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Today’s Objectives

12-Jun-2006. Today’s Objectives. Announcements Turn in Homework #1 Homework #2 is posted and it is due on 21-Jun Review Quiz #1 Pointers and C-style strings (Ch. 5) Pointer variables – declaration and initialization Operators * and & Pass-by-reference using pointers

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Today’s Objectives

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  1. 12-Jun-2006 Today’s Objectives • Announcements • Turn in Homework #1 • Homework #2 is posted and it is due on 21-Jun • Review Quiz #1 • Pointers and C-style strings (Ch. 5) • Pointer variables – declaration and initialization • Operators * and & • Pass-by-reference using pointers • Using const with pointers • Arrays, pointers, and dynamic allocation of arrays with new • C-style strings • C++ string class (Ch. 18) • Using a vector as a data member of a class • Bonus Lab

  2. Review Quiz #1 Answers are posted in the “Files” area on the class discussion group site

  3. Pointers Chapter 8

  4. Pointers and C-Style Strings (Deitel, 402) Regular Variables Variables are used to keep track of data in the computer’s memory. Declaring a regular variable = Allocating a location in memory to store a value • Example:int myInt = 0; //Allocates enough space in memory //to store an int and puts 0 there • The location in memory has an address • Use operator & to get the address • Example:cout << "Address of myInt = " << &myInt << endl;

  5. Pointers and C-Style Strings (Deitel, 402) Pointer Variables A pointer variable is used to hold an address • Use operator * to declare a pointer variable • Example:int *pMyInt; //Declares an empty pointer • An address must be assigned to the pointer variable before it can be used int myInt = 0; //Allocates memory, stores 0 int *pMyInt; //Declares an empty pointer pMyInt = &myInt; //Puts an address in the pointer

  6. Pointers and C-Style Strings (Deitel, 405) Dereferencing a Pointer • Accessing the object addressed by the pointer • Operator * used with the name of the pointer, after it is declared and initialized • Examples int myInt = 0; //Allocates memory, stores 0 int *pMyInt; //Declares an empty pointer pMyInt = &myInt; //Puts address in the pointer cout << *pMyInt << endl; //Prints 0 *pMyInt = 5; //puts 5 into myInt cout << myInt << endl; //Prints 5 cout << *pMyInt << endl; //Also prints 5 cout << pMyInt << endl; //What prints?

  7. Pointers and C-Style Strings Where are the errors? int main(){ int m; int *pm; *pm = 5; int n; int *pn = &n; pn = 5; }

  8. Pointers and C-Style Strings Where are the errors? int main(){ int m; int *pm; *pm = 5; int n; int *pn = &n; pn = 5; } ERROR! No address in pm //Correction pm = &m; *pm = 5;

  9. Pointers and C-Style Strings Where are the errors? int main(){ int m; int *pm; *pm = 5; int n; int *pn = &n; pn = 5; } ERROR! No address in pm //Correction pm = &m; *pm = 5; ERROR! Missing operator* //Correction *pn = 5;

  10. Pointers and C-Style Strings Pointer Exercises Handout

  11. Pointers and C-Style Strings (Deitel, 407) Pass-by-Reference Using Pointers • Pass-by-reference • Means that the function arguments are used to pass data both into and out of a function • Changes to the variable in the function are passed to the calling variable. • One way to do it is by using a reference parameter or “alias” void change( int &rn ){ //rn is a reference rn = rn + rn; } • Pointers can also be used for pass-by-reference void change( int *pn ){ //pn is a pointer *pn = *pn + 1; }

  12. Pointers and C-Style Strings (Deitel, 407) Pass-by-Reference Using Pointers void change( int *pn ){ //pn is a pointer *pn += *pn; //Dereferencing with operator * } int main(){ int num = 5; change( &num ); //Using operator & cout << num << endl; //What prints? int *pNum; pNum = &num; change( pNum ); //Using a pointer argument cout << num << endl; //What prints? }

  13. Pointers and C-Style Strings (Deitel, 411) Using const with Pointers • We use pass-by-reference to make our programs more efficient. In this approach, no copies of the arguments are made inside the function. • Sometimes we want the efficiency of using pass-by-reference, but we don’t want to change the argument that is passed, so we use “const” to make the argument unchangeable void noChange( const int *pn ){ //Now *pn cannot be changed cout << "Inside the function *pn = " << *pn << endl; } int main(){ int num = 5; noChange( &num ); cout << num << endl;}

