Pointers Pointer Variables. Pointer Variables Getting the Address of a Variable. Each variable in program is stored at a

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1 3.1. Getting the Address of a Variable Pointers (read Chapter 9. Starting Out with C++: From Control Structures through Objects, Tony Gaddis.) Le Thanh Huong School of Information and Communication Technology Hanoi University of Technology Each variable in program is stored at a unique address Use address operator & to get address of a variable: int num = -99; cout << &num; // prints address in hexadecimal Le Thanh Huong - SoICT - HUT Pointer Variables A pointer is a variable that holds the address of something else. Pointer points to the data. int foo; int *x; //same as int* x; //or int * x; foo = 123; x = &foo; MEMORY Address foo x Pointer Variables int *x; x is a pointer to an integer. You can use the integer x points to in a C++ expression like this: the int x points to y = *x + 17; *x = *x +1; Le Thanh Huong - SoICT - HUT 3 Le Thanh Huong - SoICT - HUT 4 1

2 Pointer Variables In C++ you can get the address of a variable with the & operator. MEMORY int foo; foo = 123; x = &foo; foo Address &foo means the address of foo Example // This program stores the address of a variable in a pointer. int x = 25; // int variable int *ptr; // Pointer variable, can point to an int ptr = &x; // Store the address of x in ptr cout << "The value in x is " << x << endl; cout << "The address of x is " << ptr << endl; Le Thanh Huong - SoICT - HUT 5 Exercise 3.1. Complete the following program int x = 25, y = 50; // Two int variables int *ptr; // Pointer variable // Display the contents of x and y. cout<< x << <<y; // Use the pointer to manipulate x and y. // Store the address of x in ptr. ptr = &x; // Add 100 to the value in x. x +=100; // *ptr = *ptr +100; // Store the address of y in ptr. ptr = &y; // Add 100 to the value in y. y +=100; *ptr = *ptr + 100; // Display the contents of x and y. cout << x << << y; Exercise Write a program that includes the following functions: Input values for an array Increase all values of the array by 2 Print out the new array You should use pointers to access the array. The array is passed as function parameters Le Thanh Huong - SoICT - HUT 8 2

3 Dereference a pointer The indirection operator (*) dereferences a pointer. It allows you to access the item that the pointer points to. int foo = 3; int *x = &foo; cout << *x << endl; // this prints 3 A pointer must have a value before you can dereference it (follow the pointer). int *x; *x=3; int foo; int *x = &foo; *x=3; Le Thanh Huong - SoICT - HUT 9 // This program demonstrates the use of the indirection operator. int x = 25; // int variable int *ptr; // Pointer variable, can point to an int ptr = &x; // Store the address of x in ptr // Use both x and ptr to display the value in x. cout << "Here is the value in x, printed twice:\n"; cout << x << endl; // Displays the contents of x cout << *ptr << endl; // Displays the contents of x // Assign 100 to the location pointed to by ptr. This will // actually assign 100 to x. *ptr = 100; // Use both x and ptr to display the value in x. cout << "Once again, here is the value in x:\n"; cout << x << endl; // Displays the contents of x cout << *ptr << endl; // Displays the contents of x Pointers to anything int *x; int **y; y x some *int some int some int double *z; z some double Le Thanh Huong - SoICT - HUT 12 3

4 3.3. The Relationship Between Arrays and Pointers Array name is starting address of array int a[] = 4, 7, 11, 5, 23; Starting address of a: 0x4a00 cout << a; // displays 0x4a00 cout << a[0]; // display The Relationship Between Arrays and Pointers Array name can be used as a pointer constant: int a[] = 4, 7, 11, 5, 23; cout << *a; // displays 4 Pointer can be used as an array name: int *ptr = a; cout << ptr[1]; // displays 7 Le Thanh Huong - SoICT - HUT 13 Le Thanh Huong - SoICT - HUT 14 Pointer in expressions Integer math operations can be used with pointers. If you increase a pointer, it will be increased by the size of whatever it points to. int *ptr = a; *(ptr+2) *(ptr+4) *ptr a[0] a[1] a[2] a[3] a[4] int a[5]; Printing an array void print_array(int a[], int len) for (int i=0;i<len;i++) cout << "[" << i << "] = " << a[i] << endl; void print_array(int *a, int len) for (int i=0;i<len;i++) cout << "[" << i << "] = " << *a++ << endl; For clarity, use array notation instead of pointer notation when manipulating arrays. Le Thanh Huong - SoICT - HUT 15 Le Thanh Huong - SoICT - HUT 16 4

5 3.4. Pointer Arithmetic int a[] = 4,7,11,5,23; int *ptr = a; Operation Example ++, -- ptr++; // points at 7 ptr--; // now points at 4 +, - (pointer and int) cout << *(ptr + 2); // 11 +=, -= (pointer and int) ptr = a; // points at 4 ptr += 2; // points at 11 - (pointer from pointer) cout << ptr a; //difference (number of ints) between ptr and val <, <=, >, >=, //compare address in pointers ==,!= if(ptr1 == ptr2) //compare addresses if(*ptr1 == *ptr2) //compare contents const int SIZE = 8; int set[size] = 5, 10, 15, 20, 25, 30, 35, 40; int *numptr; // Pointer int count; // Counter variable for loops // Make numptr point to the set array. numptr = set; // Use the pointer to display the array contents. cout << "The numbers in set are:\n"; for (count = 0; count < SIZE; count++) cout << *numptr << " "; numptr++; // Display the array contents in reverse order. cout << "\nthe numbers in set backward are:\n"; for (count = 0; count < SIZE; count++) numptr--; cout << *numptr << " "; Le Thanh Huong - SoICT - HUT 17 Exercise 3.2. Complete the following program const int SIZE = 5; // Size of the array int numbers[size]; // Array of integers int count; // Counter variable cout << "Enter " << SIZE << " numbers: "; for (count = 0; count < SIZE; count++) // Get a value to store in numbers[count]. // Use pointer notation instead of subscripts. cout << "Here are the numbers you entered:\n"; for (count = 0; count < SIZE; count++) // Display a value of the array. // Use pointer notation instead of subscripts. cout << endl; 3.5. Initializing Pointers Can initialize at definition time: int num, *numptr = &num; int a[5], *ptr = a; Cannot mix data types: double cost; int *cptr = &cost; //won t work Can test for an invalid address for a pointer with: if (!cptr)... Le Thanh Huong - SoICT - HUT 20 5

