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CSc212 AB Data Structures Lecture 10

CSc212 AB Data Structures Lecture 10. M. P. Johnson mpjohnson@gmail.com http://www-cs.ccny.cuny.edu/~mjohnson/212/ City College of New York, Spring, 2005. Outline: review session. Review some material Interspersed with sample problems Discuss homework Discuss exam topics

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CSc212 AB Data Structures Lecture 10

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  1. CSc212AB Data StructuresLecture 10 M. P. Johnson mpjohnson@gmail.com http://www-cs.ccny.cuny.edu/~mjohnson/212/ City College of New York, Spring, 2005 M.P. Johnson, DS, CCNY, Spring 2005

  2. Outline: review session • Review some material • Interspersed with sample problems • Discuss homework • Discuss exam topics • More sample problems • Take questions M.P. Johnson, DS, CCNY, Spring 2005

  3. Chapter 1 • The Phases of Software Development • Basic design strategy • Pre-conditions and post-conditions • Run time analysis M.P. Johnson, DS, CCNY, Spring 2005

  4. Basic Design Strategy • Basic Design Strategy – four steps (Reading: Ch.1 ) • Specify Input/Output (I/O) • Design data structures and algorithms • Implement in a language such as C++ • Test and debug the program (Reading Ch 1.3) • Design Technique • Decomposing the problem M.P. Johnson, DS, CCNY, Spring 2005

  5. Specifications: pre and post-condition • Frequently a programmer must communicate precisely what a function accomplishes, without any indication of how the function does its work • One way to specify such requirements is with a pair of statements about the function. • The precondition statement indicates what must be true before the function is called. • The postcondition statement indicates what will be true when the function finishes its work. M.P. Johnson, DS, CCNY, Spring 2005

  6. Run time analysis • Counts the number of basic operations of the program • Big-O Notation – the order of the algorithm • Use the largest term in a formula • Ignore the multiplicative constant • Worse case analysis M.P. Johnson, DS, CCNY, Spring 2005

  7. Example Q: • Write the simplest big-O expression to describe the number of operations required for the following algorithm: for (i = 1; i < N; ++i) { ...statements that require exactly 7 operations... } M.P. Johnson, DS, CCNY, Spring 2005

  8. Q • Write the simplest big-O expression to describe the number of operations required for the following algorithm: for (i = 1; i < N; ++i) { ...statements that require exactly N operations... } M.P. Johnson, DS, CCNY, Spring 2005

  9. Q • Write the simplest big-O expression to describe the number of operations required for the following algorithm: for (i = 1; i < N; ++i) { ...statements that require exactly i operations... } M.P. Johnson, DS, CCNY, Spring 2005

  10. Q • Which of these is used to stop the program execution when a precondition is not met. • A. assert(); • B. exit(); • C. return(); • D. void(); M.P. Johnson, DS, CCNY, Spring 2005

  11. Q • Which of the following formulas in big-O notation best represent the expression n²+35n+6? • A. O(n³) • B. O(n²) • C. O(n) • D. O(42) M.P. Johnson, DS, CCNY, Spring 2005

  12. Q • Answer true or false for this statement: True or false: An algorithm with complexity O(3n) takes at least 30 operations for every input of size n=10. • TRUE. • FALSE. M.P. Johnson, DS, CCNY, Spring 2005

  13. Chapter 2 outline A Review of C++ Classes (Lecture 2) • OOP, ADTs and Classes • Class Definition, Implementation and Use • Constructors and Value Semantics More on Classes (Lecture 3) • Namespace and Documentation • Classes and Parameters (value, reference, const reference) • Operator Overloading M.P. Johnson, DS, CCNY, Spring 2005

  14. Summary of classes • Classes have member variables and member functions. An object is a variable whose data type is a class. • You should know how to declare a new class type, how to implement its member functions, how to use the class type. • Frequently, the member functions of an class type place information in the member variables, or use information that's already in the member variables. M.P. Johnson, DS, CCNY, Spring 2005