  14. Pointers and C-Style Strings (Deitel, 427) Pointers & Arrays The array name is a pointer to the first element const int CAPACITY = 5; int myArray[CAPACITY] = {0}; cout << myArray; //Prints address of myArray[0] cout << &myArray[0]; //Prints the same address int *pMyArray = myArray; //Initialize a pointer myArray[0] = 8; cout << myArray[0] << endl; //Prints 8 cout << *pMyArray << endl; //Prints 8

  15. Pointers and C-Style Strings (Deitel, 424) Pointer Arithmetic When a pointer points to an array, we can use ++, --, +=, and -= to move through the array. int myArray[] = {1,2,3,4,5}; int *pMyArray = myArray; cout << *pMyArray << endl; //Prints 1 pMyArray++; cout << *pMyArray << endl; //Prints 2 pMyArray += 2; cout << *pMyArray << endl; //Prints 4

  16. Pointers and C-Style Strings Dynamically Allocating Arrays • Arrays can be allocated with “new” • “new” allocates a section of memory for the array, and then returns a pointer to it. • Useful when you don’t know how large the array will be until the program is running int capacity, *myArray; cin >> capacity; myArray = new int[capacity];//Creates the array for( int i=0; i<capacity; ++i ) //Initialize it myArray[i] = i; cout << myArray[0] << endl; //Prints 0 delete [] myArray;

  17. Pointers and C-Style Strings Using delete • Memory allocated with “new” must always be recovered by “delete” • Always “delete” an array when you don’t need it anymore if you created it with “new” delete [] myArray;

  18. C-Style Strings Chapter 8

  19. J o h n \0 Pointers and C-Style Strings (Deitel, 443) C-style Strings • In C, strings are treated as arrays of type char that end with the null character, ‘\0’. This also works in C++ • There are several ways to initialize them char *beatle1 = "John"; char beatle2[5] = "Paul"; char beatle3[] = {'G','e','o','r','g','e','\0'}; cout << "The first three Beatles were " << beatle1 << " " << beatle2 << " " << beatle3 << endl; • If you use the char* syntax, you can change the string beatle1 = "Ringo"; 4 3 2 0 1

  20. Pointers and C-Style Strings (Deitel, 443) Input of C-style Strings • Strings can be input using cin char beatle[50]; cout << "Enter a Beatle: "; cin >> beatle; cout << "One of the Beatles was " << beatle; • Better method is to limit the size of the input const char CAPACITY = 50; char buffer[CAPACITY]; cout << "Enter all four Beatles "; cin.getline( buffer, CAPACITY ); cout << buffer << endl;

  21. Pointers and C-Style Strings (Deitel, 446) Functions for C-style Strings //Copy one string to another char marx[] = "Groucho"; strcpy( marx, "Karl"); //Compare two strings char marx1[] = "Harpo"; char marx2[] = "Chico"; int result = strcmp( marx1, marx2 );//0 if equal //Length of a string int length = strlen( marx1 );//returns 5

  22. Pointers and C-Style Strings (Deitel, 884) C++ string Class • A better, object-oriented approach #include <string> using namespace std; int main(){ string input, secret="UHCL", welcome="Welcome!"; string greeting = "Enter the password"; cout << greeting << endl; cout << "It has " << secret.size() << " letters."; cin >> input; if( input == secret ){ greeting = welcome; cout << greeting; } }

  23. Using a vector as a data member of a class Homework 2, Problem #3

  24. Using a vector as a data member Homework 2, Problem #3 • Implement the classes for Customer and Store as shown in the class diagram • Each class shall be in its own header file • Customer.h and Store.h • Create an object-oriented C++ program that implements the actions of the following menu items 1 View customer list 2 Search for a customer 3 Add a customer • Use the Gigaplex Menu program as the driver program • Add a new menu item called “Add a customer” • Instantiate a Store object in main. When the user selects “View customer list,” “Search for a customer,” or “Add a customer,” the program shall get any input from the user that might be required, and then call the appropriate operation of the Store object.

