1 / 81

C Program Design C Pointers

C Program Design C Pointers. 主講人：虞台文. Content. Introduction Point Operators Calling Functions by References Pointer Expressions and Pointer Arithmetic Pointers and Arrays Using the const Qualifier with Pointers Arrays of Pointers. C Program Design C Pointers. Introduction.

tyne
Download Presentation

C Program Design C Pointers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. C Program DesignC Pointers 主講人：虞台文

  2. Content • Introduction • Point Operators • Calling Functions by References • Pointer Expressions and Pointer Arithmetic • Pointers and Arrays • Using the const Qualifier with Pointers • Arrays of Pointers

  3. C Program DesignC Pointers Introduction

  4. What is Pointer?

  5. CPU and Memory

  6. 00000000 00000001 00000002 00000003 00000004 00000005 00000006 00000007 00000008 FFFFFFFB FFFFFFFC FFFFFFFD FFFFFFFE FFFFFFFF Memory Space • Program and data are stored in some places of memory. • Addresses (Pointers) are used to identify where the program or data are stored. • CPU accesses a piece of memory data by emitting its address on address bus. address

  7. C Pointers • Powerful, but difficult to master • Simulatecall-by-reference • C only has call-by-value • Close relationship with arrays and strings • important for handling arrays and strings

  8. C Program DesignC Pointers Pointer Variable Definitions and Initialization

  9. Variable is a piece of memory containing a value of a specific type (direct reference). Pointer Variables • Pointer variables contain memory addresses as their values • Indirect reference  • Typed pointers point to the address of a variable of a specific type • Non-typed pointers are simply addresses, i.e., they can point to an addresses of any thing

  10. Pointer Definitions datatype *pointerName Examples: int *ptr1; int *ptr2, *ptr3; char *ptr4; double *ptr5; void *ptr6, *ptr7; Typed pointers Non-typed pointers

  11. Pointer Definitions datatype *pointerName Don’t write as int *ptr2, ptr3; Examples: int *ptr1; int *ptr2, *ptr3; char *ptr4; double *ptr5; void *ptr6, *ptr7; Typed pointers Non-typed pointers

  12. ptr1 ptr2 ptr3 ptr4 ptr5 ptr6 ptr7 Pointer Initialization Where these pointers point to? int *ptr1; int *ptr2, *ptr3; char *ptr4; double *ptr5; void *ptr6, *ptr7;

  13. ptr1 ptr2 ptr3 ptr4 ptr5 ptr6 ptr7 Before pointers are actually used, it is a good habit to initialize them to NULL, meaning that they are pointing to nothing. Pointer Initialization #define NULL 0 int *ptr1=NULL; int *ptr2=NULL, *ptr3=NULL; char *ptr4=NULL; double *ptr5=NULL; void *ptr6=NULL, *ptr7=NULL;

  14. C Program DesignC Pointers Pointer Operators

  15. Point Operators • Reference operator (&) • Retrieve the address of a variable • Dereference operator (*) • Retrieve the value pointed by a pointer (address) • * and & are inverses • They cancel each other out

  16. 5 3.14 Nickname: address-of operator Reference Operator (*) • &: Returns address-of operand int n=5; double x=3.14; . . . . . int *nPtr; double *xPtr; nPtr = &n; xPtr = &x; n x . . . nPtr xPtr

  17. n 5 3.14 x . . . nPtr xPtr 範例： #include <stdio.h> int n=5; double x=3.14; int *nPtr; double *xPtr; main() { /* nPtr gets address of n */ nPtr = &n; /* xPtr gets address of n */ xPtr = &x; printf("Address of n is %p\n" "Address of x is %p\n", nPtr, xPtr); } 00427600 00427608 00427600 00427608

  18. n 5 3.14 x . . . nPtr xPtr 練習： #include <stdio.h> int n=5; double x=3.14; int *nPtr; double *xPtr; main() { /* nPtr gets address of n */ nPtr = &n; /* xPtr gets address of n */ xPtr = &x; printf("Address of n is %p\n" "Address of x is %p\n", nPtr, xPtr); } • 將上例之變數改成main之區域變數，觀察其值，並繪製類似右側之圖。 00427600 00427608 00427600 00427608

  19. n 5 3.14 x . . . nPtr xPtr Nickname: value-point-by operator Dereference Operator (*) • *: Returns value point by operand 00427600 *nPtr = ? *xPtr = ? 00427608 00427600 00427608

