1 / 70

Chapter 9 Arrays and Pointers

Chapter 9 Arrays and Pointers. Chapter 9 Arrays and Pointers. One-dimensional arrays The Relationship between Arrays and Pointers Pointer Arithmetic and Element Size Passing Arrays to Functions Two-Dimensional Arrays Multidimensional Arrays Dynamic Memory Allocation.

deshawne
Download Presentation

Chapter 9 Arrays and 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. Chapter 9 Arrays and Pointers

  2. Chapter 9 Arrays and Pointers • One-dimensional arrays • The Relationship between Arrays and Pointers • Pointer Arithmetic and Element Size • Passing Arrays to Functions • Two-Dimensional Arrays • Multidimensional Arrays • Dynamic Memory Allocation

  3. One-Dimensional Arrays • What is an array? • A sequence of data items • that are of the same type, • that can be indexed, and • that are stored contiguously.

  4. One-Dimensional Arrays #define NUM 100 #include <stdio.h> int main(void){ int grade[NUM]; int i, avg, sum = 0; printf("Input scores:\n"); for (i=0; i<NUM; i++) scanf("%d", &grade[i]); for (i=0; i<NUM; i++) sum = sum + grade[i]; avg = sum/ NUM; printf("Average=%d\n", avg); } • Declaration of an array • Data_Type: the type of elements • Size: constant integral expression • size of the array • The number of elements of the array Example: int grade[100]; float f[10000]; Data_type array_name[size]

  5. One-Dimensional Arrays #define NUM 100 #include <stdio.h> int main(void){ int grade[NUM]; int i, avg, sum = 0; printf("Input scores:\n"); for (i=0; i<NUM; i++) scanf("%d", &grade[i]); for (i=0; i<NUM; i++) sum = sum + grade[i]; avg = sum/ NUM; printf("Average=%d\n", avg); } • Access elements in an array • Use an index • int grade[100]; • grade[index], where index is an integral expression • Elements are indexed from 0 to size-1 •  index must lie in the range 0 to size-1

  6. One-Dimensional Arrays What is the potential problem of this code? #include <stdio.h> int main(void){ int a[5]; int i; for( i=0; i<5; i++) a[i]=i; printf("%d %d \n", a[1]+a[2], a[1]-1); } #include <stdio.h> int main(void){ int a[5]; int i; a[5]=1; } index must lie in the range 0 to size-1 % a.out 3 0

  7. One-Dimensional Arrays • Memory allocation of an array • the compiler assigns an appropriate amount of memory, starting from a base address. • The base address is represented by the array name.

  8. One-Dimensional Arrays • Memory allocation of an array • Example: int grade[100]; • If 4 bytes is used to represent an integer, What is the size of memory assigned to the array? 100 * 4= 400 bytes

  9. One-Dimensional Arrays • Memory assignment of an array • Example: int grade[100]; • 400 bytes is allocated to grade • The base address is represented by the array name, that is, grade. base address

  10. One-Dimensional Arrays • Initialization • Arrays can be initialized within a declaration • An array initializer is a sequence of initializing values written as a brace-enclosed, comma-separated list. • Example: • float x[4] = {-1.1, 0.2, 3.0, 4.4};

  11. One-Dimensional Arrays • Initialization (cont’d) • When a list of initializers is shorter than the number of array elements to be initialized, the remaining elements are initialized to zero #define NUM 10 #include <stdio.h> int main(void){ int grade[NUM] = {99,89,89}; printf("%d\n", grade[3]); } 0

  12. One-Dimensional Arrays • Initialization (cont’d) • If an array is declared without a size and is initialized to a series of values, it is implicitly given the size of the number of initializers. • Example: • int a[]={3,4,5,6}; What is the value of sizeof(a)? 4*4=16

  13. One-dimensional arrays • Summary • An array is a sequence of data items that are of the same type, that can be indexed, and that are stored contiguously. • Declaration of an array: • Data_type array_name[size] • Access elements in an array: • p[index] • Elements are indexed from 0 • Index should be in range [0, size-1] • Memory allocation: an appropriate amount of memory, starting from a base address, represented by the array name. • Initialization

