Chapter 11

1. Chapter 11 Class

2. Student Record • Student ID • First name • Last name • Gender • major • GPA • How do we store the data?

3. Store Student Records in Variables // For one student at a time float GPA; string firstName, lastName; string major; long id; char gender; …

4. Store Student Records in Parallel Arrays const int MAX_STUDENTS = 100; long id [MAX_STUDENTS]; string firstName[MAX_STUDENTS]; string lastName [MAX_STUDENTS]; char gender [MAX_STUDENTS]; string major [MAX_STUDENTS]; float gpa [MAX_STUDENTS];

5. floating address float double long double pointer reference C++ Data Types simple structured integral enum array struct union class char short int long bool

6. Store Student Records in Class // “Student” is the identifier of this class class Student { private: long id; string firstName; string lastName; char gender; string major; float gpa; public: long GetID ( ) { ... } void SetID ( long id ) { ... } ... }; // indicates following variables are only accessible within class Student // data fields (something new in class) // indicates following methods can be used outside class Student // methods (something new in class) // semicolon after }

7. C++ Class • Syntax: • class, private, and public are key words. • A class usually has two types of members: data fields and methods. • Data fields should always be private! • Method can be private or public. (We will explain this later.) class ClassName { private: DataMemberList; MethodMemberList1; public: MethodMemberList2; };

8. Class Object // Student is a new data type! class Student { private: long id; string firstName; ... }; Student stu1; // stu1 is an object of Student. // A class object is a variable of the class! // A Student object is a variable of Student data type!

9. Class Methods class Student { private: long id; ... public: ... void PrintStudentRecord () { cout << setprecision(2) << fixed << endl; cout << setw(8) << id << setw(15) << firstName << setw(15) << lastName << setw(8) << gender << setw(20) << major << setw(8) << gpa; } ... }; How to call this function? You can only call this function if it is public! Declare a class object and then Use the dot notation!

10. Call a Class Method int main() { Student stu; ... // assume all data fields in stu are filled with values stu.PrintStudentRecord(); ... return 0; } // Declare a Student object first. // Use DOT notation.

11. Didn’t we already know classes?! cin >> base; while ( !cin.eof() && (base < 2 || base > 9) ) { // display message cin.ignore(MAX_LINE_SIZE, ‘\n’); cin >> base: } string str1, str2; str1 = “”; while(str1.length() < MAX_LENGTH) { cin >> str2; str1 += str2; }

12. Class Scope • The identifier of a class member has class scope and can only be used in the following cases: • In a member function of that class • After the . (dot) operator applied to an object of that class (if it is declared as public) • Other situations (not covered in this class) • An identifier declared within a member function hides a declaration of the same identifier whose scope extends to or past the end of the member function's class.

13. Use a data field in a member function class Student { private: long id; string firstName; string lastName; char gender; string major; float gpa; public: void PrintStudentRecord() { cout << "Name : " << firstName<< " " << lastName << endl << "Gender: " << gender << endl << "Major : " << major << endl << "GPA : " << gpa << endl; } ... }; firstName, etc are data fields of Class Student: they can be used directly in member functions of Class Student!

14. Call a member function after DOT void main() { Student stu1; ... stu1.PrintStudentRecord(); ... return 0; } PrintStudentRecordis a public method of Class Student: it can be used anywhere outside Class Student by DOT notation.

15. Access private data in a class class Student { private: long id; string firstName; string lastName; char gender; string major; float gpa; ... } // The data fields are private to a Student object. What if we want to use their values in the main function? int main() { Student stu1; ... cout << stu1.id; ... } No! DOT notation cannot be used to access private data or methods!

16. Access private data in a class class Student { private: long id; string firstName; string lastName; char gender; string major; float gpa; public: long GetID() { return id; } } // The data fields are private to a Student object. What if we want to use their values in the main function? int main() { Student stu1; ... cout << stu1.GetID(); ... } Declare a public method to access private data!

