Applications of Pointers (1A) Young Won Lim 3/31/18

Transcription

1 (1A)

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3 Variables and their addresses address data int a; &a a int * ; & int **q; &q q 3

4 Initialization of Variables address data int a = 100; &a a = 100 int * = &a; & = &a int **q = &; &q q = & 4

5 Traditional arrow notations address data address data &a a = 100 &a a = 100 & = &a & = &a &q q = & &q q = & 5

6 Pointed addresses :, q address data int a; a int * = &a; q int **q = &; &q q = &a q = & 6

7 Dereferenced Variables : * address data int a; * * a int * = &a; & int **q = &; 7

8 Dereferenced Variables : * int a; int * = &a; int **q = &; Address assignment = &a &a *() *(&a) * a Variable aliasing * a Relations after address assignment 8

9 Dereferenced Variables : *q, **q address data int a; *q **q **q a int * = &a; q *q *q int **q = &; &q q 9

10 Dereferenced Variables : *q, **q int a; int * = &a; Address assignment = &a Variable aliasing * a int **q = &; q = & *q **q a q & *(q) *(&) * q **q * **q a Relations after address assignment 10

11 Two more ways to access a : *, **q &a a * *q **q & & q *q &q q &q q &q q a * a **q a 11

12 Two more ways to access a : *, **q address data &a & a * **q &q q 1) Read / Write a 2) Read / Write * 3) Read / Write **q 12

13 Variables int a; a can hold an integer address &a data a a = 100; a holds 100 address &a data a

14 Pointer Variables int * ; can hold an address int * ; holds an address of a int tye data & ointer to int int * ; * holds a int tye data int * 14

15 Pointer to Pointer Variable int ** q; q holds an address int ** q; q holds an address of a ointer to int tye data ointer to ointer to int &q q int * *q; ointer to int *q holds an address of a int tye data q *q int **q; **q holds a int tye data *q **q int 15

16 Pointer Variables Examles int a = 200; &a address 0x3A0 a data 200 int * = & a; & 0x3AB 0x3A0 int ** q = & ; &q 0x3CE q = 0x3AB &q 0x3CE q 0x3AB *q 0x3A0 **q

17 Pointer Variable with an arrow notation address data * &a address 0x3A0 a data 200 & & 0x3AB 0x3A0 using an arrow notation & * 0x3AB 0x3A

18 Pointer Variable q with an arrow notation address data address data *q **q &a 0x3A0 a 200 q *q & 0x3AB 0x3A0 &q q &q 0x3CE q = 0x3AB using an arrow notation &q q 0x3CE 0x3AB *q 0x3A0 **q

19 The tye view oint of ointers data address (int *) address (int **) Tyes 19

20 The different view oints of ointers **q *q (int *) *q q (int **) q &q Tyes Variables Addresses 20

21 Single and Double Pointer Examles (1) int a ; int * ; int **q ; a * a, *, and **q: int variables q q *q *q **q 21

22 Single and Double Pointer Examles (2) int a ; int * ; int * *q ; a * and *q : int ointer variables (singleointers) q q *q *q **q 22

23 Single and Double Pointer Examles (3) int a ; int * ; int ** q ; a * q : double int ointer variables q q *q *q **q 23

24 Values of double ointer variables int **, **q ; (int **) (int **) X X = q; (int *) (float *) (float) 24

25 Pointed Addresses and Data int a ; &a a =100 The variable a holds an integer data int * ; & 200 The ointer variable holds an address, at this address, an integer data is stored int * * q ; &q q *q 30 The ointer variable q holds an address, at the address q, another address *q is stored, at the address *q, an integer data **q is stored 25

26 Dereferencing Oerations * int a &a a =100 *(&a) = a * * int * & *=200 *(&) = *() = * * * * int * * q &q q q *q *q **q=30 *(&q) = q *(q) = *q *(*q) = **q 26

27 Direct Access to an integer a int a ; &a a =100 Direct Access address &a value a integer 1 memory access 27

28 Indirect Access * to an integer a int * ; & *=200 Indirect Access address & value 2 memory accesses Dereference Oerator * the content of the ointed location * 28

