGRAM 9-3Add Two Numbers Using Pointers

28. Computer Science: A Structured Programming Approach Using C28 PROGRAM 9-3Add Two Numbers Using Pointers

29. Computer Science: A Structured Programming Approach Using C29 FIGURE 9-15 Demonstrate Pointer Flexibility

30. Computer Science: A Structured Programming Approach Using C30 PROGRAM 9-4Using One Pointer for Many Variables

31. Computer Science: A Structured Programming Approach Using C31 PROGRAM 9-4Using One Pointer for Many Variables

32. Computer Science: A Structured Programming Approach Using C32 FIGURE 9-16 One Variable with Many Pointers

33. Computer Science: A Structured Programming Approach Using C33 PROGRAM 9-5Using A Variable with Many Pointers

34. Computer Science: A Structured Programming Approach Using C34 PROGRAM 9-5Using A Variable with Many Pointers

35. Computer Science: A Structured Programming Approach Using C35 9-2 Pointers for Inter-function Communication One of the most useful applications of pointers is in functions. When we discussed functions in Chapter 4, we saw that C uses the pass-by-value for downward communication. For upward communication, we normally pass an address. In this section, we fully develop the bi-directional communication. Passing Addresses Functions Returning Pointers Topics discussed in this section:

36. Computer Science: A Structured Programming Approach Using C36 FIGURE 9-17 An Unworkable Exchange

37. Computer Science: A Structured Programming Approach Using C37 FIGURE 9-18 Exchange Using Pointers

38. Computer Science: A Structured Programming Approach Using C38 Every time we want a called function to have access to a variable in the calling function, we pass the address of that variable to the called function and use the indirection operator to access it. Note

39. Computer Science: A Structured Programming Approach Using C39 FIGURE 9-19 Functions Returning Pointers

40. Computer Science: A Structured Programming Approach Using C40 It is a serious error to return a pointer to a local variable. Note

41. Computer Science: A Structured Programming Approach Using C41 9-3 Pointers to Pointers So far, all our pointers have been pointing directly to data. It is possible—and with advanced data structures often necessary—to use pointers that point to other pointers. For example, we can have a pointer pointing to a pointer to an integer.

42. Computer Science: A Structured Programming Approach Using C42 FIGURE 9-20 Pointers to Pointers

43. Computer Science: A Structured Programming Approach Using C43 FIGURE 9-21 Using Pointers to Pointers

44. Computer Science: A Structured Programming Approach Using C44 PROGRAM 9-6Using pointers to pointers

45. Computer Science: A Structured Programming Approach Using C45 PROGRAM 9-6Using pointers to pointers

46. Computer Science: A Structured Programming Approach Using C46 PROGRAM 9-6Using pointers to pointers

47. Computer Science: A Structured Programming Approach Using C47 9-4 Compatibility It is important to recognize that pointers have a type associated with them. They are not just pointer types, but rather are pointers to a specific type, such as character. Each pointer therefore takes on the attributes of the type to which it refers in addition to its own attributes. Pointer Size Compatibility Dereference Type Compatibility Dereference Level Compatibility Topics discussed in this section:

48. Computer Science: A Structured Programming Approach Using C48 PROGRAM 9-7Demonstrate Size of Pointers

49. Computer Science: A Structured Programming Approach Using C49 PROGRAM 9-7Demonstrate Size of Pointers

50. Computer Science: A Structured Programming Approach Using C50 PROGRAM 9-7Demonstrate Size of Pointers

51. Computer Science: A Structured Programming Approach Using C51 FIGURE 9-22 Dereference Type Compatibility

52. Computer Science: A Structured Programming Approach Using C52 Pointers to void A pointer can be typed as void, e.g., void* pVoid The type void with pointers is a generic type that is not associated with a reference type. It is compatible, for assignment purposes, with all pointer types. Thus a pointer to void type can assigned to a pointer of any reference type and a pointer of any reference type can be assigned to a pointer of void type. However a pointer to void type cannot be deferenced unless it is cast. In a previous example we saw the statement pc = &a which was illegal since pc was a pointer to a char and a was type int. It can be made legal as follows: pc = (char*) &a although this is not recommended.

53. Computer Science: A Structured Programming Approach Using C53 Some more examples void* pVoid; char* pChar; int* pInt; pVoid = pChar; pInt = pVoid; pInt = (int*) pChar; The above statements are all legal but not recommended since other operations that use the casted pointer must also be casted or serious problems can arise.

54. Computer Science: A Structured Programming Approach Using C54 A void pointer cannot be dereferenced. Note

55. Computer Science: A Structured Programming Approach Using C55 FIGURE 9-23 Dereference Level Compatibility

56. Computer Science: A Structured Programming Approach Using C56 9-5 Lvalue and Rvalue In C, an expression is either an lvalue or an rvalue. As you know, every expression has a value. But the value in an expression (after evaluation) can be used in two different ways. Pointer Examples Topics discussed in this section:

57. Computer Science: A Structured Programming Approach Using C57 Table 9-1lvalue Expressions

58. Computer Science: A Structured Programming Approach Using C58 The right operand of an assignment operator must be an rvalue expression. Note

59. Computer Science: A Structured Programming Approach Using C59 Table 9-2Operators That Require lvalue Expressions

60. Computer Science: A Structured Programming Approach Using C60 Table 9-3Invalid rvalue Expressions

61. Computer Science: A Structured Programming Approach Using C61 PROGRAM 9-8Convert Seconds to Hours, Minutes, and Seconds

62. Computer Science: A Structured Programming Approach Using C62 PROGRAM 9-8Convert Seconds to Hours, Minutes, and Seconds

63. Computer Science: A Structured Programming Approach Using C63 Create local variables when a value parameter will be changed within a function so that the original value will always be available for processing. Note

64. Computer Science: A Structured Programming Approach Using C64 When several values need to be sent back to the calling function, use address parameters for all of them. Do not return one value and use address Parameters for the others. Note

65. Computer Science: A Structured Programming Approach Using C65 FIGURE 9-24 A Common Program Design

66. Computer Science: A Structured Programming Approach Using C66 FIGURE 9-25 Using Pointers as Parameters

67. Computer Science: A Structured Programming Approach Using C67 PROGRAM 9-9Quadratic Roots

68. Computer Science: A Structured Programming Approach Using C68 PROGRAM 9-9Quadratic Roots

69. Computer Science: A Structured Programming Approach Using C69 PROGRAM 9-9Quadratic Roots

70. Computer Science: A Structured Programming Approach Using C70 PROGRAM 9-9Quadratic Roots

71. Computer Science: A Structured Programming Approach Using C71 PROGRAM 9-9Quadratic Roots

72. Computer Science: A Structured Programming Approach Using C72 PROGRAM 9-9Quadratic Roots

73. Computer Science: A Structured Programming Approach Using C73 PROGRAM 9-9Quadratic Roots

74. Computer Science: A Structured Programming Approach Using C74 PROGRAM 9-9Quadratic Roots

75. Computer Science: A Structured Programming Approach Using C75 9-6 Software Engineering In this chapter, we discuss a general software engineering topic, quality, which can be applied to any topic, including pointers. Quality Defined Quality Factors The Quality Circle Conclusion Topics discussed in this section:

76. Computer Science: A Structured Programming Approach Using C76 Software that satisfies the user’s explicit and implicit requirements, is well documented, meets the operating standards of the organization, and runs efficiently on the hardware for which it was developed. Note

77. Computer Science: A Structured Programming Approach Using C77 FIGURE 9-26 Streams

78. Computer Science: A Structured Programming Approach Using C78 FIGURE 9-27 Streams