1. Structured Programming Instructor: Prof. K. T. Tsang Lecture 7: Pointer 指针 1

2. Memory cells and addresses Computer memory is a sequential collection of storage-cells for data. Each cell is 1 byte in size and has an address associated with it. Typically, the addresses are numbered consecutively, starting from zero. The last address depends on the memory size. A computer with 64k memory will have its last address as 65,535. 2

3. Address of Variable When a variable is declared, the C/C++ compiler will allocate a block of cells somewhere in the memory to hold the value of the variable, either during compile-time or in run-time. The size of this block of cells will depend on the data type of the variable being declared. The address of the first cell in this block is the address of the variable. 3

4. Example In a machine with 4-byte integer, when we declare and initialize int an_int = 789; 4 adjacent memory cells are reserved for the variable an_int and occupied by the integer 789. The address of the first cell in this block is the address of an_int. 19202122232425262728 Cell address 4 bytes occupied by an_int 789 Address of an_int 4

5. 4 essential attributes 特征 of a variable name (identifier) data type (size of memory block) address value an_int 20020 789 5

6. Address Operator & The “address of ” operator ( & ) gives the memory address of the variable( 地址算符 ) Usage: &variable_name int a = 100; //To get the value, use the variable name printf(“a=%d\n”, a);//prints 100 //To get the memory address, add the address //operator before the variable name printf(“address of a: %d\n”, &a); //prints 1024 100………… Memory address:1024… 1020 a 6

7. Address Operator & #include void main(){ int a, b; a = 88; b = 100; printf("The address of a is: %x”, &a); printf("The address of b is: %x”, &b); } Try the following code to get the addresses of variables. Address is usually a large number, so print it in hex. 7

8. Pointer 指针 A pointer constant 指针常数 is the memory address of a variable. A pointer variable 指针变量 is a variable used for storing the address of a variable. Using pointer (i.e. data type and memory address), we can access the value of the variable stored in the memory indirectly. 8

9. Pointer Types C/C++ has pointer types for each type of data, e.g. Pointers to int data type Pointers to char data type Pointers to user-defined data structure Even pointers to pointers Pointers to pointers to int type 9

10. Pointer Variables A pointer variable is a specific box for storing a memory address Declaration of Pointer variables type* pointer_name; //style 1 or type *pointer_name; //style 2 or type * pointer_name; //style 3 where type is the type of data pointed to (e.g. int, char, double) by the pointer. 10

11. Declaration of Pointers The style 2 (in previous slide) is more popular because it is more convenient to have multiple declaration, and accessing the target value, e.g. int i1, i2, *ip; //ip is a pointer ip = &i1; //initialize ip by address of i1 *ip = 23; //assign 23 to i1 float f1 = 1.33; ip = &f1; //error, why? 11

12. Pointer Variables int a = 100; int *p = &a; printf(“%d %x\n”, a, &a); printf(“%x %x\n”, p, &p); The value of pointer p is the address of variable a. A pointer is also a variable, so it has its own memory address. 10088…1024… Memory address:0x10240x1034… 0x1020 ap 12

13. De-reference Operator * 取值算符 We can access to the value stored in the variable pointed to by preceding the pointer with the dereference (indirection) operator (*), int a = 100; int *p = &a; printf(“%d %x\n”, a, &a); printf(“%d %x %x\n”, *p, p, &p); 10088…1024… Memory address:0xa23b0xa24b… ap Result is: 100 a23b 100 a23b a24b 13

14. Example: * operator int numberA, *p, m; numberA = 145; p = &numberA;//initialize p m = *p;//assign 145 to m *p = 31;//assign 31 to variable pointed to by p printf(“%d\n”, numberA );//31 14

15. Don ’ t get confused Declaring a pointer means only that it is a pointer: int *p; //p is an integer pointer Don’t be confused with the de-reference operator, which is also written with an asterisk ( * ). They are simply two different tasks represented with the same sign int a = 100, b = 88, c = 8; int *p1 = &a, *p2, *p3 = &c; p2 = &b;// p2 points to b p1 = p2; // p1 points to b b = *p3;//assign c to b printf(“%d%d%d\n”, a, b, c); //result is: 888 15

16. Pointer Example #include int main (){ int value1 = 5, value2 = 15; int *p1, *p2; p1 = &value1; // p1 = address of value1 p2 = &value2; // p2 = address of value2 *p1 = 10; // value pointed to by p1=10 *p2 = *p1; // value pointed to by p2= value // pointed to by p1 printf("value1=%d ; value2=%d\n“, value1, value2); p1 = p2; // p1 = p2 (pointer value copied) *p1 = 20; // value pointed to by p1 = 20 printf("value1=%d ; value2=%d\n“, value1, value2); return 0; } Result is: value1=10 ; value2=10 value1=20 ; value2=20 16