17. Input values to data fields Student stu1; ... stu1.firstName = “Alex”; // NO! firstName is private! stu1.SetFirstName(“Alex”); // Yes! Declare a Set method to input values to private data fields! class Student { private: string firstName; ... public: void SetFirstName(string fname) { firstName = fname; } ... }

18. Initialize a class object // Declare variable with initial value int numStudents = 0; // Can we declare a class variable with initial value? Student s1; Use Class Constructors!

19. Class Constructor class Student { private: long id; char gender; string firstName, lastName, major; float gpa; public: Student() { id = 1001; firstName = “Alice”; lastName = “Wonderland”; gender = ‘F’; major = “ComputerScience”; gpa = 3.2; } }; // Syntax for constructor // No returned data type! // Default constructor: no parameters!

20. Class Constructor With Parameters class Student { private: long id; char gender; string firstName, lastName, major; float gpa; public: Student( long stuId, string fName, string lName, char stuGender, string stuMajor, float stuGpa ) { id = stuId; firstName = fName; lastName = lName; gender = stuGender; major = stuMajor; gpa = stuGpa; } }; The parameter names cannot be the same as data fields! why?

21. Multiple Class Constructors class Student { ... public: Student( ) { id = 1001; firstName = "Alice"; lastName = "Wonderland"; gender = 'F'; major = "ComputerScience"; gpa = 3.2; } Student( long id, string fName, string lName, char stuGender, string stuMajor, float stuGpa ) { id = id; firstName = fName; lastName = lName; gender = stuGender; major = stuMajor; gpa = stuGpa; } ... }; • You can define multiple constructors for the same class. • Different Constructors Must Have Different Parameters. • Which one to use depends on the parameters during the object declaration.

22. Invoke Class Constructor • Constructor is called implicitly when class objects are declared. // Default constructor is called Student stu1; // Parameterized constructor is called Student stu2( 1002, "Johnny", "Depp", 'M', "SoftwareEngineering", 2.8 ); //OR //Student stu2; //stu2 = ( 1002, "Johnny", "Depp", 'M', "SoftwareEngineering", 2.8 ); // The default constructor has set the // values for data members stu1.PrintStudentRecord(); // The parameterized constructor has set the // values for data members stu2.PrintStudentRecord(); // This will change the data fields stu1.SetID();

23. Different parameters for different constructors // will this work? class Student { private: long id; ... public: Student( long stuID ) { id = stuID; } Student( long sID) { id = sID; gender = ‘F’; } ... }; NO! YES! // will this work? class Student { private: long id; ... public: Student( long stuID ) { id = stuID; } Student( int stuID ) { id = stuID; } ... }; DIFFERENT refers to different data types and number of parameters, not names!

24. Default Constructor // Default constructor is called Student stu1; • If you don’t define any constructor: • There is a default constructor doing nothing generated by the compiler • If you define a constructor without any parameter: • This is your new default constructor • If you define a constructor with parameters: • There is NO default constructor generated by the compiler anymore!

25. Copy Constructor • Declare an object which is exactly the same as another object: class Student { ... public: Student( Student stu ) { id = stu.id; firstName = stu.firstName; lastName = stu.lastName; gender = stu.gender; major = stu.major; gpa = stu.gpa; } ... }; // Copy constructor: // gets initial values from object stu // Use the DOT notation to access // the data field of stu!

26. Constructor Summary • Define a constructor: • NO returned-data type! • Constructors have the SAME name as the class! • Purpose: initialize a class object • Default constructor: • NO parameters. • If no constructor is defined, a default constructor doing NOTHING will be automatically generated. • If there is any parameterized constructor defined, no automatic generated default constructor anymore. • Copy constructor: • Initialize one object by copying data fields of another object. • Use the DOT notation directly to access data fields of another object. • Multiple constructors: • Different constructors have DIFFERENT parameters! • Different constructors are called based on the parameters during object declaration. • Invoke constructor: • Student stu1; // call the default constructor • Student stu2( 1002, "Johnny", "Depp", 'M', "SoftwareEngineering", 2.8 ); • Student stu4; stu4 = Student( 1004, “Brad”, “Pitt”, ‘M’, “CS”, 3.6 );

27. Array of Class Objects Student stuList[MAX_STUDENTS]; // An array of Student objects • Each element is treated the same way as a Student object! • Everything applied to an integer array applied to an object array!