29 Double Indirect Access **q to an integer a int * * q ; &q q q *q *q **q=30 Double Indirect Access address &q value q 3 memory accesses Dereference Oerator * the content of the ointed location q *q Dereference Oerator * the content of the ointed location *q **q 29

30 Values of Variables int a ; &a a =100 address &a value a integer int * ; & *=200 address value & * address integer int * * q ; &q q q *q *q **q=30 address value &q q q *q *q **q address address integer 30

31 Swaing ointers - ass by reference - double ointers 31

32 Swaing integer ointers & = &a & = &b &q q = &b &q q = &a a = 111 a = 111 b = 222 b =

33 Swaing integer ointers & = &a & = &b &q q = &b &q q = &a int *, *q; swa_ointers( &, &q ); swa_ointers( int **, int ** ); function call function rototye 33

34 Pass by integer ointer reference void swa_ointers (int **m, int **n) { int* tm; int ** m int * *m int ** n int * *n } tm = *m; *m = *n; *n = tm; int * tm int a, b; int *, *q; =&a, q=&b; swa_ointers( &, &q ); 34

35 Array of Pointers 35

36 Array of Pointers int a [4]; int * b [4]; int a [4] No. of elements = 4 Tye of each element int * No. of elements = 4 b [4] Tye of each element 36

37 Array of Pointers variable view int a [4]; int * b [4]; a b b[0] *b[0] = 11 a[0] = 11 b[0] a[1] = 22 b[1] b[1] *b[1] = 22 a[2] = 33 b[2] a[3] = 44 b[3] b[2] *b[2] = 33 b[3] *b[3] = 44 37

38 Array of Pointers tye view int a [4]; int * b [4]; (int *) (int * *) (int *) (int *) (int *) (int *) 38

39 Pointer to Arrays 39

40 Pointer to array (1) int m ; int *n ; an integer ointer int a [4] int (*) [4] an integer array ointer int func (int a, int b) ; int (* f) (int a, int b) ; a function ointer 40

41 Pointer to array (1) int int * an integer ointer int [] int (*) [] an integer array ointer int (int, int) int (*) (int, int) a function ointer 41

42 Pointer to array (1) int a [4]; int a [4] (int []) a int (*) [4] ointer to the array of 4 elements a[0] a[1] a[2] a[3] int m ; an integer variable int *n ; a ointer variable int func (int a, int b) ; a rototye int (* f) (int a, int b) ; a function's tye int * f (int a, int b) ; function ointer 42

43 Pointer to array (2) int (*) [4] ; (int (*) []) int a [4] (int []) * a * = a (*) = a &(*) = &a = &a sizeof()= 4 bytes sizeof(*)= 16 bytes a[0] a[1] a[2] a[3] an array with 4 integer elements 43

44 Pointer to array (3) & int (*) [4] ; int c[4] [4] (int (*) []) = c (*) [ i ][ j ]; a 2-d array with 4 rows and 4 columns (int (*) []) (int []) (int []) (int []) (int []) c * c[0] c[1] c[2] c[3] *(+0) *(+1) *(+2) *(+3) c[0][0] c[0][1] c[0][2] c[0][3] c[1][0] c[1][1] c[1][2] c[1][3] c[2][0] c[2][1] c[2][2] c[2][3] c[3][0] c[3][1] c[3][2] c[3][3] 44

45 Pointer to array (4) int c [4][4]; int (*) [4]; = c; func(,... ); void func(int (*x)[4],... ) { } x[r][c] = void func(int x[ ][4],... ) { } x[r][c] = 45

46 2-d Arrays 46

47 Addresses of 4 element integer arrays int A [4] int c[4] [4] int c [4] [4]; A c[i] i = 0, 1, 2, 3 47

48 A 2-D Array a variable view int c [4] [4]; c c[0] c[1] c[2] c[3] c[0] c[1] c[2] c[3] c[0][0] c[0][1] c[0][2] c[0][3] c[1][0] c[1][1] c[1][2] c[1][3] c[2][0] c[2][1] c[2][2] c[2][3] c[3][0] c[3][1] c[3][2] c[3][3] 48

