C Programming : Pointers and Arrays, Pointers and Strings

Programming and Linear Data Structures, Pointers and Arrays,
Pointers and Strings

1. UNIT I : Pointers and Arrays, Pointers and Strings By Mr.S.Selvaraj Asst. Professor (SRG) / CSE Kongu Engineering College Perundurai , Erode, Tamilnadu , India Thanks to and Resource from : Sumitabha Das, “Computer Fundamentals and C Programming”, 1st Edition, McGraw Hill, 2018. 20CST21 – Programming and Linear Data Structures

2. Preamble and Course Outcomes 4/21/2021 1.0 _ Pointers and Arrays 2

3. Syllabus – Unit Wise 4/21/2021 1.0 _ Pointers and Arrays 3

4. List of Exercises 4/21/2021 1.0 _ Pointers and Arrays 4

5. Text Book and Reference Book 4/21/2021 1.0 _ Pointers and Arrays 5

6. Unit I : Contents Pointers- Introduction Pointers and 1D array Passing an array to a function Returning an array from function NULL pointers Array of pointers Pointer-to-pointer Pointers and 2D array Generic pointers Dangling Pointer Using Pointers for string manipulation Two dimensional array of strings Array of pointers to strings. 4/21/2021 6 1.0 _ Pointers and Arrays

7. Pointers- Introduction Every variable in C language has a name and a value associated with it. When a variable is declared , a specific block of memory within the computer is allocated to hold the value of that variable. Each memory location has a distinct address . 4/21/2021 1.0 _ Pointers and Arrays 7

8. Pointers- Introduction For example, we live in a house and our house has an address , which helps other people to identify our house. The same way the value of the variable is identified using the address of that variable. This address of the variable or memory location can be stored in a pointer. 4/21/2021 1.0 _ Pointers and Arrays 8

9. Pointers- Introduction Using pointer we can have direct access to a memory location because they store address of a variable or memory location. Pointers are closely related to low level memory operations. Before introducing the pointers in programming, let us understand what happens during a variable definition. 4/21/2021 1.0 _ Pointers and Arrays 9

10. Pointers- Introduction Let us consider the following statement : When the above statement is compiled and executed, the following will happen: The compiler allocates memory to store an integer . Next , compiler assigns a label to the allocated memory location as n . Finally, the constant 10 is stored in that location. 4/21/2021 1.0 _ Pointers and Arrays 10

11. Advantages of Pointers Pointers provide direct access to memory . Pointers are used to return more than one value from a function. Pointers allow dynamic memory allocation and deallocation . Reduces the storage space required by the program. Pointers are more efficient in handling arrays and structures. Pointers are used to get reference of a variable or function . Pointers increases program execution speed. Pointer allows to create complex data structures such as linked list, stack, queues, trees, graphs etc. 4/21/2021 1.0 _ Pointers and Arrays 11

12. Pointers- Introduction Pointer is a variable which stores the address of another variable. It is used to allocate memory dynamically . Memory allocation done at run time is known as Dynamic memory allocation . Pointer variable may be any valid C data type such as int , float, double, char etc. For any type of pointers , only four bytes of memory is allocated for storing the address. 4/21/2021 1.0 _ Pointers and Arrays 12

13. Example 4/21/2021 1.0 _ Pointers and Arrays 13

14. Pointer Declaration 4/21/2021 1.0 _ Pointers and Arrays 14 Pointer can be declared with a variable preceded by * . Pointer can be declared using the following syntax:

15. Pointer operators Reference or Address of operator ( & ) Dereference operator or indirection operator or Value at Address Operator ( * ) 4/21/2021 1.0 _ Pointers and Arrays 15

16. Reference Operator - Example 4/21/2021 1.0 _ Pointers and Arrays 16

17. Dereference operator (*) To access the value at the address stored in the pointer variable, we can use dereference operator (*) . This is an unary operator * that returns the value of the variable located at the address specified by its operand. 4/21/2021 1.0 _ Pointers and Arrays 17