  20. n 5 3.14 x . . . nPtr xPtr Nickname: value-point-by operator Dereference Operator (*) • *: Returns value point by operand #include <stdio.h> int n=5; double x=3.14; int *nPtr; double *xPtr; main() { /* nPtr gets address of n */ nPtr = &n; /* xPtr gets address of n */ xPtr = &x; printf("integer value in address %p is %d\n" "double value in address %p is %lf\n", nPtr, *nPtr, xPtr, *xPtr); } 00427600 *nPtr = ? *xPtr = ? 00427608 00427600 00427608

  21. 範例： Showing that * and & are complements of each other. #include <stdio.h> int main( void ) { int a; /* a is an integer */ int *aPtr; /* aPtr is a pointer to an integer */ a = 7; aPtr = &a; /* aPtr set to address of a */ printf( "The address of a is %p" "\nThe value of aPtr is %p", &a, aPtr ); printf( "\n\nThe value of a is %d" "\nThe value of *aPtr is %d", a, *aPtr ); printf( "\n\nShowing that * and & are complements of " "each other\n&*aPtr = %p" "\n*&aPtr = %p\n", &*aPtr, *&aPtr ); return 0; /* indicates successful termination */ } /* end main */

  22. 0012FF7C a 7 aPtr 0012FF7C 範例： Showing that * and & are complements of each other. #include <stdio.h> int main( void ) { int a; /* a is an integer */ int *aPtr; /* aPtr is a pointer to an integer */ a = 7; aPtr = &a; /* aPtr set to address of a */ printf( "The address of a is %p" "\nThe value of aPtr is %p", &a, aPtr ); printf( "\n\nThe value of a is %d" "\nThe value of *aPtr is %d", a, *aPtr ); printf( "\n\nShowing that * and & are complements of " "each other\n&*aPtr = %p" "\n*&aPtr = %p\n", &*aPtr, *&aPtr ); return 0; /* indicates successful termination */ } /* end main */

  23. 0012FF7C a 7 aPtr 0012FF7C • What is the data type of &aPtr? • Modify the following program to also show the address of aPtr. 練習： #include <stdio.h> int main( void ) { int a; /* a is an integer */ int *aPtr; /* aPtr is a pointer to an integer */ a = 7; aPtr = &a; /* aPtr set to address of a */ printf( "The address of a is %p" "\nThe value of aPtr is %p", &a, aPtr ); printf( "\n\nThe value of a is %d" "\nThe value of *aPtr is %d", a, *aPtr ); printf( "\n\nShowing that * and & are complements of " "each other\n&*aPtr = %p" "\n*&aPtr = %p\n", &*aPtr, *&aPtr ); return 0; /* indicates successful termination */ } /* end main */

  24. C Program DesignC Pointers Calling Functions by Reference

  25. C Simple Swap Program #include <stdio.h> main() { int a = 23, b = 47; int t; printf("Before. a: %d, b: %d\n", a, b); t = a; a = b; b = t; printf("After. a: %d, b: %d\n", a, b); }

  26. C Simple Swap Program #include <stdio.h> main() { int a = 23, b = 47; int t; printf("Before. a: %d, b: %d\n", a, b); t = a; a = b; b = t; printf("After. a: %d, b: %d\n", a, b); } define a function to do swapping.

  27. void swap(int a, int b) { int t; t = a; a = b; b = t; } C Simple Swap Program #include <stdio.h> main() { int a = 23, b = 47; printf("Before. a: %d, b: %d\n", a, b); swap(a, b); printf("After. a: %d, b: %d\n", a, b); } Workable?

  28. void swap(int a, int b) { int t; t = a; a = b; b = t; } C Simple Swap Program #include <stdio.h> main() { int a = 23, b = 47; printf("Before. a: %d, b: %d\n", a, b); swap(a, b); printf("After. a: %d, b: %d\n", a, b); } Workable?

  29. void swap(int *a, int *b) { int t; t = *a; *a = *b; *b = t; } Calling Function by Passing References #include <stdio.h> main() { int a = 23, b = 47; printf("Before. a: %d, b: %d\n", a, b); swap(&a, &b); printf("After. a: %d, b: %d\n", a, b); } Workable?

  30. void swap(int *a, int *b) { int t; t = *a; *a = *b; *b = t; } Calling Function by Passing References #include <stdio.h> main() { int a = 23, b = 47; printf("Before. a: %d, b: %d\n", a, b); swap(&a, &b); printf("After. a: %d, b: %d\n", a, b); } Workable?

