C Pointers. ENGG1002 Computer Programming and Applica;ons Dr. Hayden Kwok Hay So Week 2. GeGng Hold of Memory. Sta;c: When we define a variable

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1 C Pointers ENGG1002 Computer Programming and Applica;ons Dr. Hayden Kwok Hay So Week 2 GeGng Hold of Memory Sta;c: When we define a variable int gvar; d c b a gvar Allocate memory in global storage Allocate memory in local storage int square(int ) { int d; d = * return d; int main() { int a; int b; int c; int ; a = 2; b = 5; c = a + b; 1

2 Calculate c = a + b c b a gvar => Take the value from the loca;on represen;ng a 2. Take the value from the loca;on represen;ng b 3. Add the two values. Store the result at the loca;on represen;ng c Q: How do we find the loca;on represen;ng a? Find the Variables A: The compiler knows eactly how many and where to find each variable if they are defined sta0cally. square() main() global area d c b a gvar int gvar; int square(int ) { int d; d = * return d; int main() { int a; int b; int c; int ; a = 2; b = 5; c = a + b; 2

3 Passing the Variable Loca;ons Around d c b a gvar &a 112 &gvar 100 &? How do you tell something else where can it find a par;cular variable? We use the ampersand symbol (&) to get the address of a variable. &b Note: It is similar to the C++ s concept of reference, but not eactly the same. You can then use the address like any other value, e.g. Get the net address net to b net_address = &b + 1 Pass the address to a func;on so that it knows how to find b. find(&b); Pointers Loosely speaking, the address of a variable is called a pointer. A pointer to something can be thought as the address of something.. E.g. a pointer to an integer is essen;ally the address of an integer. The C/C++ type of a pointer to a type T is denoted as T* The type pointer to an integer : int* The type pointer to a struct of image : struct image* int *iptr; float *fp; struct image *img; 3

4 Using Pointers Pointers are just like any other variables You can assign them to different values: int* ptr1; int* ptr2; int ; ptr1 = &; ptr2 = ptr1; A special value NULL means it points to nothing ptr2 = NULL; You can compare them: if (ptr2 == ptr1) You can perform arithme;c on them: ptr2 = ptr1 + 1; Pointer Arithme;c Q: What does +1 means when used with a pointer to a type T? A: The memory loca;on aher this element of type T that this pointer is poin;ng to. C uses sizeof() to find the size int *ip; int *jp; int i = 237; ip = &i; jp = ip + 1; cout << Loc of i: << ip << endl;; cout << Value of jp: << jp << endl; Loc of i: 100 Value of jp: 10

5 Dereferencing Pointers int *ip; int *ptr; int i = 237; int k; ip = &i; k = *ip; cout << k << endl; ptr = ip; k = *ptr; cout << k << endl; Recall that we get the address of a variable by the & operator The opposite opera;on is called dereferencing a pointer. We use the * operator *p means get the value at loca;on pointed to by p. What will this code print? *ptr = i + 1; cout << *ptr << endl; cout << i << endl; GeGng Hold of Memory (Revisit) Sta;c: When variables are defined. Dynamic: used malloc() int* bigregion = malloc( ); We need dynamic memory alloca;on because in many cases, we don t know how much memory do we need when we write our programs. E.g. We don t know how many customer orders we will get in a day. We don t know how many images we need to load for a web page, etc Pointers are used to point to these allocated memory regions. They hold the address of the allocated memory int* bigregion = (int*) malloc(100 * sizeof(int)); 5

6 GeGng Memory for Arrays Sta;c: int primes[5] = {2,3,5,7,11; Dynamic: When you don t know the size when you write your program E.g. To put all the customer orders in an array Use malloc() int num_orders; int orders[]; orders = (int*) malloc(num_orders * sizeof(int)); C++ has some etra synta Dangerous, not recommended! int num_orders; int orders[num_orders]; cin >> num_orders; for (int i = 0; i < num_orders; i++) orders[0] =... Pointer vs Array (1) Arrays in C are implemented using Pointers The defini;on int v[10] essen;ally means: 1. allocate a memory region for 10 integers 2. define a variable called v with type int* and point it to the beginning of the memory region Now, to get the value v[2], need to find the memory loca;on of v[2], how? From last week: v[2] is located at address(v) + 2 * sizeof(int) From this week (Pointer arithme;c): v[2] is located at v + 2 6

7 Pointer vs Array (2) Q: We know the loca;on, how do we find the value? A: Recall: pointer dereferencing. E.g. *ptr = 8; So pugng together, v[2] = 8 is equivalent to *(v + 2) = 8. In fact, it is how array is implemented in C In C/C++, you may use the array name just as if it is a pointer. Pointer vs Array (3) You can pass arrays as pointers #define N 100 void square(int* values, int num) { for (int i = 0; I < num ; i++) values[i] = values[i] * values[i]; int main() { int v[n];... square(v, N) Note that, however, the dimension informa;on is lost when you pass an array like this. 7

8 Pointer vs Array () You can point a pointer to the middle of an array int mat[3][]; int* ptr; ptr = mat+ Then ptr will point to the 2 nd row of values 2 ways to iterate through an array #define N 100 int i; int v[n] for (i = 0 ; i < N ; i++) { v[i] = i * 2; Slow #define N 100 int i; int v[n] int* p; for (i = 0, p = v; i < N ; i++, p++) { *p = *p * 2; fast 8