18. Dereference Operator - Example 4/21/2021 1.0 _ Pointers and Arrays 18

19. Example 1 4/21/2021 1.0 _ Pointers and Arrays 19

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21. Swapping using function and pointers we have to use pointers concept. Bz , Parameters are always passed by value in C and function can not return more than one value . 4/21/2021 1.0 _ Pointers and Arrays 21

22. Example 2 4/21/2021 1.0 _ Pointers and Arrays 22

23. Example 3 4/21/2021 1.0 _ Pointers and Arrays 23

24. Pointer Arithmetic A pointer is a variable which holds the address of another variable. Address is represented by a numeric value . We cannot perform every type of arithmetic operation on pointers. The following arithmetic operations are the only valid arithmetic operations performed on pointers: 4/21/2021 1.0 _ Pointers and Arrays 24

25. Pointer Arithmetic For example, assume the integer pointer ptr which contains the address 1000 . If we are incrementing the pointer ptr by 1 ( using ++ ptr or ptr ++ ), we will get 1002 ( i.e 1000 + 1 * 2 ) Instead of 1001 because the size of the int data type is 2 bytes (Note: Size of the datatype changes depends on the compiler used). If we are using a system where the size of int is 4 bytes then we would get 1004 ( i.e 1000 + 1 * 4 ). 4/21/2021 1.0 _ Pointers and Arrays 25

26. Pointer Arithmetic - Example Suppose the address of the variables i, f and ch are 1000, 2000 , 3000 respectively. Therefore iptr , fptr and cptr will have initial values of 1000, 2000 and 3000 . Assume 1, 2 and 4 bytes of memory is allocated for char, int and float variables respectively. 4/21/2021 1.0 _ Pointers and Arrays 26

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30. Example 4 - Print all the alphabets using pointer 4/21/2021 1.0 _ Pointers and Arrays 30

31. Example 5 - Add two integers using pointers 4/21/2021 1.0 _ Pointers and Arrays 31

32. Example 6 - Add two integers using function by passing address of two numbers as arguments to function. 4/21/2021 1.0 _ Pointers and Arrays 32

33. Pointer Arithmetic between Two Pointers Arithmetic operation between two pointers is performed only on same type of pointers. Let us take two integer pointers ptr1 and ptr2 with addresses 1000 and 1008 respectively. The expression ptr2 - ptr1 will give the result as 4 , since the memory location allocated for integer data type is 2 bytes . If we subtract ptr2 from ptr1 i.e the expression ( ptr1 - ptr2 ) will give the result -4 . 4/21/2021 1.0 _ Pointers and Arrays 33

34. Example 7 4/21/2021 1.0 _ Pointers and Arrays 34

35. Thank you 4/21/2021 1.0 _ Pointers and Arrays 35

36. UNIT I : Pointers and Arrays, Pointers and Strings By Mr.S.Selvaraj Asst. Professor (SRG) / CSE Kongu Engineering College Perundurai , Erode, Tamilnadu , India Thanks to and Resource from : Sumitabha Das, “Computer Fundamentals and C Programming”, 1st Edition, McGraw Hill, 2018. 20CST21 – Programming and Linear Data Structures

37. Unit I : Contents Pointers- Introduction Pointers and 1D array Passing an array to a function Returning an array from function NULL pointers Array of pointers Pointer-to-pointer Pointers and 2D array Generic pointers Dangling Pointer Using Pointers for string manipulation Two dimensional array of strings Array of pointers to strings. 4/21/2021 37 2.0 _ Pointers and Arrays

38. Pointers and Arrays We are already familiar with arrays , which store multiple values of the same data type . Array elements are stored in contiguous memory locations. When an array is declared, compiler allocates required amount of contiguous memory locations to hold all the elements of the array. Memory is allocated by the compiler during compilation time itself. Base address of the array is the address of the first element of the array. We can create a pointer which can be used to point an array . In an one dimensional array, we can use single subscript to access the array elements. Name of the 1-D array or &array[0] points to the address of the first element of an array. 4/21/2021 2.0 _ Pointers and Arrays 38