  14. Outline • One-dimensional arrays • The Relationship between Arrays and Pointers • Pointer Arithmetic and Element Size • Passing Arrays to Functions • Two-Dimensional Arrays • Multidimensional Arrays • Dynamic Memory Allocation

  15. The relationship between arrays and pointers Memory • An array name: • the base address of the array • the initial location in memory where the array is stored; • the address of the first element (index 0) of the array. grade[99] grade[2] grade[1] grade[0] Base address: grade

  16. The relationship between arrays and pointers • An array name is the base address of the array • the initial location in memory where the array is stored; • the address of the first element (index 0) of the array. •  an address, or pointer value • Difference betw. an array name and a pointer • A pointer is a variable that takes addresses as values. • The value of a pointer can be changed. • An array name is a particular fixed address that can be thought of as a constant pointer. • The value of an array name cannot be changed.

  17. Class on Nov 8

  18. Outline • One-dimensional arrays • The Relationship between Arrays and Pointers • Pointer Arithmetic and Element Size • Passing Arrays to Functions • Two-Dimensional Arrays • Multidimensional Arrays • Dynamic Memory Allocation

  19. Pointer Arithmetic and Element Size • Pointer Arithmetic • If p is a pointer to a particular type, then the expression p+1 yields • the machine address for the next variable of that type • Given an array int a[10], • &(a[i]) is equivalent to a+i

  20. Pointer Arithmetic and Element Size If p is a pointer to a particular type, • p+1 is the machine address for the next variable of that type #define NUM 5 #include <stdio.h> int main(void){ int *p, grade[NUM]={100,80,90,90,80}; int sum = 0; p = grade; printf("p : %u\n", p); printf("p+1 : %u\n", p+1); printf("p+2 : %u\n", p+2); printf("*(p+1): %d\n", *(p+1)); } % a.out p : 4290705240 p+1 : 4290705244 p+2 : 4290705248 *(p+1): 80

  21. Pointer Arithmetic and Element Size • Pointer Arithmetic • p+i and ++p and p+=i are defined in a similar fashion If p is a pointer to a particular type, • p+1 is the correct machine address for storing or accessing the next variable of that type

  22. Pointer Arithmetic and Element Size If p is a pointer to a particular type, • p+1 is the machine address for the next variable of that type #define NUM 5 #include <stdio.h> int main(void){ int *p, grade[NUM]={90,80,70,60,50}; int sum = 0; p = grade; printf("%d\n", *(p++)); printf("%d\n", *(++p)); printf("%d\n", *(p+=2)); } % a.out 90 70 50

  23. Pointer Arithmetic and Element Size • Programming Problem • int grades[5]; • Write a for loop to access each element of this array using pointer arithmetic. int *p; /* expr1: p points to the first element */ /* condition: p points to an element in the array */ /* expr3: p points to the next element */ for( expr1; condition; expr3) { …… }

  24. Pointer Arithmetic and Element Size #define NUM 5 #include <stdio.h> int main(void){ int *p, grade[NUM]={100,100,100,100,100}; int sum = 0; for(p=grade; p <= grade+NUM-1; ++p) sum += *p; printf("sum = %d\n", sum); return 0; } % a.out sum = 500

  25. Pointer Arithmetic and Element Size • Pointer Arithmetic • If p and q are both pointing to elements of an array, then what is the value of (p – q) the int value representing the number of array elements between p and q. If p is a pointer to a particular type, • p+1 is the correct machine address for storing or accessing the next variable of that type

  26. Pointer Arithmetic and Element Size Assume a double is stored in 8 bytes. #include <stdio.h> int main(void){ double a[2], *p, *q; p = &a[0]; q = p +1; printf("%d\n", q-p); printf("%d\n", (int)q-(int)p); return 0; } % a.out 1 8 The value printed by the last statement is system-dependent. On many system a double is stored in eight bytes.

  27. Pointer Arithmetic and Element Size • Summary • If p is a pointer to a particular type, • p+1: • yields the correct machine address for storing or accessing the next variable of that type • p++, ++p, p+=i : • defined in a similar fashion. • p – q: • the int value representing the number of array elements between p and q.

