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Engineering Problem Solving with C++, Etter/Ingber

Engineering Problem Solving with C++, Etter/Ingber. Chapter 9 An Introduction to Pointers. An Introduction to Pointers. Addresses and Pointers Pointers to Array Elements Dynamic Memory Allocation Data Structures and the STL. address operator pointer assignment pointer arithmetic.

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Engineering Problem Solving with C++, Etter/Ingber

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  1. Engineering Problem Solving with C++, Etter/Ingber Chapter 9 An Introduction to Pointers Engineering Problem Solving with C++, second edition, J. Ingber

  2. An Introduction to Pointers • Addresses and Pointers • Pointers to Array Elements • Dynamic Memory Allocation • Data Structures and the STL. Engineering Problem Solving with C++, second edition, J. Ingber

  3. address operator pointer assignment pointer arithmetic Addresses and pointers Engineering Problem Solving with C++, second edition, J. Ingber

  4. Addresses and Pointers • A pointer is an object that holds the memory address of another object. • If a variable p contains the address of another variable q, then p is said to point to q. • If q is a variable at location 100 in memory, then p would have the value 100 (q’s address). • Memory snapshot: p q Engineering Problem Solving with C++, second edition, J. Ingber

  5. Address Operator • The operator & is called the address operator. • When the & operator is applied to an object, the result is the address of the object. • Example: int x=75; cout << "x is " << x; cout << "\ntheaddres of x is " << &x; x [0x7fff8164] 75 OUTPUT: x is 75 the address of x is 0x7fff8164 Engineering Problem Solving with C++, second edition, J. Ingber

  6. Pointer Assignment • Pointer types are declared using the pointer operator*, also called the dereferencing operator. • Syntax - • type *variable_name, *variable_name; • type* variable_name; • When declaring more than one pointer variable, the * operator must precede each identifier. Engineering Problem Solving with C++, second edition, J. Ingber

  7. Example int *iPtr; double* dPtr; • The variable iPtr is declared to be of type pointer to int. • The variable dPtr is declared to be of type pointer to double. • Neither variable in this example has been initialized. iPtr dPtr ? ? Engineering Problem Solving with C++, second edition, J. Ingber

  8. Example int *iPtr, i=6;char* s, str[] = "example";double *dPtr, d=1.25; iPtr s dPtr i str d 6 "example" 1.25 Engineering Problem Solving with C++, second edition, J. Ingber

  9. Initialization and Assignment • Pointer types may be initialized at the time they are declared. • Pointer types may be assigned new values using the assignment operator. Engineering Problem Solving with C++, second edition, J. Ingber

  10. Example – Initial Pointers to NULL int *iPtr=0; char *s=NULL; //predefined constant in iostream double *dPtr=NULL; iPtr s dPtr Engineering Problem Solving with C++, second edition, J. Ingber

  11. x xp ip Assignment • The assignment operator (=) is defined for pointers of the same base type. • The right operand of the assignment operator can be any expression that evaluates to the same type as the left operand. Example: int x, *xp, *ip; xp = &x; ip = xp; Engineering Problem Solving with C++, second edition, J. Ingber

  12. The Base Type • The base type of a pointer refers to the type of object the pointer is referencing. • The base type of a pointer defines the size of the object the pointer is referencing. • The size of a pointer is independent of its base type. • p and q are the same ( 4 bytes**), but p points to 4 bytes** and q points to 8 bytes** **compiler dependent Engineering Problem Solving with C++, second edition, J. Ingber

  13. Example: int p(5), *iPtr=&p; double q, *dPtr=&q; p iPtr 5 dPtr q ? ? • sizeof p is 4 • sizeof q is 8 • sizeof iPtr is 4 • sizeof dPtr is 4 Engineering Problem Solving with C++, second edition, J. Ingber

  14. Base Type • A pointers base type determines how the object referenced by the pointer will be interpreted. • The declaration: int *p; declares p to be a pointer to int. What ever p points to will be interpreted as an int, ie 4 bytes. • Base type also defines pointer arithmetic. Engineering Problem Solving with C++, second edition, J. Ingber

  15. Pointer Arithmetic • Four arithmetic operations are supported: +, -, ++, -- • Arithmetic is performed relative to the base type of the pointer. • When applied to pointers, ++ means increment pointer to point to next object. Example:p++; • if p is defined as int *p, p will be incremented by 4 (bytes). • if p is defined as double *p, p will be incremented by 8(bytes). Engineering Problem Solving with C++, second edition, J. Ingber

  16. Example int *p; cout << "size of char is " << sizeof(char) << endl; cout << "size of int is " << sizeof(int) << endl; cout << "size of double is " << sizeof(double) << endl; cout << "size of float is " << sizeof(float) << endl; cout << "the size of p is " << sizeof(p) << endl; Output: size of char is 1 size of int is 4 size of double is 8 size of float is 4 the size of p is 4 Engineering Problem Solving with C++, second edition, J. Ingber