  15. Constructors • Constructor is a member function which • automatically called whenever a variable of the class is declared • the name must be the same as the class name • arguments must be given after the variable name (when declared) • A way to improve the initialize function • by providing an initialization function that is guaranteed to be called • You may declare as many constructors as you like – one for each different way of initializing an object • Each constructor must have a distinct parameter list so that the compiler can tell them part M.P. Johnson, DS, CCNY, Spring 2005

  16. (Automatic) Default constructor • What happens if you write a class without any constructors? • The compiler automatically creates a simple default constructor • which only calls the default constructors for the member variables that are objects of some other classes • Programming Tip: Always provide your own constructors, and better with a default constructor M.P. Johnson, DS, CCNY, Spring 2005

  17. Value semantic of a class • Value semantics determines how values are copied from one object to another • Consists of two operations in C++ • The assignment operator • The copy constructor • Document the value semantics • When you implement an ADT, the document should include a comment indicating that the value semantics is safe to use. M.P. Johnson, DS, CCNY, Spring 2005

  18. Q: Constructors & assignment op • point p1; • // Default constructor • point p2(p1); • //Copy constructor • point p3 = p2; • //Copy constructor • p3 = p1; • //Assignment operator M.P. Johnson, DS, CCNY, Spring 2005

  19. Classes and parameters • Default parameters • when no or only part of the parameters are provided in calling function • Types of parameters • value parameters • reference parameters • constant reference parameters • Return value is a class M.P. Johnson, DS, CCNY, Spring 2005

  20. Default arguments • A default argument is a value that will be used for an argument when a programmer does not provide an actual argument when calling a function • Default arguments may be listed in the prototype of a function • Syntax: Type_name var_name = default_value M.P. Johnson, DS, CCNY, Spring 2005

  21. Value parameters • A value parameter is declared by writing • type-nameparameter-name • Any change made to the formal parameter within the body of the function does not change the actual argument from the calling program • The formal parameter is implemented as a local variable of the function, and the class’s copy constructoris used to initialize the formal parameter as a copy of the actual argument M.P. Johnson, DS, CCNY, Spring 2005

  22. Reference parameters • A reference parameter is declared by writing • type-name &parameter-name • Any use of the formal parameter within the body of the function will access the actual argument from the calling program; change made to the parameter in the body of the function will alter the argument • The formal parameter is merely another name of the argument used in the body of the function! M.P. Johnson, DS, CCNY, Spring 2005

  23. const reference parameters • A const reference parameter is declared by writing • const type-name &parameter-name • A solution that provides the efficiency of a reference parameter along with the security of a value parameter. • Example ( newpoint.cxx) • double distance (const point& p1, const point& p2) • point p1 and p2 cannot be changed (TEST!) M.P. Johnson, DS, CCNY, Spring 2005

  24. Class as return value • The type of a function’s return value may be a class • Often the return value will be stored in a local variable of the function (such as midpoint), but not always (could be in a formal parameter) • C++ return statement uses the copy constructor to copy the function’s return value to a temporary location before returning the value to the calling program • Example ( Ch 2.4, Look into newpoint.cxx) • point middle(const point& p1, const point& p2) M.P. Johnson, DS, CCNY, Spring 2005

  25. Operator Overloading • Binary functions and binary operators • Overloading arithmetic operations • Overloading binary comparison operations • Overloading input/output functions • Friend functions – when to use M.P. Johnson, DS, CCNY, Spring 2005

  26. Chapter 2 sample questions • Write one clear sentence telling me when it would be appropriate to use a const reference parameter. • Suppose that you define a new class called foo. For two foo objects x and y, you would like the expression x+y to be a new foo object. What is the prototype of the function that you must write to enable expressions such as x+y? . M.P. Johnson, DS, CCNY, Spring 2005

  27. Q • Which kind of functions can access private member variables of a class? • A. Friend functions of the class • B. Private member functions of the class • C. Public member functions of the class • D. All of the above • E. None of the above M.P. Johnson, DS, CCNY, Spring 2005