  31. Review: Bubble Sort void BubbleSort(int data[], int n) { int tmp, i, j; for(i=0; i<n-1; i++) for(j=0; j<n-i-1; j++) if(data[j] > data[j+1]){ tmp = data[j]; data[j] = data[j+1]; data[j+1] = tmp; } }

  32. void swap(int *a, int *b) { int t; t = *a; *a = *b; *b = t; } Bubble Sort void BubbleSort(int data[], int n) { int i, j; for(i=0; i<n-1; i++) for(j=0; j<n-i-1; j++) if(data[j] > data[j+1]) swap(&data[j], &data[j+1]); }

  33.  BubbleSort(&vals[0], 5); vals 0 100 1 30 2 20 3 75 4 15 Pointers vs. Arrays void swap(int *a, int *b) { int t; t = *a; *a = *b; *b = t; } void BubbleSort(int data[], int n) { int i, j; for(i=0; i<n-1; i++) for(j=0; j<n-i-1; j++) if(data[j] > data[j+1]) swap(&data[j], &data[j+1]); } main() { int vals[5]={100, 30, 20, 75, 15}; BubbleSort(vals, 5); /* ............................ */ } int * int * int *

  34.  BubbleSort(&vals[0], 5); vals 0 100 1 30 2 20 3 75 4 15 Pointers vs. Arrays void swap(int *a, int *b) { int t; t = *a; *a = *b; *b = t; } void BubbleSort(int data[], int n) { int i, j; for(i=0; i<n-1; i++) for(j=0; j<n-i-1; j++) if(data[j] > data[j+1]) swap(&data[j], &data[j+1]); } main() { int vals[5]={100, 30, 20, 75, 15}; BubbleSort(vals, 5); /* ............................ */ } int * void BubbleSort(int *data, int n) int *

  35. C Program DesignC Pointers Pointer Expressions and Pointer Arithmetic

  36. Pointer Arithmetic • Increment/decrement pointer • ++ or –- • Add/substract an integer to a pointer • + or += , - or -= • Pointers may be subtracted from each other

  37. Pointer Arithmetic May be meaningless unless performed on an array. • Increment/decrement pointer • ++ or –- • Add/substract an integer to a pointer • + or += , - or -= • Pointers may be subtracted from each other

  38. data data[9] data[8] data[0] data[1] data[2] data[3] data[4] data[5] data[6] data[7] Pointer Arithmetic • Increment/decrement pointer (++ or –-) • Add/substract an integer to a pointer (+ or += , - or -=) • Pointers may be subtracted from each other int data[10]; int i; for(i=0; i<10; i++) data[i] = i; 0 1 2 3 4 5 6 int data[10]; int i, *p; for(i=0, p=data; i<10; i++){ *p = i; p++; } 7 8 9

  39. data data[9] data[8] data[0] data[1] data[2] data[3] data[4] data[5] data[6] data[7] Pointer Arithmetic Which one is performance more effective? • Increment/decrement pointer (++ or –-) • Add/substract an integer to a pointer (+ or += , - or -=) • Pointers may be subtracted from each other int data[10]; int i; for(i=0; i<10; i++) data[i] = i; 0 1 2 3 4 5 6 int data[10]; int i, *p; for(i=0, p=data; i<10; i++){ *p = i; p++; } 7 8 9

  40. data data[9] data[0] data[1] data[2] data[3] data[4] data[5] data[6] data[7] data[8] Pointer Arithmetic int data[10]; int i, *p; for(i=0, p=data; i<10; i++) *p++ = i; • Increment/decrement pointer (++ or –-) • Add/substract an integer to a pointer (+ or += , - or -=) • Pointers may be subtracted from each other 0 1 2 3 4 5 6 int data[10]; int i, *p; for(i=0, p=data; i<10; i++){ *p = i; p++; } 7 8 9

  41. data data[9] data[0] data[1] data[2] data[3] data[4] data[5] data[6] data[7] data[8] Pointer Arithmetic int data[10]; int i, *p; for(i=0, p=data; i<10; i++) *p++ = i; • Increment/decrement pointer (++ or –-) • Add/substract an integer to a pointer (+ or += , - or -=) • Pointers may be subtracted from each other 0 1 2 3 4 5 6 7 int data[10]; int i, *p; for(i=0, p=data; i<10; *p++ = i++) ; 8 9

  42. data 0 1 2 3 4 5 6 7 8 9 data 0 1 2 5 3 4 5 6 7 8 data 0 1 2 5 3 5 6 7 8 8 範例：Insert/Delete Data into/from Arrays void insertAt(int array[], int size, int at, int val); void deleteAt(int array[], int size, int at); insertAt(data, 10, 3, 5); deleteAt(data, 10, 5);