6 3.6. Pointers as Function Parameters Example A pointer can be a parameter Works like reference variable to allow change to argument from within function Requires: asterisk * on parameter in prototype and heading void getnum(int *ptr); //ptr is pointer to an int asterisk * in body to deference the pointer cin >> *ptr; address as argument to the function getnum(&num); //pass address of num to getnum void swap(int *x, int *y) int temp = *x; *x = *y; *y = temp;... int num1 = 2; num2 = 3; swap(&num1, &num2);... Le Thanh Huong - SoICT - HUT 21 Le Thanh Huong - SoICT - HUT 22 (continued) // This program uses two functions that accept addresses of // variables as arguments. // Function prototypes void getnumber(int *); void doublevalue(int *); int number; // Call getnumber and pass the address of number. getnumber(&number); // Call doublevalue and pass the address of number. doublevalue(&number); // Display the value in number. cout << "That value doubled is " << number << endl; //*************************************************************** // Definition of getnumber. The parameter, input, is a pointer. * // This function asks the user for a number. The value entered * // is stored in the variable pointed to by input. * //*************************************************************** void getnumber(int *input) cout << "Enter an integer number: "; cin >> *input; //*************************************************************** // Definition of doublevalue. The parameter, val, is a pointer. * // This function multiplies the variable pointed to by val by * // two. * //*************************************************************** void doublevalue(int *val) *val *= 2; 6

7 3.7. Pointers to Constants Constant Pointers To store the address of a constant in a pointer Example: const int SIZE = 6; const double b[size] = 18.4, 17.4, 12.8, 14.7, 20.0, 18.9 b is an array of constant doubles. void displayb(const double *b, int size) This is what b points to for(int count=0; count<size; count++) cout << *(b + count) << endl; The parameter, b, is a pointer to const double A constant pointer is a pointer that is initialized with an address, and cannot point to anything else. Example: int value = 22; * const indicates that ptr is a constant pointer int * const ptr = &value; This is what ptr points to Le Thanh Huong - SoICT - HUT 25 Le Thanh Huong - SoICT - HUT 26 Constant Pointers to Constants A constant pointer to a constant is: a pointer that points to a constant a pointer that cannot point to anything except what it is pointing to Example: int value = 22; * const indicates that ptr is a constant pointer const int * const ptr = &value; This is what ptr points to 3.8. Dynamic Memory Allocation Uses new operator to allocate memory int *iptr; iptr = new int; It returns address of memory location To allocate array: arrayptr = new int[100]; Access array: For(i=0;i<100;i++) i<100 i++) arrayptr[i]=i*i; //or *(arrayptr+i)=i*i; Program will terminate if not enough memory available to allocate Le Thanh Huong - SoICT - HUT 27 Le Thanh Huong - SoICT - HUT 28 7

8 Releasing Dynamic Memory Use delete to free dynamic memory: delete fptr; Use [] to free dynamic array: delete [] arrayptr; Le Thanh Huong - SoICT - HUT 29 Prevent code from using the pointer to access the area of memory that was freed. 8

9 Returning Pointers from Functions Pointer can be the return type of a function: int* newnum(); The function must not return a pointer to a local variable in the function A function should only return a pointer: To data that was passed to the function as an argument, or To dynamically allocated memory int *getrandomnumbers(int num) int *array; // Array to hold the numbers // Return null if num is zero or negative. if (num <= 0) return NULL; // Dynamically allocate the array. array = new int[num]; // Seed the random number generator by passing // the return value of time(0) to srand. srand( time(0) ); // Populate the array with random numbers. for (int count = 0; count < num; count++) array[count] = rand(); Le Thanh Huong - SoICT - HUT 33 // Return a pointer to the array. return array; Exercise 3.3. Implement the function my_strcat : Input parameters: two strings, s1 and s2 Output: a pointer to a dynamically allocated concatenation For example: The concatenation of hello_ and world! is the string hello_world! Write a program to test your function Some policies! Exercises that have not been given solutions during the class are considered as homework. Students are expected to prepare exercises of exercise hours beforehand. These exercises are also considered as homework. Only some of them are corrected during the class. Students have to submit their own individual homework to the lecturer by (to huongthanh@gmail.com) one week after the homework is given in class. If the homework of two students are similar, marks will be reduced by 50%. If the homework is submitted one week late, mark is reduced by 30%. After that, no submission is accepted. Le Thanh Huong - SoICT - HUT 36 9

9.2 Pointer Variables. Pointer Variables CS Pointer Variables. Pointer Variables. 9.1 Getting the Address of a. Variable

9.2 Pointer Variables. Pointer Variables CS Pointer Variables. Pointer Variables. 9.1 Getting the Address of a. Variable CS 1400 Chapter 9 9.1 Getting the Address of a Variable A variable has: Name Value Location in a memory Type The location in memory is an address Use address operator & to get address of a variable: int