  17. Practice! int q=6; int *iPtr = &q; cout << "iPtr is " << iPtr << endl; cout << "*iPtr is " << *iPtr << endl; cout << "++*iPtr, is " << ++*iPtr << endl; cout << "q is " << q << endl; cout << "iPtr is " << iPtr << endl; cout << "*iPtr++ is " << *iPtr++ << endl; cout << "iPtr is " << iPtr << endl; cout << "q is " << q << endl; Complete the output: iPtr is 0x7fff2f14 Engineering Problem Solving with C++, second edition, J. Ingber

  18. Result of Practice iPtr is 0x7fff2f14 *iPtr is 6 ++*iPtr is 7 q is 7 iPtr is 0x7fff2f14 *iPtr++ is 7 iPtr is 0x7fff2f18 q is 7 Engineering Problem Solving with C++, second edition, J. Ingber

  19. Comparing Pointers • You may compare pointers using relational operators • Common comparisons are: • check for null pointer (p == NULL) • Note: since NULL evaluates as false, and any other pointer evaluates as true, checking for a null pointer can be done as (!p) • check if two pointers are pointing to the same object (p == q) • Note: (*p == *q) means they are pointing to equivalent, but not necessarily the same data. Engineering Problem Solving with C++, second edition, J. Ingber

  20. one-dimensional arrays pointers as arguments to functions Pointers to Array Elements Engineering Problem Solving with C++, second edition, J. Ingber

  21. One Dimensional Arrays • The name of an array is the address of the first element (i.e. a pointer to the first element). • Arrays and pointers may often be used interchangeably. Example int num[4] = {1,2,3,4}, *p; p = num; //same as p = &num[0]; cout << *p <<endl; ++p; cout << *p; output: 1 2 Engineering Problem Solving with C++, second edition, J. Ingber

  22. Arrays and Pointers • You can index a pointer using [] operator. Example: char myString[] = "This is a string"; char *str; str = myString; for(int i =0; str[i]; i++) //look for null cout << str[i]; What does this segment print? This is a string Engineering Problem Solving with C++, second edition, J. Ingber

  23. Arrays and Pointers • When an array is defined, memory is allocated according to the specified size of the array. • The name of an array is a pointer to the first element. However, the value of the pointer can not be changed. It will always point to the same memory location. • When a pointer is defined, 4 bytes* are allocated to store a memory address. • The value assigned to a pointer can be modified (reassigned to point to a different object). Engineering Problem Solving with C++, second edition, J. Ingber

  24. 0xfff4c252 myString T h i s i s a s t r i n g \0 strPtr 0xfff4c252 Practice! What does this print? char myString[ ] = "This is a string"; char *strPtr; strPtr = myString; cout << *myString << endl; cout<<myString << endl; cout << *(myString + 1) << endl; strPtr++; cout << *++strPtr << endl; myString++; //not legal T This is a string h i Engineering Problem Solving with C++, second edition, J. Ingber

  25. Arrays of Pointers • You may define arrays of pointers like any other data type in C++ int num=8; //declare an array of 10 pointers to int int *iPtrs[10]; //first element is assigned a value iPtrs[0] = &num; //output the value of num cout << *iPtrs[0]; Engineering Problem Solving with C++, second edition, J. Ingber

  26. Pointers As Arguments to Functions • Pointers may be passed either "by value" or "by reference". • In either case, the pointer can be used to modify the object to which it is pointing. • Only when passed by reference can the pointer argument itself be modified. Engineering Problem Solving with C++, second edition, J. Ingber

  27. Common Pointer Problems • Using uninitialized pointers int *iPtr; *iPtr = 100; iPtr has not been initialized. The value 100 will be assigned to some memory location. Which one determines the error. • Failing to reset a pointer after altering it’s value. • Incorrect/unintended syntax. Engineering Problem Solving with C++, second edition, J. Ingber

  28. Practice! • #include <iostream> • int main() • { char *aString = "What happens here?"; • int len=0; • while(*aString++ != '\0') • len++; • std::cout << len << ": " << aString; • return 0; • } • Does this compile? If not, why? • If it does, what is the output? Explain? Yes 18: Engineering Problem Solving with C++, second edition, J. Ingber

  29. new delete dynamically allocated arrays Dynamic memory allocation Engineering Problem Solving with C++, second edition, J. Ingber

  30. Stack Heap Global data Program code Dynamic Allocation Using new and delete • Storage for data is allocated as needed from the free memory area known as the heap. • Run-time stack • Local variables • Formal parameters • Managed by the compiler • Heap • Dynamic storage • Managed by storage allocator Engineering Problem Solving with C++, second edition, J. Ingber

  31. Dynamic Memory Allocation • Dynamically allocated memory is defined at runtime. • Dynamic allocation of memory allows for more efficient use of a finite resource. • Dynamic allocation is often used to support dynamic data structures such as stacks, queues, linked lists and binary trees. • Dynamically allocated memory should be freed during execution when it is no longer needed. Engineering Problem Solving with C++, second edition, J. Ingber

  32. Operator new • Replaces malloc() used in C • Allocates a block of memory from the heap. • Returns the address of the first byte. • If allocation fails, new throws and exception that terminates the program (or returns NULL on older compilers). Engineering Problem Solving with C++, second edition, J. Ingber