39. Pointers and 1D Arrays Suppose we declare an integer array “ A ” with 5 elements and assuming that the base address of the array a is 1000 and each integer element requires two bytes of memory. The five elements will be stored as follows: Here array name “ A ” acts as a constant pointer which will give the base address of the array and points to the first element of the array , A[0]. i.e., the value of A and &A[0] are same . 4/21/2021 2.0 _ Pointers and Arrays 39

40. Pointers and 1D Arrays We can declare another pointer of type int to point to the array A . Now we can access every element of the array A using pointer p. (*p) is used to access the element A[0] which has the value 1 in the given example. To access the next element in the array A , increment the pointer p by 1 using (p++) or (++p). Now we can give *p to get A[1]. It gives the value 2 . To get the next element in the array A, increment the pointer p by 1 and use *p. 4/21/2021 2.0 _ Pointers and Arrays 40

41. Example 1- Program to demonstrates the use of pointer to access the array elements 4/21/2021 2.0 _ Pointers and Arrays 41

42. Example 2- use array name as a pointer to access the array elements 4/21/2021 2.0 _ Pointers and Arrays 42

43. In the above program, A is a constant pointer which always points to first element A[0] . We cannot move this pointer to next element using ++ operator since base address of the array cannot be changed. To access the elements of the array using a constant pointer A, we can use the following statements: 4/21/2021 2.0 _ Pointers and Arrays 43

44. Example 3- Program to display the elements of an array in reverse order using pointer. 4/21/2021 2.0 _ Pointers and Arrays 44

45. Example 4 – Program to add the elements of an array using pointer 4/21/2021 2.0 _ Pointers and Arrays 45

46. Example 5 – Program to demonstrate increment and decrement operators with pointers and arrays. 4/21/2021 2.0 _ Pointers and Arrays 46

47. Example 6 - Program to copy an array to another array using pointers 4/21/2021 2.0 _ Pointers and Arrays 47

48. Thank you 4/21/2021 2.0 _ Pointers and Arrays 48

49. UNIT I : Pointers and Arrays, Pointers and Strings By Mr.S.Selvaraj Asst. Professor (SRG) / CSE Kongu Engineering College Perundurai , Erode, Tamilnadu , India Thanks to and Resource from : Sumitabha Das, “Computer Fundamentals and C Programming”, 1st Edition, McGraw Hill, 2018. 20CST21 – Programming and Linear Data Structures

50. Unit I : Contents Pointers- Introduction Pointers and 1D array Passing an array to a function Returning an array from function Pointers and 2D array NULL pointers Array of pointers Pointer-to-pointer Generic pointers Dangling Pointer Using Pointers for string manipulation Two dimensional array of strings Array of pointers to strings. 4/21/2021 50 2.0 _ Passing and Returing an array to_from a function

51. Passing an array to a function We can pass an individual elements or entire array to a function as an argument. To pass an entire array to a function , only the name of the array is passed as an argument. When address is passed as an argument , the function definition should have an array or a pointer as a parameter to receive the passed address. When an array is passed to a function, the changes made by the function affect the original array . 4/21/2021 2.0 _ Passing and Returing an array to_from a function 51

52. Function Definition with array as a parameter We can receive an array in a function using one of the following two methods. The name of the array is passes as an argument , which is the address of the first element of the array. 4/21/2021 2.0 _ Passing and Returing an array to_from a function 52

53. Example 1 – Program to demonstrate the average of all the elements of an array. 4/21/2021 2.0 _ Passing and Returing an array to_from a function 53

54. Method 1 4/21/2021 2.0 _ Passing and Returing an array to_from a function 54

55. Method 2 4/21/2021 2.0 _ Passing and Returing an array to_from a function 55

56. Method 3 4/21/2021 2.0 _ Passing and Returing an array to_from a function 56

57. Example 2 - Write a program in C to sort an array using Pointer 4/21/2021 2.0 _ Passing and Returing an array to_from a function 57

58. Returning an array from a function C does not allow to return an entire array from a function. However, we can return a pointer to the base address(address of first element of array) of an array by specifying the name of the array without an index . 4/21/2021 2.0 _ Passing and Returing an array to_from a function 58

59. Returning an array from a function If we want to return a 1-D array from a function, we have to declare a function returning a pointer as in the following example : 4/21/2021 2.0 _ Passing and Returing an array to_from a function 59

