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Understanding One-Dimensional Arrays in C Programming

Learn about one-dimensional arrays in C, including their declaration, memory allocation, array initialization, and potential problems. Understand the relationship between arrays and pointers in memory management.

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Understanding One-Dimensional Arrays in C Programming

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  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

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