Data Structures (Second Part) Lecture 2 : Pointers Bong-Soo Sohn Assistant Professor School of Computer Science and Engineering Chung-Ang University

Pointer Pointers Variables that contain memory addresses Pointer Operators * (dereference operator) : Unary * takes the contents of a pointer (dereferences a pointer) & (reference operator) : Unary & gives the address of (pointer to) something Declaring variables of pointer types Declare which data type a pointer is going to point to (ex) int* a; // ‘a’ is a variable that stores an address of // an integer type variable. float* b; void swap(int* x, int* y); NULL Pointer?

C++ example 1 // more pointers #include using namespace std; int main () { int firstvalue = 5, secondvalue = 15; int * p1, * p2; p1 = &firstvalue; // p1 = address of firstvalue p2 = &secondvalue; // p2 = address of secondvalue *p1 = 10; // value pointed by p1 = 10 *p2 = *p1; // value pointed by p2 = value pointed by p1 p1 = p2; // p1 = p2 (value of pointer is copied) *p1 = 20; // value pointed by p1 = 20 cout << "firstvalue is " << firstvalue << endl; cout << "secondvalue is " << secondvalue << endl; return 0; } Output : firstvalue is 10 secondvalue is 20

Pointers and Arrays Array variable is a constant pointer the identifier of an array is equivalent to the address of its first element (ex) int numbers[20]; int *p; p=numbers; // OK numbers=p; // NOT OK! numbers[5]=0; // numbers [offset of 5] = 0; *(numbers+5)=0; // value pointed by (numbers+5) = 0; *p=1; p++; *p=2;

C++ example 2 // more pointers #include using namespace std; int main () { int numbers[5]; int * p; p = numbers; *p = 10; p++; *p = 20; p = &numbers[2]; *p = 30; p = numbers + 3; *p = 40; p = numbers; *(p+4) = 50; for (int n=0; n<5; n++) cout << numbers[n] << ", "; return 0; } Output : 10, 20, 30, 40, 50,

Pointers to Pointers C++ allows the use of pointers that point to pointers char a; char * b; char ** c; a = 'z'; b = &a; c = &b; c has type char** and a value of 8092 *c has type char* and a value of 7230 **c has type char and a value of 'z'