60. Returning an array from a function We should not return base pointer of a local array declared inside a function because as soon as control returns from a function all local variables gets destroyed . If we want to return a local array then we should declare it as a static variable so that it retains it's value after function call. 4/21/2021 2.0 _ Passing and Returing an array to_from a function 60

61. Example 2 - Program to generate first N even numbers and return as an array. 4/21/2021 2.0 _ Passing and Returing an array to_from a function 61

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63. Thank you 4/21/2021 2.0 _ Passing and Returing an array to_from a function 63

64. UNIT I : Pointers and Arrays, Pointers and Strings By Mr.S.Selvaraj Asst. Professor (SRG) / CSE Kongu Engineering College Perundurai , Erode, Tamilnadu , India Thanks to and Resource from : Sumitabha Das, “Computer Fundamentals and C Programming”, 1st Edition, McGraw Hill, 2018. 20CST21 – Programming and Linear Data Structures

65. Unit I : Contents Pointers- Introduction Pointers and 1D array Passing an array to a function Returning an array from function Pointers and 2D array Array of pointers NULL pointers Pointer-to-pointer Generic pointers Dangling Pointer Using Pointers for string manipulation Two dimensional array of strings Array of pointers to strings. 4/21/2021 65 1.4 _ Pointers and 2D Arrays

66. Pointers and 2D array In a two dimensional array , we can access each element by using two subscripts. first subscript represents the row number and second subscript represents the column number Consider the following declaration statement, 4/21/2021 1.4 _ Pointers and 2D Arrays 66

67. Pointers and 2D array The above declaration statement creates a 3x4 matrix initialized with the following values: 4/21/2021 1.4 _ Pointers and 2D Arrays 67

68. Pointers and 2D array The element at i th row , j th column can be accessed using a[i][j]. To access the value 7, we have to use a[1][2]. Let us see how pointer is used to access the 2-D array. As we know, name of the array gives the base address of the array i.e., address of a[0][0] . To access a[i][j] using base pointer of the array, we can use the following generalized form: 4/21/2021 1.4 _ Pointers and 2D Arrays 68

69. Pointers and 2D array The following figure shows how the above 2-D array will be stored in memory . Array elements are stored in memory row by row. 4/21/2021 1.4 _ Pointers and 2D Arrays 69

70. Pointers and 2D array In a 2-D array, each row will be considered as a one dimensional array . Hence 2-D array can be considered as a collection of several one dimensional arrays . In this example, each row is an array of 4 integers. We know that the name of an array is a constant pointer that points to th row which contains address 1000 . 4/21/2021 1.4 _ Pointers and 2D Arrays 70

71. Pointers and 2D array 4/21/2021 1.4 _ Pointers and 2D Arrays 71

72. Pointers and 2D array Consider a two dimensional array mat[ROWS] [COLS] where ROWS and COLS are some integer value . We can use any one of the following two notations to access the two dimensional array. The first int mat[][COLS] is general array notation. Whereas int (*mat)[COLS] is a pointer to array . 4/21/2021 1.4 _ Pointers and 2D Arrays 72

73. Example - read and display a 3x3 matrix 4/21/2021 1.4 _ Pointers and 2D Arrays 73

74. Passing a 2D array to a function For passing multidimensional arrays , First array dimension does not have to be specified. The second dimension must be explicitly specified or a pointer to pointer as a parameter to receive the address. 4/21/2021 1.4 _ Pointers and 2D Arrays 74

75. Example - print 2-D array using function 4/21/2021 1.4 _ Pointers and 2D Arrays 75

76. Example - add two 3 x 3 matrices using pointer 4/21/2021 1.4 _ Pointers and 2D Arrays 76

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81. Example - display Lower triangular and Upper triangular of a given matrix of size mxn 4/21/2021 1.4 _ Pointers and 2D Arrays 81

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86. Example - to print and find sum of diagonal elements of a given matrix. 4/21/2021 1.4 _ Pointers and 2D Arrays 86