17. Pointer vs. reference Both are used to indirectly access variables. Reference always refers to a data object. It is an error to define a reference without initializing it, while it is OK to do so for a pointer. It is an error to define a reference to another reference, but it is OK to define a pointer to another pointer. 17

18. Pointer vs. reference - 2 Once initialized, a reference always refers to the same data. Pointer can point to different data by re-assignment. int i1=17, i2=210; int &r1=i1, &r2=i2; r1=r2; //assigns 210 to i1 int *p1=&i1, *p2=&i2; p1=p2; //p1 now points to i2 18

19. Pointer to other pointer Pointer variable has its own address. So we can define a pointer points to another pointer. int ival = 67; int *p = &ival; //pointer to ival int **q; // declaring a pointer to other pointer q = &p;//pointer to pointer p 19

20. Pointer to Pointer What is the output? 58 58 58 20

21. More Pointer to Pointer int a = 58; int *p = &a; int **q = &p; int ***r = &q; int ****s = &r; q = &a//illegal! s = &q//illegal! 58 2234333444345534 Int a pqrs Pointer to intPointer to pointer to int Pointer to pointer to pointer to int Pointer to pointer to pointer to int 223433344434 5534 addresses 21

22. Pointer and array When we use the name of an array in an expression, that name is automatically converted into a pointer to the first element of the array. int ia[] = {5, 6, 1, 3}; int *p = ia; //p points to ia[0] int *q = ＆ ia[4]; //q points to ia[4] 22

23. Array Name is a pointer constant #include void main (){ // Demonstrate array name is a pointer constant int a[5]; printf("Address of a[0]: %x\n”, &a[0]); printf("Name as pointer: %x\n“, a); } /* example of result (system dependent): Address of a[0]: 0x0065FDE4 Name as pointer: 0x0065FDE4 */ 23

24. Pointer Arithmetic -- to compute a pointer to an array element by adding /subtracting an integral value to/from a pointer to another array element int ia[] = {5, 6, 4, 1, 3}; int *p1 = ia; //p1 points to ia[0] int *p2 = ia + 4; //p2 points to ia[4] int *p3 = ia + 6; //error: ia + 6 is invalid address 24

25. Pointer Arithmetic Given a pointer p, p+n refers to the element that is offset from p by n positions. int a[5] = {2,4,6,8,22}; int *p = &a[1]; 2 4 8 6 22 a a + 2 a + 4 a + 3 a + 1 p p + 2 p + 3 p - 1 p + 1 25

26. *(a+n) is identical to a[n] Dereferencing Array Pointers a[3] or *(a + 3) 2 4 8 6 22 a a + 2 a + 4 a + 3 a + 1 a[2] or *(a + 2) a[1] or *(a + 1) a[0] or *(a + 0) a[4] or *(a + 4) 26

27. Use of array pointer const int arr_sz = 6; int ia[arr_sz] = {5, 6, 1, 2, 4, 3}; for (int *p1 = ia; p1 <= ia+arr_sz; ++p1) printf(“%d ”, *p1); //print current element printf(“\n”); 27

28. Array of Pointers & Pointers to Array a b c An array of Pointers p int a = 1, b = 2, c = 3; int *p[5]; p[0] = &a; p[1] = &b; p[2] = &c; int list[5] = {9, 8, 7, 6, 5}; int *p; P = list;//points to 1 st entry P = &list[0];//points to 1 st entry P = &list[1];//points to 2 nd entry P = list + 1; //points to 2 nd entry A pointer to an array 28

29. NULL pointer NULL is a special value that indicates an empty pointer If you try to access a NULL pointer, you will get an error “NULL” is defined as a preprocessor constant in cstdlib int *p = NULL ;//ok p = 0;//ok, literal ‘0’ for null pointer cout << p << endl; //prints 0 cout << &p << endl;//prints address of p cout << *p << endl;// Error! int zero = 0; int *p1 = zero;// error, cannot use integer zero instead of ‘0’ int *p2 = 7889;// error, cannot be assigned with a fixed number 29

30. Good programming practice tips Using un-initialized pointer is dangerous. If you have to define a pointer without assigning a pointer constant to it, set it to NULL first. 30

31. String pointer char s1[ ] = “A string"; while (*s1) { printf(“%c”, *s1); ++s1; } printf(“\n”); char *s2 = “Another string"; char *s3 = s2; printf(“%s %s %s\n”, s1, s2, s3) This is the old way we learned to declare & initialize a string. S1 is the pointer to the first element of the character array. This creates a string to hold all the characters and stores the address of the first character in the string pointer variable s2. Assign s2 to another string pointer variable s3 31

32. String copy using pointer #include char* str_copy(char *s1, char *s2); int main() { char *s3 = “This is a string!"; printf(“s3= %s\n”, s3); char *s4; s4 = str_copy(s4, s3); printf(“s4= %s\n”, s4); return 0; } char* str_copy(char *s1, char *s2) { int i = 0; while (*s2 != '\0') { *s1 = *s2; ++s1; ++s2; i++; } *s1 = *s2; return s1-i; } 32