  28. Outline • One-dimensional arrays • The Relationship between Arrays and Pointers • Pointer Arithmetic and Element Size • Passing Arrays to Functions • Two-Dimensional Arrays • Multidimensional Arrays • Dynamic Memory Allocation

  29. Passing Arrays to Functions • Programming Problem: • Write a function to increment each element of an array by one. • How to pass the values of elements of an array to a function? • How to modify the values of elements of the array in the function?

  30. Write a function to increment each element by one? #include <stdio.h> void inc(int a[], int n){ int i; for (i=0; i<n; ++i){ a[i]=a[i]+1; } } int main(void){ int a[]= {7, 3, 6, 2}; int i; for(i=0;i<3;i++) printf("%4d", a[i]); printf("\n"); inc(a, 4); for(i=0;i<3;i++) printf("%4d", a[i]); printf("\n"); } • How to pass the values of elements of an array to a function? • How to modify the values of elements of the array in the function?

  31. Passing Arrays to Functions • Passing Arrays to Functions • In function definition, • a formal parameter that is declared as an array is actually a pointer.

  32. Write a function to increment each element by one? #include <stdio.h> void inc(int a[], int n){ int i; for (i=0; i<n; ++i){ a[i]=a[i]+1; } } int main(void){ int a[]= {7, 3, 6, 2}; int i; for(i=0;i<3;i++) printf("%4d", a[i]); printf("\n"); inc(a, 4); for(i=0;i<3;i++) printf("%4d", a[i]); printf("\n"); } • a formal parameter: that is declared as an array is actually a pointer. •  a is a pointer

  33. Passing Arrays to Functions • Passing Arrays to Functions • a formal parameter that is declared as an array is actually a pointer. • When an array is being passed, its base address is passed call-by-value. The array elements themselves are not copied. • By using the base address, we can access and update the elements in an array

  34. inc: a inc: n #include <stdio.h> void inc(int a[], int n){ int i; for (i=0; i<n; ++i){ a[i]=a[i]+1; } } int main(void){ int a[]= {7, 3, 6, 2}; int i; for(i=0;i<3;i++) printf("%4d", a[i]); printf("\n"); inc(a, 4); for(i=0;i<3;i++) printf("%4d", a[i]); printf("\n"); } Memory a[3] 2 3 6 a[2] 7 4 a[1] 3 4290705256 a[0] 8 7 4290705256 4 a[i]: the ith element (indexed from 0) in the array with based address a.

  35. Passing Arrays to Functions • Summary • In function definition, a formal parameter that is declared as an array is actually a pointer. • When an array is being passed, its base address is passed call-by-value. The array elements themselves are not copied. • By using the base address, we can access and update the elements in an array

  36. Outline • One-dimensional arrays • The Relationship between Arrays and Pointers • Pointer Arithmetic and Element Size • Passing Arrays to Functions • Two-Dimensional Arrays • Multidimensional Arrays • Dynamic Memory Allocation

  37. One-dimensional arrays • Summary • An array is a sequence of data items • that are of the same type, • that can be indexed, and • that are stored contiguously. • Declaration of an array: • Data_type array_name[size]

  38. One-dimensional arrays • Summary • Access elements in an array: • p[index] • Elements are indexed from 0 • Index should be in range [0, size-1] • The compiler assigns an appropriate amount of memory, starting from a base address, represented by the array name. • Initialization

  39. One-dimensional arrays • Summary • The Relationship between Arrays and Pointers • The array name is the base address of the array • Difference betw. an array name and a pointer • A pointer is a variable that takes addresses as values. • The value of a pointer can be changed. • An array name is a particular fixed address that can be thought of as a constant pointer. • The value of an array name is decided by the system and cannot be changed.

  40. One-dimensional arrays • Summary • Pointer Arithmetic and Element Size If p is a pointer to a particular type, • p+1: • yields the machine address for the next variable of that type • p++, ++p, p+=i : • defined in a similar fashion. • p – q: • the int value representing the number of array elements between p and q.