28. Operations on an object array element Student stuList[MAX_STUDENTS]; • Initialization: stuList[i] = Student(stuList[i-1]); • Assignment: stuList[i].SetID(1001); • Computation: totalGPA += stuList[i].GetGPA();

29. Pass an object array as a parameter Student stuList[MAX_STUDENTS]; // Given a student id, find the index of the student // in the Student array. Return -1 if not found. // Parameters: in, in, in int FindIndex ( const Student sList[], int size, long id ) { for ( int i = 0; i < size; i ++ ) if ( sList[i].GetID() == id ) return i; return -1; }

30. StudentList Class class StudentList { private: Student stuList[MAX_STUDENTS]; int numStudents; public: StudentList() { numStudents = 0; } ... }; // An array of Students // Treat an element of stuList the same way as a Student object! // Initialize a stuList element using constructor. // Print a stuList element by its PrintRecord() method.

31. StudentList Class class StudentList { private: Student stuList[MAX_STUDENTS]; int numStudents; public: StudentList() { numStudents = 0; } StudentList( int size ) { numStudents = 0; Read(size); } void Print() { for( int i = 0; i < numStudents; i++ ) stuList[i].PrintRecord(); } void Read( int size ) { long id; string firstName, lastName, major; char gender; float gpa; if ( size <= MAX_STUDENTS ) while ( numStudents < size ) { cin >> id >> firstName >> lastName >> gender >> major >> gpa; stuList[numStudents] = Student(id, firstName, lastName, gender, major, gpa); numStudents ++; } } }; Read() is used in the constructor before its definition!

32. StudentList Class class StudentList { private: Student stuList[MAX_STUDENTS]; int numStudents; public: void Read( int size ) { long id; string firstName, lastName, major; char gender; float gpa; if ( size <= MAX_STUDENTS ) while ( numStudents < size ) { cin >> id >> firstName >> lastName >> gender >> major >> gpa; stuList[numStudents] = Student(id, firstName, lastName, gender, major, gpa); numStudents ++; } } void Print() { for( int i = 0; i < numStudents; i++ ) stuList[i].PrintRecord(); } StudentList() { numStudents = 0; } StudentList( int size ) { numStudents = 0; if ( size <= MAX_STUDENTS ) Read(size); } };

33. Another way to read into a Student List // Suppose Student has a ReadRecord method void Read( int size ) { if ( size <= MAX_STUDENTS ) while ( numStudents < size ) { stuList[numStudents].ReadRecord(); numStudents ++; } } // This is a better approach because // operations related to one object // should be done by that object! // DECOMPOSITION!!

34. Information Hiding • Hide data fields and supportive methods under private. • Only methods that will be used by other classes or stand alone functions are public. • Treat your class object as a BLACK BOX: Class Object data fields Helper methods are those only used by member methods in the same class. supportive methods public methods

35. Employee Class class Employee { private: string employeeID; string firstName; string lastName; float payRate; float weeklyHours; float weeklySalary; void CalcSalary() { if ( weeklyHours <= OVERTIME ) weeklySalary = payRate * weeklyHours; else weeklySalary = OVERTIME_RATE * payRate * (weeklyHours - OVERTIME ) + payRate * OVERTIME; } public: void Print () { CalcSalary(); cout << "ID : " << employeeID << endl << "Name : " << firstName << " " << lastName << endl << "Pay Rate: " << payRate << endl << "Hours : " << weeklyHours << endl << "Salary : " << weeklySalary << endl << endl; } }

36. Class Method Modifier const class Student { private: long id; string firstName, lastName, major; char gender; float gpa; public: void PrintRecord() { cout << "Name : " << firstName << " " << lastName << endl << "Gender: " << gender << endl << "Major : " << major << endl << "GPA : " << gpa << endl; } void ReadRecord () { cin >> id >> firstName >> lastName >> gender >> major >> gpa; } }; // The method will NOT change any data members: const. // It cannot call a non-const member functions! const // The method will change some data members: no const.