  25. Using a vector as a data member UML Class Diagram Class name (The top section) Data members are in the middle section. Member functions are in the bottom section. Datatype class Customer { private: string ID; string firstName; string lastName; public: Customer(); string getID() const { return ID; } void setID( string id ) { ID = id; } }; Customer – ID : string – firstName : string – lastName : string + Customer() + getID() : string + setID( in id : string ) Scope private public Return datatype Parameter

  26. Customer - ID : string - firstName : string - lastName : string + Customer() + getID() : string + setID(in id : string ) Using a vector as a data member UML Class Diagram Composition The “has-a” relationship Multiplicities 1 Store objecthas 0, 1, or many Customers Store - customers : vector<Customer> +viewCustomerList() +searchForCustomer(in id: string ) : Customer* +addCustomer(in cust : Customer& ) 1 0..* class Store{ private: //The Store has-a Customer vector<Customer> customers; public: void viewCustomerList(); Customer* searchForCustomer( string id ); void addCustomer( Customer &cust ); };

  27. Using a vector as a data member Instantiating a vector Object #include <vector> using namespace std; int main(){ vector<char> coll; Instantiate a vector object that will hold char data The object name Name of the class Template parameter – type of data the vector will hold

  28. Using a vector as a data member Adding Data to a vector Object #include <vector> using namespace std; int main(){ vector<char> coll; coll.push_back('c'); coll.push_back('a'); coll.push_back('b'); Add some data If we need to print the data, we can loop through the data in the vector by using an iterator.

  29. Using a vector as a data member (Josuttis, 83–86) STL Iterators • An iterator is a class used to create objects that give us access to the elements inside a container, such as a vector • They are called “iterators” because they are often used to sequentially iterate or “loop” through all the elements in a container • All container classes have their own iterators that are implemented as part of the container class

  30. Using a vector as a data member (Josuttis, 83–86) Using an STL Iterator vector<char> coll; coll.push_back('c'); coll.push_back('a'); coll.push_back('b'); Create a vector of chars and put some chars in it

  31. Using a vector as a data member (Josuttis, 83–86) Using an STL Iterator vector<char> coll; coll.push_back('c'); coll.push_back('a'); coll.push_back('b'); vector<char>::iterator pos; Instantiate an iterator that can be used with a vector of chars

  32. Using a vector as a data member (Josuttis, 83–86) Using an STL Iterator vector<char> coll; coll.push_back('c'); coll.push_back('a'); coll.push_back('b'); vector<char>::iterator pos; for( pos = coll.begin(); pos != coll.end(); ++pos) Create a for loop

  33. Using a vector as a data member (Josuttis, 83–86) Using an STL Iterator vector<char> coll; coll.push_back('c'); coll.push_back('a'); coll.push_back('b'); vector<char>::iterator pos; for( pos = coll.begin(); pos != coll.end(); ++pos) Initialization Assign a starting value to the iterator Every collection class has a begin() member function that returns an iterator representing its first element.

  34. Using a vector as a data member (Josuttis, 83–86) Using an STL Iterator vector<char> coll; coll.push_back('c'); coll.push_back('a'); coll.push_back('b'); vector<char>::iterator pos; for( pos = coll.begin(); pos != coll.end(); ++pos) Condition Loop is executed only if this is true Every collection class has a end() member function that returns an iterator representing the position after the last element.

  35. Using a vector as a data member (Josuttis, 83–86) Using an STL Iterator vector<char> coll; coll.push_back('c'); coll.push_back('a'); coll.push_back('b'); vector<char>::iterator pos; for( pos = coll.begin(); pos != coll.end(); ++pos) In the expression evaluated at the end of each loop, the iterator behaves like a pointer.

  36. Using a vector as a data member (Josuttis, 83–86) Using an STL Iterator vector<char> coll; coll.push_back('c'); coll.push_back('a'); coll.push_back('b'); vector<char>::iterator pos; for( pos = coll.begin(); pos != coll.end(); ++pos) { cout << *pos << " "; } In the loop, we can use the iterator like a pointer again, so that we can get the value stored at this position.

  37. Bonus Lab #2Using a vector as a data member in the GradeBook class • Optional • Procedure • First, a demo by the instructor • Then you will complete the assignment on your own • Print it and hand it in before the end of class • Rules • Do your own work, but you may help each other • You may ask the instructor for help • You may leave if you finish early or if you do not wish to do this assignment • 1 bonus point added to a quiz grade for each correctly completed lab handed in before the end of class

  38. References Deitel, H. M., and P. J. Deitel, C++ How to Program, Fifth Edition. Upper Saddle River, NJ: Prentice Hall, 2005. Josuttis, Nicolai M., The C++ Standard Library, A Tutorial and Reference. Boston: Addison-Wesley, 1999.

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