  28. Q • Here is the start of a class declaration: class foo { public: void x(foo f); void y(const foo f); void z(foo f) const; ... Which of the three member functions can alter the PRIVATE member variables of the foo object that activates the function? M.P. Johnson, DS, CCNY, Spring 2005

  29. Chapter 3 – container classes • A container classis a data type that is capable of holding a collection of items. • In C++, container classes can be implemented as a class, along with member functions to add, remove, and examine items. • Bag and sequence class using partially filled static array. M.P. Johnson, DS, CCNY, Spring 2005

  30. The invariant of a class • Two rules for our bag implementation • The number of items in the bag is stored in the member variable used; • For an empty bag, we don’t care what is stored in any of data; for a non-empty bag, the items are stored in data[0] through data[used-1], and we don’t care what are stored in the rest of data. • The rules that dictate how the member variables of a (bag) class are used to represent a value (such as a bag of items) are called the invariant of the class M.P. Johnson, DS, CCNY, Spring 2005

  31. run time analysis of the bag class • count – the number of occurrence • erase_one – remove one from the bag • erase – remove all • += - append • b1+b2 - union • insert – add one item • size – number of items in the bag M.P. Johnson, DS, CCNY, Spring 2005

  32. Containers - summary • A container class is a class that can hold a collection of items. • Container classes can be implemented with a C++ class. • The class is implemented with • a header file (containing documentation and the class definition) bag1.h and • an implementation file (containing the implementations of the member functions) bag1.cxx. • Other details are given in Section 3.1, which you should read, especially the real bag code M.P. Johnson, DS, CCNY, Spring 2005

  33. Chapter 3 – sample questions • Suppose that I have the following declarations: int data[100]; size_t i; Write a small segment of C++ code that will shift data[51]...data[99] down one spot to the locations data[50]...data[98]. The value of data[99] remains at its original value. M.P. Johnson, DS, CCNY, Spring 2005

  34. Q • Suppose that the bag class is efficiently implemented with a fixed array with a capacity of 4000, as in Chapter 3 of the class text. We execute these statements: • bag b; • b.insert(5); • b.insert(4); • b.insert(6); • b.erase_one(5); • What are the values of used and data afterward? M.P. Johnson, DS, CCNY, Spring 2005

  35. Chapter 4: pointers and dynamic arrays • Pointers • *(asterisk) and &(ampersand) operators • Dynamic Variables and new Operator • Dynamic Arrays and Dynamic Objects • Stack (local) vs. heap (dynamic) memory • Garbage Collection and delete Operator • Parameters revisited • Pointers and Arrays as Parameters M.P. Johnson, DS, CCNY, Spring 2005

  36. Operator * Pointer declaration int *i_ptr; dereferencing operator cout << *i_ptr; Two different meanings! Operator & Reference parameter void funct(int& i); “address of ” operator i_ptr = &i; Just coincidence? Will see in parameter passing Operators * and & M.P. Johnson, DS, CCNY, Spring 2005

  37. Assignment Operators with Pointers • p1 = p2 • *p1 = *p2 M.P. Johnson, DS, CCNY, Spring 2005

  38. Use array notation the 1st entry p1[0] = 18; the 3rd entry p1[2] = 20; the ith entry p1[i-1] = 19; Use pointer notation the 1st entry *p1 = 18; the 3rd entry *(p1+2) = 20; the ith entry *(p1+i-1) = 19; Accessing Dynamic Array M.P. Johnson, DS, CCNY, Spring 2005

  39. Failure of the new Operator • Dynamic memory via new operator comes from heap of a program • Heap size from several K to 1GB, however fixed • Could run out of room therefore cause a bad_alloc exception • error message and program halts • Good practice 1: document which functions uses new • Good practice 2: garbage collection by delete operator M.P. Johnson, DS, CCNY, Spring 2005

