Accessing the elements of the array

ACCESSING THE ELEMENTS OF AN ARRAY

Storing related data items in a single array enables the programmers to develop concise and efficient programs. But there is no single function that can operate on all the elements of an array. 

To access all the elements, we must use a loop. That is, we can access all the elements of an array by varying the 

value of the subscript into the array. But note that the subscript must be an integral value or an expression that evaluates to an integral value. As shown in above Fig., the 

first element of the array marks[10] can be accessed by writing marks[0]. Now to process all the elements of the array, we use a loop as shown in Fig. below.

// Set each element of the array to –1

int i, marks[1 ];

for(i= ;i<1 ;i++)

marks[i] = –1;

Fig: Code to initialize each element of the 

array to –1

The code accesses every individual 

element of the array and sets its value to –1. In the for loop, first the value of marks[0] is set to –1, then the value of the index (i) is incremented and the next value, that is, marks[1] is set to –1. procedure continues until all the 10 elements of the array are set to –1.

Fig: Array marks after executing the code

Calculating the Address of Array Elements

You must be wondering how C gets to know where an individual element of an array is located in the memory. The answer is that the array name is a symbolic reference to the address of the first byte of the array. When we use the array name, we are actually referring to the first byte of the array.The subscript or the index represents the offset from the beginning of the array to the element 

being referenced. That is, with just the array name and the index, C can calculate the address of any element in the array.

Since an array stores all its data elements in consecutive memory locations, storing just the base address, that is the address of the first element in the array, is sufficient. The address of other data elements can simply be calculated using the base address. The formula to perform this calculation is,

Address of data element, A[k] = BA(A) + w(k – lower_bound)

Here, A is the array, k is the index of the element of which we have to calculate the address, BA is the base address of the array A, and w is the size of one element in memory, for example, size of int is 2.

Example: Given an array int marks[]={99,67,78,56,88,90,34,85}, calculate the address of marks[4] if the base address = 1000.

Solution

We know that storing an integer value requires 2 bytes, therefore, its size is 2 bytes.marks[4] = 1000 + 2(4 – 0)

                           = 1000 + 2(4) = 1008

Calculating the Length of an Array

The length of an array is given by the number of elements stored in it. The general formula to calculate the length of an array is

Length = upper_bound – lower_bound + 1

where upper_bound is the index of the last element and lower_bound is the index of the first element in the array

Example: Let Age[5] be an array of integers such that Age[0] = 2, Age[1] = 5, Age[2] = 3, Age[3] = 1, Age[4] = 7

Show the memory representation of the array and calculate its length.

Solution

The memory representation of the array Age[5] is given as below.

Length = upper_bound – lower_bound + 1

Here, lower_bound = 0, upper_bound = 4

Therefore, length = 4 – 0 + 1 = 5