  33. Example int *iPtr; iPtr = new int; // 4 bytes are allocated // iPtr points to 1st byte double *dPtr; dPtr = new double[20]; // 160 bytes allocated // dPtr points to 1st byte iPtr dPtr heap Engineering Problem Solving with C++, second edition, J. Ingber

  34. Initializing Dynamically Allocated Memory • To initialize a dynamically allocated object, the initial value is provided inside parentheses following the type. • Example: int *ptr; ptr = new int(100); //4 bytes allocated //initial value: 100 Engineering Problem Solving with C++, second edition, J. Ingber

  35. Practice: • int main() • { • int *iPtr, *jPtr, i; • iPtr = new int; • jPtr = new int(3); • double *dPtr; • dPtr = new double[6]; • *iPtr = 7; • cout << *iPtr << ',' << *jPtr << endl; • for(i=0; i<6; i++) • dPtr[i] = 5; • for(i=0; i<6; i++) • cout << (*dPtr)++ << ' '; • cout << endl; • for(i=0; i<6; i++) • cout << dPtr[i] << ' '; • return 0; • } OUTPUT 7 , 3 5 6 7 8 9 10 11 5 5 5 5 5 EXPLAIN Engineering Problem Solving with C++, second edition, J. Ingber

  36. Operatordelete • Replaces free() used in C. • The delete operator frees memory allocated by new. • Using delete to attempt to free any other type of address will result in errors. Engineering Problem Solving with C++, second edition, J. Ingber

  37. 100 delete Example int *ptr; ptr = new int (100); cout << *ptr; //see if it worked ptr delete ptr; //free the memory ptr ? value of ptr is now undefined Engineering Problem Solving with C++, second edition, J. Ingber

  38. Example: Dynamically Allocated Arrays double *dptr; const int SIZE = 10; dptr = new double[SIZE];//80 bytes for(int i=0; i<SIZE; ++i) cin >> dptr[i]; fun1(dptr, SIZE); // pass array to fun1 delete [] dptr; //free all 10 elements Engineering Problem Solving with C++, second edition, J. Ingber

  39. Example using delete • int main() • { • int *iPtr, *jPtr; • iPtr = new int(2); • jPtr = new int(3); • cout << *iPtr << ',' << *jPtr << endl; • delete iPtr; • delete jPtr; • cout << *iPtr << ',' << *jPtr << endl; • int *dPtr; • dPtr = new int(5); • cout << iPtr << ',' << jPtr << endl; • cout << *iPtr << ',' << *jPtr << endl; • cout << dPtr << endl; • return; • } • What output would you expect from this code? Engineering Problem Solving with C++, second edition, J. Ingber

  40. list stack queue Data structures and the STL Engineering Problem Solving with C++, second edition, J. Ingber

  41. List • A list is a data structure organized as a collection of elements, or nodes, that are linked by pointers. • Elements can be added at any position in a list in constant time by reassigning pointers. • List have many useful applications in the organization large amounts of data. Engineering Problem Solving with C++, second edition, J. Ingber

  42. The list class • list is a class template defined in the header file list. #include<list> list<string> word; //list of strings Engineering Problem Solving with C++, second edition, J. Ingber

  43. list Methods • Elements can be added to and removed from any position in a list using the appropriate method and an iterator. • An iterator is similar to a pointer, but is usually implemented as a class object. • Common Methods: bool empty() iterator insert() iterator remove() iterator begin() iterator end() Engineering Problem Solving with C++, second edition, J. Ingber

  44. Example list<string> wordList; list<string>::iterator iter = wordList.begin(); iter = wordList.insert(iter,"hello"); wordList.insert(iter,"world"); for(iter=wordList.begin(); iter!= wordList.end(); iter++) cout << *iter << " "; output: hello world Engineering Problem Solving with C++, second edition, J. Ingber

  45. Queue • A queue is known as a first-in-first-out (FIFO) data structure. • Items are added to the end of a queue and removed from the front of the queue. • Queues have many useful applications, including processing requests. Engineering Problem Solving with C++, second edition, J. Ingber

  46. The queue class • queue is a class template defined in the header file queue. #include<queue> queue<int> printIDs; //queue of ints Engineering Problem Solving with C++, second edition, J. Ingber

  47. queue Methods bool empty() void pop()//remove from front void push()//add to end data-type front()//return front data-type back()//return end Engineering Problem Solving with C++, second edition, J. Ingber

  48. Example #include<queue> … queue<int> theQueue; theQueue.push(10); theQueue.push(20); cout << theQueue.front(); //output 10 cout << theQueue.back(); //output 20 Engineering Problem Solving with C++, second edition, J. Ingber

  49. Stack • A stack is known as a last-in-first-out (LIFO) data structure. • Items are added to and removed from the top of the stack. • Stacks are essential in the design of compilers and for implementing recursion. Engineering Problem Solving with C++, second edition, J. Ingber

  50. The stack class • stack is a class template defined in the header file stack. #include<stack> stack<int> theStack; //stack of ints Engineering Problem Solving with C++, second edition, J. Ingber

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