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91. Returning a 2D array from Function Returning a 2D array from function is not as straight as passing an array to a function. Arrays in C are passed by reference, hence any changes made to array passed as argument persists after the function. So you can accept the output array you need to return, as a parameter to the function. The following program demonstrates how a 2-D array is returned from a function by passing resultant array also as a parameter. In the following program, main() receives two matrices as input and these two matrices along with resultant matrix are passed as arguments to the function sum(). The sum() function adds the two matrices and return the resultant matrix as result to the main() function. 4/21/2021 1.4 _ Pointers and 2D Arrays 91

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97. Further Reference To Pass and Return an array(1D & 2D) to / from function with/without using pointers please refer this link...... https://codeforwin.org/2017/12/pass-return-array-function-c.html#return-array-multi 4/21/2021 1.4 _ Pointers and 2D Arrays 97

98. Thank you 4/21/2021 1.4 _ Pointers and 2D Arrays 98

99. UNIT I : Pointers and Arrays, Pointers and Strings By Mr.S.Selvaraj Asst. Professor (SRG) / CSE Kongu Engineering College Perundurai , Erode, Tamilnadu , India Thanks to and Resource from : Sumitabha Das, “Computer Fundamentals and C Programming”, 1st Edition, McGraw Hill, 2018. 20CST21 – Programming and Linear Data Structures

100. Unit I : Contents Pointers- Introduction Pointers and 1D array Passing an array to a function Returning an array from function Pointers and 2D array Array of pointers NULL pointers Pointer-to-pointer Generic pointers Dangling Pointer Using Pointers for string manipulation Two dimensional array of strings Array of pointers to strings. 4/21/2021 100 1.5 _ Pointers Concepts

101. Array of Pointers When we want to maintain an array to store the address of memory locations of type int or float or char or any other data type available, we have to create an array of pointers . Here is the syntax to declare an array of pointers which is almost same as that of declaring an array of int , float or char etc., 4/21/2021 1.5 _ Pointers Concepts 101

102. Array of Pointers Here ptr is an array of 5 integer pointers ie ., the integer pointers are ptr [0], ptr [1] …. ptr [4]. Each element in this array can hold the address of an integer variable. 4/21/2021 1.5 _ Pointers Concepts 102

103. Example 4/21/2021 1.5 _ Pointers Concepts 103

104. NULL Pointer Pointer is assigned a value NULL if it is not assigned any valid address of the memory. NULL pointer is a pointer which points to nothing. Pointer is initialized with a value NULL or regardless of the pointer type. 4/21/2021 1.5 _ Pointers Concepts 104

105. Example 4/21/2021 1.5 _ Pointers Concepts 105

106. Uses of NULL pointer If no valid memory address is available to store it in a pointer, initialize a pointer with NULL. If pointer is used in a function block , before using it, check whether the pointer is not a NULL value. Make a habit of initializing a pointer before using it. Initialize a pointer with a valid address or NULL. 4/21/2021 1.5 _ Pointers Concepts 106

107. Example 4/21/2021 1.5 _ Pointers Concepts 107

108. Pointer to a Pointer C supports a pointer that points to another pointer. This pointer in turn can point to another pointer, and so on. At the simplest level, a pointer to a pointer is declared in this manner: 4/21/2021 1.5 _ Pointers Concepts 108

109. Pointer to a Pointer 4/21/2021 1.5 _ Pointers Concepts 109

110. Pointer to a Pointer This pointer currently points nowhere. It can be completely dereferenced after (i) pointing q to a pointer variable , say, p, and (ii) pointing p to another object , say, the simple variable x. Thus, if p = &x and q = &p , then the value of x can be evaluated using double indirection: 4/21/2021 1.5 _ Pointers Concepts 110

111. Pointer to a Pointer This expression can be interpreted as *(*q), which means using the dereferencing operator twice. You need to use an extra * for every additional level of indirection. Thus, you can create a pointer to q, say , r, and then use ***r or **q or *p to evaluate x . 4/21/2021 1.5 _ Pointers Concepts 111