  40. new & delete • Release any dynamic memory (heap memory) that is no longer needed int *i_ptr; double *d_ptr; point *p_ptr; i_ptr = new int; d_ptr = new double[20]; p_ptr = new point(1, 2); //… … delete i_ptr; //empty brackets delete [] d_ptr; delete p_ptr; • Questions( true or false): • delete resets these pointers • delete removes dynamic objects pointed by the pointers • nothing happens to the pointers themselves X V V M.P. Johnson, DS, CCNY, Spring 2005

  41. Pointers and Arrays as Parameters • Value parameters that are pointers • Array parameters • Pointers and arrays as const parameters • Reference parameters that are pointers M.P. Johnson, DS, CCNY, Spring 2005

  42. Array Parameters • Compare ordinary and Dynamic arrays void make_all_20(int data[], size_t size) { for (int i = 0 ; i< size; i++) data[i] = 20; } Calling programs: int *ages; ages = new int[30] make_all_20(ages, 30); int ages[30]; make_all_20(ages, 30); • An array parameter automatically treated as pointer to the first entry (– value or reference?) • In the function prototype and implementation, size of the array is not specified inside bracket but by another parameter M.P. Johnson, DS, CCNY, Spring 2005

  43. Dynamic Classes New Features (Ch 4.3–4) • Pointers Member Variables • Dynamic Memory Allocation (where and how) • Value Semantics (what’s new?) • assignment operator overloading • your own copy constructor • Introducing Destructor • Conclusion: the Law of the Big Three M.P. Johnson, DS, CCNY, Spring 2005

  44. The Static bag The Dynamic bag Pointer Member Variable // From bag1.h in Sect. 3.1 class bag { public: static const size_t CAPACITY = 20; //... private: value_type data[CAPACITY]; size_type used; }; // From bag2.h class bag { public: //... private: value_type *data; size_type capacity; size_type used; }; M.P. Johnson, DS, CCNY, Spring 2005

  45. Invariant of the Dynamic bag • the number of items is in the member variable used • The actual items are stored in a partially filled array. The array is a dynamic array, pointed to by the pointer variable data • The total size of the dynamic array is the member variable capacity • Invariant is about rules of implementation... M.P. Johnson, DS, CCNY, Spring 2005

  46. Allocate Dynamic Memory: Where? • In Old Member Functions • constructor – how big is the initial capacity? • insert – if bag is full, how many more? • +/+= operators – how to combine two bags? • New Member Functions • resize – explicitly adjust the capacity • Example • constructor with default size M.P. Johnson, DS, CCNY, Spring 2005

  47. Allocate Dynamic Memory: How? // From bag2.h class bag { public: static const size_t DEFAULT_CAPACITY = 20; bag(size_type init_cap = DEFAULT_CAPACITY); //... private: value_type *data; size_type capacity; size_type used; }; • Constructor: • why initialize? • Default • specific size // From implementation file bag2.cxx bag::bag(size_type init_cap) { data = new value_type[init_cap]; capacity = init_cap; used = 0; } M.P. Johnson, DS, CCNY, Spring 2005

  48. Value Semantics • Copy constructor • bag y = x; // or: • bag y(x); • Assignment operator • y = x; • Automatic assignment operator and copy constructor • copy all the member variables (data, used, capacity) from object x to object y • but our days of easy contentment are done! M.P. Johnson, DS, CCNY, Spring 2005

  49. Failure in auto assignment operator capacity used data bag x(4), y(5); x.insert(18); x.insert(19); y=x; x.insert(20); x [0] [1] [2] [3] y [0] [1] [2] [3] [4] Question: What will happen after executing code on left? M.P. Johnson, DS, CCNY, Spring 2005

  50. Failure in auto assignment operator capacity used data bag x(4), y(5); x.insert(18); x.insert(19); y=x; x.insert(20); x [0] [1] [2] [3] y [0] [1] [2] [3] [4] Question: What will happen after executing code on left? M.P. Johnson, DS, CCNY, Spring 2005

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