An array is defined as the collection of similar type of data items stored at contiguous memory locations. Arrays are the derived data type in C programming language which can store the primitive type of data such as int, char, double, float, etc. It also has the capability to store the collection of derived data types, such as pointers, structure, etc. The array is the simplest data structure where each data element can be randomly accessed by using its index number.

Why we need Array in Programming?

Consider a scenario where we need to find out the average of 100 integer numbers entered by user. In C language, we have two ways to do this: 

1. Define 100 variables with int data type and then perform 100 scanf() operations to store the entered values in the variables and then at last calculate the average of them. 
2. Have a single integer array to store all the values, loop the array to store all the entered values in array and later calculate the average.

It is clear that second solution is better than first. It is convenient to store same data types in one single variable and later access them using index.

Properties of Array

• Each element of an array is of same data type and carries the same size, i.e., int = 4 bytes.
• Elements of the array are stored at contiguous memory locations where the first element is stored at the smallest memory location.
• Elements of the array can be randomly accessed since we can calculate the address of each element of the array with the given base address and the size of the data element.

Array declaration in C

• Array variables are declared identically to variables of their data type, except that the variable name is followed by one pair of square [ ] brackets for each dimension of the array.
• Uninitialized arrays must have the dimensions of their rows, columns, etc. listed within the square brackets.
• Dimensions used when declaring arrays in C must be positive integral constants or constant expressions.

Array declaration syntax in C

dataType array1DName[arraySize];
dataType array2DName[arraySize1][arraySize2]
dataType arrayMultiDName[arraySize1][arraySize2][arraySize3].......[arraySizeN]

Example of array declaration in C

        int i, j, intArray[ 10 ], number;
        float floatArray[ 1000 ];
        int tableArray[ 3 ][ 5 ];      /* 3 rows by 5 columns */        
        int NROWS = 100;      
        int NCOLS = 200;      
        float matrix[ NROWS ][ NCOLS ]; /* NROWS  rows by NCOLS columns */

Array Initialization in C

• Arrays may be initialized when they are declared, just as any other variables
• Place the initialization data in curly {} braces following the equals sign
• An array may be partially initialized, by providing fewer data items than the size of the array.  The remaining array elements will be automatically initialized to default value of data type.
• If an array is to be completely initialized, the dimension of the array is not required.  The compiler will automatically size the array to fit the initialized data.

Example of array Initialization in C

int i =  5, intArray[ 6 ] = { 1, 2, 3, 4, 5, 6 }, k;
float sum  = 0.0f, floatArray[ 100 ] = { 1.0f, 5.0f, 20.0f };
double  piFractions[ ] = { 3.141592654, 1.570796327, 0.785398163 };

Accessing Array Elements

• Elements of an array are accessed by specifying the index ( offset ) of the desired element within square [ ] brackets after the array name.
• Array subscripts / index must be of integer type
• Array indices start at zero in C, and go to one less than the size of the array.

Example of Array In C programming to find out the average of 4 integers

#include <stdio.h>
int main()
{
    int avg = 0;
    int sum =0;
    int x=0;

    /* Array- declaration – length 4*/
    int num[4];

    /* We are using a for loop to traverse through the array
     * while storing the entered values in the array
     */
    for (x=0; x<4;x++)
    {
        printf("Enter number %d \n", (x+1));
        scanf("%d", &num[x]);
    }
    for (x=0; x<4;x++)
    {
        sum = sum+num[x];
    }

    avg = sum/4;
    printf("Average of entered number is: %d", avg);
    return 0;
}

Output

Enter number 1 
10
Enter number 2 
10
Enter number 3 
20
Enter number 4 
40
Average of entered number is: 20

Advantages of Array in C

• Random access of elements using array index.
• Use of less line of code as it creates a single array of multiple elements.
• Easy access to all the elements.
• Traversal through the array becomes easy using a single loop.
• Sorting becomes easy as it can be accomplished by writing less line of code.

Disadvantages of Array in C

• Allows a fixed number of elements to be entered which is decided at the time of declaration. Unlike a linked list, an array in C is not dynamic.
• Insertion and deletion of elements can be costly since the elements are needed to be managed in accordance with the new memory allocation.