  43. 範例：Insert/Delete Data into/from Arrays /* arrayOp.c */ /* insertAt: insert an element into an int array */ void insertAt(int array[], int size, int at, int val) { int *p, *q; p = &array[at]; q = &array[size-1]; while(q > p){ *q = *(q-1); q--; } *p = val; } /* deleteAt: delete an element from an int array */ void deleteAt(int array[], int size, int at) { int *p, *q; p = &array[at]; q = &array[size-1]; while(p < q){ *p = *(p+1); p++; } }

  44. 範例：Insert/Delete Data into/from Arrays /* main.c */ void insertAt(int array[], int size, int at, int val); void deleteAt(int array[], int size, int at); void listElements(int array[], int size); main() { int data[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}; printf("Origin data:\n"); listElements(data, 10); insertAt(data, 10, 3, 5); printf("After calling insertAt(data, 10, 3, 5):\n"); listElements(data, 10); deleteAt(data, 10, 5); printf("After calling deleteAt(data, 10, 5):\n"); listElements(data, 10); }

  45. data[9] data[8] data[6] data[5] data[4] data[7] data[2] data[1] data[0] data[3] data 0 1 2 3 4 5 6 7 8 9 範例：Pointer Arithmetic #include <stdio.h> main() { int data[]={0, 1, 2, 3, 4, 5, 6, 7, 8, 9}; int *p, *q; printf("data[]={0, 1, 2, 3, 4, 5, 6, 7, 8, 9}\n\n"); p = q = data; printf("p = q = data => p = q = %p\n", data); printf("q - p = %d\n\n", q - p); p++; q += 5; printf("p++; q += 5 => p = %p, q = %p\n", p, q); printf("q - p = %d\n\n", q - p); p -= 3; q--; printf("p -= 3; q-- => p = %p, q = %p\n", p, q); printf("q - p = %d\n\n", q - p); }

  46. data[9] data[8] data[6] data[5] data[4] data[7] data[2] data[1] data[0] data[3] data 0 1 2 3 4 5 6 7 8 9 範例：Pointer Arithmetic #include <stdio.h> main() { int data[]={0, 1, 2, 3, 4, 5, 6, 7, 8, 9}; int *p, *q; printf("data[]={0, 1, 2, 3, 4, 5, 6, 7, 8, 9}\n\n"); p = q = data; printf("p = q = data => p = q = %p\n", data); printf("q - p = %d\n\n", q - p); p++; q += 5; printf("p++; q += 5 => p = %p, q = %p\n", p, q); printf("q - p = %d\n\n", q - p); p -= 3; q--; printf("p -= 3; q-- => p = %p, q = %p\n", p, q); printf("q - p = %d\n\n", q - p); }

  47. Pointer Comparison • <, <=, ==, >=, > • Usually, they apply to pointers of array’s element • p  q is true iff p - q0is true, e.g., p < q is true iff p – q < 0 is true p > q is true iff p – q > 0 is true • NULL == p  Test the validity of p, e.g., #define NULL 0 int *p = NULL; /* p may point to somewhere in the following */ . . . . . . . . . . . . . . . if (NULL==p) printf("p is an invalid pointer"); else printf("%d\n", *p);

  48. Pointer Casting • Pointers of the same type can be assigned to each other • If not the same type, a cast operator must be used • Example: main() { int m=5; int *p, *q; char *r; p = &m; /* ok */ q = p; /* ok */ r = p; /* warning */ }

  49. Pointer Casting • Pointers of the same type can be assigned to each other • If not the same type, a cast operator must be used • Example: main() { int m=5; int *p, *q; char *r; p = &m; /* ok */ q = p; /* ok */ r = (char *) p; /* ok */ }

  50. m p q r 範例：Pointer Casting main() { int m; int *p, *q; char *r; m = 0x1ff; /* 511 */ printf("integer m = %d\n\n", m); p = &m; /* ok */ q = p; /* ok */ r = (char *) p; /* ok */ printf("p, q, r all point to m:\n"); printf("p=%p, q=%p, r=%p\n", p, q, r); printf("*p=%d, *q=%d, *r=%d\n\n", *p, *q, *r); *r = '\0'; printf("After setting the 1st byte of m zero, then\n"); printf("*p=%d, *q=%d, *r=%d\n", *p, *q, *r); } 00 ff 01 00 00

More Related