49 A 2-D Array a tye view int c [4] [4]; (int (*)[4]) (int *) (int *) (int *) (int *) 49

50 A 2-D Array an index view int c [4] [4]; c *(c+i)+j *(*(c+i)+j) (c+i) *(c+i) 50

51 A 2-D Array via a double indirection int c [4] [4]; int c[4] [4] (c [i])[j] (*(c+i))[j] *(*(c+i)+j) (c [i]) = (*(c+i)) (_)[j] = *((_)+j) 51

52 A 2-D Array via an array ointer int c [4] [4]; int (*) [4]; (c [i])[j] (*(c+i))[j] *(*(c+i)+j) = c ( [i])[j] (*(+i))[j] *(*(+i)+j) 52

53 A 2-D Array via a double ointer int c [4] [4]; int **, *q[4]; (c [i])[j] (*(c+i))[j] *(*(c+i)+j) = q; q[0]=c[0], q[1]=c[1], q[2]=c[2], q[3]=c[3]; ( [i])[j] (*(+i))[j] *(*(+i)+j) 53

54 2-D array as a 1-D array int c [4] [4]; c[0] c c[0] c[1] c[2] c[3] c[i][j] c[1] c[2] c[3] c[0][0] c[0][1] c[0][2] c[0][3] c[1][0] c[1][1] c[1][2] c[1][3] c[2][0] c[2][1] c[2][2] c[2][3] c[3][0] c[3][1] c[3][2] c[3][3] [i*4+j] 0=[0*4+0] 1=[0*4+1] 2=[0*4+2] 3=[0*4+3] 4=[1*4+0] 5=[1*4+1] 6=[1*4+2] 7=[1*4+3] 8=[2*4+0] 9=[2*4+1] 10=[2*4+2] 11=[2*4+3] 12=[3*4+0] 13=[3*4+1] 14=[3*4+2] 15=[3*4+3] 54

55 Accessing a 2-D array via a single ointer int c [4] [4]; c[0] c c[0] c[1] c[2] c[3] c[i][j] c[1] c[2] c[3] [0*4+0] [0*4+1] [0*4+2] [0*4+3] [1*4+0] [1*4+1] [1*4+2] [1*4+3] [2*4+0] [2*4+1] [2*4+2] [2*4+3] [3*4+0] [3*4+1] [3*4+2] [3*4+3] int *; =c[0]; [i*4+j] 55

56 2-D array index vs 1-D array index int c [4] [4]; c[0] int *=c[0]; c[i][j] [i*4+j] c[1] c[2] c[3] c[0][0] c[0][1] c[0][2] c[0][3] c[1][0] c[1][1] c[1][2] c[1][3] c[2][0] c[2][1] c[2][2] c[2][3] c[3][0] c[3][1] c[3][2] c[3][3] [0*4+0] [0*4+1] [0*4+2] [0*4+3] [1*4+0] [1*4+1] [1*4+2] [1*4+3] [2*4+0] [2*4+1] [2*4+2] [2*4+3] [3*4+0] [3*4+1] [3*4+2] [3*4+3] 56

57 2-D Array Dynamic Memory Allocation (1) int ** d ; d = (int **) malloc (4 * size of (int *)); for (i=0; i<4; ++i) d[i] = (int *) malloc(4 * sizeof); (int **) d (int *) (int *) (int *) (int *) d[0] d[1] d[2] d[3] 57

58 2-D Array Dynamic Memory Allocation (2) int ** d ; d = (int **) malloc (4 * size of (int *)); for (i=0; i<4; ++i) d[i] = (int *) malloc(4 * sizeof); &d (int **) d (int *) (int *) (int *) (int *) d[0] d[1] d[2] d[3] d[0][0] d[0][1] d[0][2] d[0][3] d[1][0] d[1][1] d[1][2] d[1][3] d[2][0] d[2][1] d[2][2] d[2][3] d[3][0] d[3][1] d[3][2] d[3][3] 58

59 References [1] Essential C, Nick Parlante [2] Efficient C Programming, Mark A. Weiss [3] C A Reference Manual, Samuel P. Harbison & Guy L. Steele Jr. [4] C Language Exress, I. K. Chun