33. Using const to protect argument so that its value cannot be changed float mean (int n, const float *a) { float sum = 0.0; for (int i=0; i<n; i++) sum += *(a+i); return (sum/(float)n); } In general if we prototype: void fun1(const int *p) then inside the definition of fun1 we cannot have a statement like: *p = 10; 33

34. Exercise Following last example, use pointer to write a function with the prototype: char* strcat(char* s1, const char* s2); to concatenate( 连接 ) 2 strings s1 & s2 together. Use this function in a program to show that it works. 34

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38. Pointer as function arguments: Compare this with call by reference in Lecture 7 example: swap void exchange (int *, int *); main() { int x = 100, y = 28; printf(“before exchange: x=%d y=%d”, x, y); exchange ( &x, &y); printf(“after exchange: x=%d y=%d”, x, y); } void exchange (int *a, int *b) { int t = *a; *a = *b;*b = t; } 38

39. Bubble Sort using pointers void sort (int m, int *x) { int i, j, temp; for (i=1; i<m; i++) for (j=1; j<=m-i; j++) { if (*(x+j-1) >= *(x+j) ) { temp = *(x+j-1); *(x+j-1) = *(x+j) ; *(x+j) = temp ; } 39

40. Bubble Sort using pointers void sort (int m, int *x) { int i, j, temp; for (i=1; i<m; i++) for (j=1; j<=m-i; j++) { if (*(x+j-1) >= *(x+j) ) exchange(x+j-1, x+j); } 40

41. Using bubble sort with pointer #include void sort (int m, int *x); void main() { int score[ ] = { 11, 9, 19, 15, 22, 7}; for (int i=0; i<6; i++) printf(“%d ”, score[i]); printf(“\n”); int *ia = score; sort(6, ia); for (int i=0; i<6; i++) printf(“%d ”, score[i]); printf(“\n”); } 41

42. Bubble Sort using array void sort (int m, int x[ ]) { int i, j, temp; for (i=1; i<m; i++) for (j=1; j<=m-i; j++) { if (x[j-1] >= x[j] ) { temp = x[j-1]; x[j-1] = x[j] ; x[j] = temp ; } 42

43. Using array bubble sort #include void sort (int m, int x[ ]); void main() { int score[ ] = { 31, 9, 19, 15, 22, 7}; for (int i=0; i<6; i++) printf(“%d ”, score[i]); printf(“\n”); sort(6, score); for (int i=0; i<6; i++) printf(“%d ”, score[i]); printf(“\n”); } 43

44. Library function for input - scanf “scanf” read input data from the keyboard, similar to “printf” output data to the terminal. First argument to “scanf” is the format string (enclosed by “ ”) similar to the format string used by “printf”. Second (and 3rd…) argument to “scanf” is an address to the input data. int number, u, v; printf (“Please input an integer.\n”); scanf ( “%i”, &number); printf (“Please input 2 positive integers.\n”); scanf (“%d%d”, &u, &v); printf (“Got 2 positive integers: %d %d\n”, u, v); 44

45. Important scanf (“format string”, arg1, arg2, arg3,…) where arg1…argn are addresses to data. scanf ( “Enter a number\n%d”, &number); Input data must be separated by space, tab or newline. If you forget to put “&” in front of the variables in the arguments of “scanf”, the result could be a “segmentation violation core dumped” or “illegal memory access” error. 45

46. Example: find greatest common divisor Given 2 positive integers u & v. If v equals zero, then the gcd is u. Find temp=u%v, then set u=v, v=temp, and go back to previous step. #include void main () { int u, v, temp; printf (“Please input 2 positive integers.\n”); scanf (“%i %i”, &u, &v); if(u v printf (“2 positive integers are: %d %d\n”, u, v); while (v != 0) { temp = u%v; u = v; v = temp; } printf (“The greatest common divisor is: %d\n”, u); } 46

47. Inputting character string with scanf “scanf” can be used with %s format to read in a string of characters up to a blank space, tab or newline character. char str[99]; scanf (“%s”, str);//no ‘&’ needed The 2 statements above have the effect of reading a character string typed into the terminal and storing it in the array str. 47

48. More examples: inputting strings char s1[99], s2[99], s3[33]; scanf (“%s %s %s”, s1, s2, s3); With the input text line: “happy new year\n” results ins1 = “happy” s2 = “new” s3 = “year” 48

49. Summary —what you should have learned from this Lecture Variable address Pointer uses the variable address to access the variable value indirectly. “Address of” operator: & “De-reference” or “Value contained” operator: * Array name as a pointer Pointer arithmetic Pointer vs. reference String pointer: char* Using “scanf” to input data from terminal 49