37. Class Method Modifier const class Student { private: long id; string firstName, lastName, major; char gender; float gpa; public: void Write() const float GetGPA() const string GetFirst() const // No const void Read() void SetGPA( float value ) void UpdateGPA( float amount ) . . . };

38. Comparing Objects Student stu1, stu2; stu1.ReadRecord(); stu2.ReadRecord(); // Check if stu1’s GPA is higher than stu2 if ( stu1.GetGPA() > stu2.GetGPA() ) cout << stu1.GetFirstName() << " has higher GPA than " << stu2.GetFirstName() << endl;

39. Comparing Method class Student { ... public: // Use a Student object as a parameter! boolHasHigherGPA ( Student stu ) const { return ( gpa > stu.gpa ); } };

40. Pass Objects as Parameters • Basic Data Types (char, int, float, string, bool) Pass by Value (without & in prototype) Pass by Reference (with & in prototype) • Arrays (of any data type) Always Pass by Reference (Never &) • Class Object (and structure) C++ allows pass by value (no &) and by reference (&) Our Rule: Always pass by reference with & Use const for in parameter

41. Comparing Method class Student { ... public: // Pass by reference // Use const to make sure stu will not be changed boolHasHigherGPA ( const Student&stu ) const { return ( gpa > stu.gpa ); } };

42. Re-Visit Copy Constructor • class Student • { • ... • public: // Copy constructor: // gets initial values from object s // Will change data members: no const. // Will not change parameter s: const with & • Student( const Student& stu ) • { • id = stu.id; • firstName = stu.firstName; • lastName = stu.lastName; • gender = stu.gender; • major = stu.major; • gpa = stu.gpa; • } • ... • };

43. More methods in StudentList class StudentList { ... public: int Find( const Student& stu ) const int Find( long stuID ) const int Find( string fName, string lName ) const // Similar as multiple constructors, a class can also // have multiple methods with the same name, // but DIFFERENT parameters! void GetStats( float& max, float& min, float& average ) const void UpdateGPA ( long id, float gpa ) ... }; // See studentList3.cpp for details

44. Design your class • What should be included in your class (Student)? • treat your class as a real world entity. • attributes related to the class: gpa, major, etc. • operations that can be initiated by the class: read, write, get, set, calculate, etc. • For class Student, should “find”, “add”, “delete”, “sort” be its methods? • NO! They are not initiated and manageable by Student.

45. Summary • class • private and public members • data fields and operations • class constructor • class objects • array of class objects • pass object as a function parameter • class method modifier const • class for a list

46. switch statement const float A_GRADE = 4.0; const float B_GRADE = 3.0; const float C_GRADE = 2.0; const float D_GRADE = 1.0; const float F_GRADE = 0.0; char grade; cin >> grade; cout << endl << "Your grade is " << fixed << setprecision(1); if ( grade == ‘A’) cout << A_GRADE; else if ( grade == ‘B’ ) cout << B_GRADE; else if ( grade == ‘C’ ) cout << C_GRADE; else if ( grade == ‘D’ ) cout << D_GRADE; else if ( grade == ‘F’ ) cout << F_GRADE; else cout << “not valid!”; char grade; cin >> grade; cout << endl << "Your grade is " << fixed << setprecision(1); switch( grade ) { case 'A': cout << A_GRADE; break; case 'B': cout << B_GRADE; break; case 'C': cout << C_GRADE; break; case 'D': cout << D_GRADE; break; case 'F': cout << F_GRADE; break; default : cout << "not valid!"; }

47. switch statement • Syntax: • switch, case and default are key words. • break; is used to stop execution and go directly to the end of the statement block. (optional) switch (IntegralOrEnumExpression) { caseConstantExpression: statement1 ... default: statementN }

48. floating address float double long double pointer reference C++ Data Types simple structured integral enum array struct union class char short int long bool

49. Store Student Records in Structure struct Student { long id; string firstName; string lastName; char gender; string major; float gpa; }; // Now Student is a Data Type!

50. C++ Struct (Record) • Syntax: • Struct is a heterogeneous structured data type: • each individual component is called a field • they can be of different data types • Declare a struct • before the function prototypes • after the constant declarations structTypeName { MemberList; }; Everything needs to be declared before used!