  41. One-dimensional arrays • Summary • Passing Arrays to Functions • Function definition: a parameter that is declared as an array is actually a pointer. • When an array is being passed, its base address is passed call-by-value. The array elements themselves are not copied. • By using the base address, we can access and update the elements in an array

  42. Outline • One-dimensional arrays • The Relationship between Arrays and Pointers • Pointer Arithmetic and Element Size • Passing Arrays to Functions • Two-Dimensional Arrays • Multidimensional Arrays • Dynamic Memory Allocation

  43. Two-Dimensional Array • An array is a sequence of data items • that are of the same type, • that can be indexed, and • that are stored contiguously. • An array can be of any type. • Question: What is an array of arrays? • Two-Dimensional array

  44. Two-Dimensional Array • Two-dimensional array: • Arrays of arrays, • Example: • int a[3][4]; • A two-dimensional array of int variables • a[i][j]: the element in the ith row, jth column of the array (counting from 0). a[0][0] a[0][1] a[0][2] a[0][3] a[1][0] a[1][1] a[1][2] a[1][3] a[2][0] a[2][1] a[2][2] a[2][3]

  45. Two-Dimensional Array #include <stdio.h> int main(void){ int a[3][4], i, j, sum =0; for (i=0; i<3; ++i) for (j=0; j<4; ++j) a[i][j]=i+j; for (i=0; i<3; ++i){ for (j=0; j<4; ++j) printf("a[%d][%d] = %d ", i, j, a[i][j]); printf("\n"); } return 0; } a[0][0] a[0][1] a[0][2] a[0][3] a[1][0] a[1][1] a[1][2] a[1][3] a[2][0] a[2][1] a[2][2] a[2][3]

  46. Two-Dimensional Array #include <stdio.h> int main(void){ int a[3][4], i, j, sum =0; for (i=0; i<3; ++i) for (j=0; j<4; ++j) a[i][j]=i+j; for (i=0; i<3; ++i){ for (j=0; j<4; ++j) printf("a[%d][%d] = %d ", i, j, a[i][j]); printf("\n"); } return 0; } % a.out a[0][0] = 0 a[0][1] = 1 a[0][2] = 2 a[0][3] = 3 a[1][0] = 1 a[1][1] = 2 a[1][2] = 3 a[1][3] = 4 a[2][0] = 2 a[2][1] = 3 a[2][2] = 4 a[2][3] = 5

  47. Two-Dimensional Array #include <stdio.h> int main(void){ int a[3][2], i, j; for (i=0;i<3;i++) for (j=0;j<2;j++) a[i][j]=i*10+j; printf("%d\n", a[2][1]/2); printf("%d\n", a[1][1] * (a[0][0]+2)); printf("%d\n", a[3][1]/2); } % a.out 10 22 -2130880

  48. Two-Dimensional Array • Two dimensional array: array of arrays • DataType array_name[n1][n2]; • array of n1 arrays • each of these n1 arrays is an array of n2 values of DataType • Example: int a[3][4]; • a is an array that has 3 arrays • each of these 3 arrays is an array of 4 ints;

  49. a[2] a[1] a[0] Two-Dimensional Array Memory Example: int a[3][4]; a[i], i=0,1,2 : an array of 4 ints, • a[0]: the 0th row • array: a[0][0], a[0][1], a[0][2], a[0][3] • a[0] is the base address of this array • a[1]: the 1st row • array: a[1][0], a[1][1], a[1][2], a[1][3] • a[1] is the base address of this array • a[2]: the 2nd row • array: a[2][0], a[2][1], a[2][2], a[2][3] • a[2] is the base address of this array a[2][3] a[2][2] a[2][1] a[2][0] a[1][3] a[1][2] a[1][1] a[1][0] a[0][3] a[0][2] a[0][1] a[0][0]

  50. a[2] a[1] a[0] Two-Dimensional Array Memory #include <stdio.h> int main(void){ int a[3][4], i, j, *p; for (i=0;i<3;i++) for (j=0;j<4;j++) a[i][j]=i*10+j; p=a[0]; for (j=0;j<4;j++){ printf("%2d ", p[j]); } } a[2][3] a[2][2] a[2][1] a[2][0] a[1][3] a[1][2] a[1][1] a[1][0] a[0][3] a[0][2] a[0][1] a[0][0] % a.out 0 1 2 3

More Related