112. Example 1 4/21/2021 1.5 _ Pointers Concepts 112

113. Example 2 4/21/2021 1.5 _ Pointers Concepts 113

114. Uses and Caution of **P Why do we need pointer-to-pointers? Can a function that swaps two regular variables be used to swap two pointers? No, it can’t because the function will swap the copies of these pointers. The solution obviously lies in the use of pointer-to-pointers as function arguments . double-indirection pointers are Implicitly used in handling 2D arrays. Explicitly used in applications that need to manipulate pointer values inside a function . 4/21/2021 1.5 _ Pointers Concepts 114

115. Generic Pointer (or) void Pointer There is no data type associated with generic pointer. It can be used to store address of any data type and it can be converted to any type using type casting . void pointers cannot be dereferenced . Generic pointer can be declared using the keyword void . 4/21/2021 1.5 _ Pointers Concepts 115

116. Example 4/21/2021 1.5 _ Pointers Concepts 116

117. Dangling Pointer 4/21/2021 1.5 _ Pointers Concepts 117 Even though the memory block is freed , it is still accessible because the pointer to it is not lost ( dangling pointer ). Dangling pointers arise during  object destruction , when an object that has an incoming reference is deleted or deallocated , without modifying the value of the pointer, so that the pointer still points to the memory location of the deallocated memory.

118. Dangling Pointer Freeing a memory block with free(p) doesn’t necessarily make it inaccessible because p continues to point to the block. p is now a dangling pointer . As a safety measure, it should be set to NULL immediately after invoking free. Because this memory—partially or totally —may have already been allocated , a subsequent attempt to access it using p could produce erroneous results or even cause the program to crash. Setting p to NULL solves the problem easily. 4/21/2021 1.5 _ Pointers Concepts 118

119. Dangling Pointer A pointer pointing to a memory location that has been deleted (or freed) is called dangling pointer. There are  three  different ways where Pointer acts as dangling pointer: De-allocation of memory Function Call Variable goes out of scope 4/21/2021 1.5 _ Pointers Concepts 119

120. Example – De-allocation of memory 4/21/2021 1.5 _ Pointers Concepts 120

121. Example – Function Call 4/21/2021 1.5 _ Pointers Concepts 121

122. Example – Function Call - Solution 4/21/2021 1.5 _ Pointers Concepts 122

123. Example – Variable goes out of scope 4/21/2021 1.5 _ Pointers Concepts 123

124. Wild Pointer A pointer which has not been initialized to anything (not even NULL ) is known as wild pointer. The pointer may be initialized to a non-NULL garbage value that may not be a valid address. 4/21/2021 1.5 _ Pointers Concepts 124

125. Thank you 4/21/2021 1.5 _ Pointers Concepts 125

126. UNIT I : Pointers and Arrays, Pointers and Strings By Mr.S.Selvaraj Asst. Professor (SRG) / CSE Kongu Engineering College Perundurai , Erode, Tamilnadu , India Thanks to and Resource from : Sumitabha Das, “Computer Fundamentals and C Programming”, 1st Edition, McGraw Hill, 2018. 20CST21 – Programming and Linear Data Structures

127. Unit I : Contents Pointers- Introduction Pointers and 1D array Passing an array to a function Returning an array from function Pointers and 2D array Array of pointers NULL pointers Pointer-to-pointer Generic pointers Dangling Pointer Using Pointers for string manipulation Two dimensional array of strings Array of pointers to strings. 4/21/2021 127 1.6 _ Pointers and Strings

128. Pointers and Strings Strings are defined as arrays of characters . In order to allow variable length strings , the string should be terminated by ‘ \0 ’ which is used to indicate the end of a string . For example, the following program segment declares a string s of 10 characters and assign the starting address of string s into pointer ptr . 4/21/2021 1.6 _ Pointers and Strings 128

129. Pointers and Strings Three things will happen when we create the below array. 10 memory locations are allocated for the array. The string “ Hello” is stored in first 5 locations and in the sixth location ‘\0’ is automatically stored to indicate the end of the string. The pointer ptr points at the first location of the allocated memory 4/21/2021 1.6 _ Pointers and Strings 129

130. Example – program to use pointer to print the string array 4/21/2021 1.6 _ Pointers and Strings 130

131. display the string using pointer with “%s” as the formatting character 4/21/2021 1.6 _ Pointers and Strings 131

132. Example 2 - to count vowels and consonants in a string using pointer 4/21/2021 1.6 _ Pointers and Strings 132

133. Example 3 - to find the length of the given string 4/21/2021 1.6 _ Pointers and Strings 133

134. 4/21/2021 1.6 _ Pointers and Strings 134

135. create a list of strings When we want to create a list of strings, We can use the following methods: Two dimensional character array Array of Pointers to Strings 4/21/2021 1.6 _ Pointers and Strings 135

136. Two dimensional character array The following program demonstrates the use of 2-D array to store the list of cities . It allocates memory for storing 4 cities and for each city 10 locations are allocated. 4/21/2021 1.6 _ Pointers and Strings 136

137. Example 1 4/21/2021 1.6 _ Pointers and Strings 137

138. Example 2 - to sort the list of names in alphabetical order 4/21/2021 1.6 _ Pointers and Strings 138

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140. Array of Pointers to Strings When we want to store multiple strings , we can create array of pointers and we can make each pointer points to one string . Suppose there are N elements in an array , we can make each element to point to one string . 4/21/2021 1.6 _ Pointers and Strings 140

141. Array of Pointers to Strings we have learned how we can use an array of strings or 2-D array of characters . It may appear to you whenever you need to store more than one string then an array of strings is the way to go, unfortunately, this is not the case. Consider the following example. 4/21/2021 1.6 _ Pointers and Strings 141

142. Array of Pointers to Strings 4/21/2021 1.6 _ Pointers and Strings 142

143. Array of Pointers to Strings As you can see not all strings are long enough to fill all the rows of the array , that's why compiler fills these empty spaces (highlighted using light grey color ) with the null characters ('\0'). The total size of the sports array is  75 bytes but only  34 bytes is used ,  41 bytes is wasted .  41 bytes may not appear much but in a large program, a considerable amount of bytes would be wasted . What we need is a jagged array : A 2-D array whose rows can be of different length . C doesn't provide jagged arrays but we can simulate them using an array of pointer to a string . 4/21/2021 1.6 _ Pointers and Strings 143

144. Array of Pointers to Strings An array of pointers to strings is an array of character pointers where each pointer points to the first character of the string or the base address of the string. Let's see how we can declare and initialize an array of pointers to strings. 4/21/2021 1.6 _ Pointers and Strings 144

145. Array of Pointers to Strings 4/21/2021 1.6 _ Pointers and Strings 145

146. Array of Pointers to Strings Here  sports  is an array of pointers to strings . If initialization of an array is done at the time of declaration then we can omit the size of an array . So the above statement can also be written as: 4/21/2021 1.6 _ Pointers and Strings 146

147. Array of Pointers to Strings It is important to note that each element of the sports array is a string literal and since a string literal points to the base address of the first character . The 0th element i.e   arr [0]  points to the base address of string " golf" . Similarly, the 1st element i.e   arr [1]  points to the base address of string  "hockey"  and so on. 4/21/2021 1.6 _ Pointers and Strings 147

148. Here is how an array of pointers to string is stored in memory. 4/21/2021 1.6 _ Pointers and Strings 148 Array of Pointers to Strings

149. Array of Pointers to Strings 34 + 20 = 54 . In this case, all string literals occupy  34 bytes and  20 bytes are occupied by the array of pointers i.e sports. So, just by creating an array of pointers to string instead of array 2-D array of characters we are saving 21 bytes ( 75-54=21 ) of memory 4/21/2021 1.6 _ Pointers and Strings 149

150. Example 1 4/21/2021 1.6 _ Pointers and Strings 150

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152. Example 2 - Array of Pointers to Strings The following program demonstrates the use of array of pointers to store the list of cities . Each pointer in the array can point to one city name. It allocates memory for 1) storing the address of four strings ( 4 x 4 = 16 locations) 2) required memory locations ( ie ., Number of characters in the string + 1 location for storing ‘\0’ character) are allocated for each city. 4/21/2021 1.6 _ Pointers and Strings 152

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156. Thank you 4/21/2021 1.6 _ Pointers and Strings 156