Introduction

C provides arrays to store a series of elements of one data type. Let us start with an analogy. Suppose that you have a cupboard in your house. You would like to keep books on each shelf of the cupboard. It would be nice to keep books related to one subject on one rack, books of another subject on another rack. A rack is an array. A rack is used to store books of the same subject. Similarly, an array can store data of the same type. When you declare an array, you must specify the type of data this array will hold. Every element of the array must be of that data type.

The syntax for declaring an array is:

data_type array_name[size];

For example:

int x[5];

Is an array of integers, with size 5, called x. Similarly,

float y[6];

is an array of floating point numbers, with size 6, called y.

To access the elements of an array, you must use array notation. The first element of an array is at position 0, the second at position 1, etc. The last element of an array is at position size - 1.

x[1] = 10;

As you can see, once you have selected an element using array notation, you can manipulate that element as you would any variable. Here is a more complex example:

int z[2];
z[0] = 10;
z[1] = 50;
z[0] = z[0] + z[1]; /* z[0] now contains the integer 60 */

Array Initialization I

When an array is declared, it is initially 'empty'; it does not contain any values. We can initialise an array as follows:

int x[5] = {5, 7, 2, 3, 8};

{5, 7, 2, 3, 8} is called an array-initialization block. When we use an array initialization block, we do not need to specify the size of the array. The following is equivalent:

int x[] = {5, 7, 2, 3, 8};

When have created an array with 5 elements. The first element is 5, the second is 7, etc.

Array Initialization II

Let us look at a small code snippet that prints the number of days per month.

#include<stdio.h>
#include<conio.h>
#define MONTHS 12

int main(void)
{
     int days[MONTHS] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
     int i;

     for(i = 0; i < MONTHS; i++)
     {
          printf("Month %d has %d days.\n", i + 1, days[i]);
     }

     return 0;
}

OUTPUT

Month 1 has 31 days.
Month 2 has 28 days.
Month 3 has 31 days.
Month 4 has 30 days.
Month 5 has 31 days.
Month 6 has 30 days.
Month 7 has 31 days.
Month 8 has 31 days.
Month 9 has 30 days.
Month 10 has 31 days.
Month 11 has 30 days.
Month 12 has 31 days.

(You may not have seen #define before. It is a preprocessor instruction that defines a constant called MONTHS and assigns it the value 12.)

If you lack faith in your ability to count, we can let the computer give us the size of an array, by using sizeof. Just replace

for(i = 0; i < MONTHS; i++)

with

for (i = 0; i < sizeof days / sizeof (int); i++)

Assigning Array Values

We can assign values to array members by using array index.

int x[5];
int i;

for(i = 0; i < 5; i++)
{  
     x[i] = i + 1;
}

Thus the array x will contain the elements 1, 2, 3, 4, and 5.

Passing Arrays to a Function

Arrays cannot be passed to functions. If an array name is used as an argument in a function call, the address of the first element of the array is passed. The function can access the array through that address.

Suppose we want to write a function that returns the sum of elements of the array.

#include <stdio.h>

/* prototype */
int sum(int a[]);

int main(void) 
{
     int marbles[] = {5, 6, 2, 3, 7};
     int ans;

     ans = sum(marbles);
     printf("The total number of marbles: %d", ans);

     return 0;
}

int sum(int arr[])
{
     int i, s = 0;
     for(i = 0; i < 5; i++)
     {
          s = s + arr[i];
     }

     return s;
}

Note that the array name marbles is used as an argument to the function. The function is expecting an array because its parameter is defined as int arr[]. What is actually passed is the address of marbles[0]. The function then can access the array using array notation.

How Are Arrays Stored in Memory?

When we declare an array, space is reserved in the memory of the computer for the array. The elements of the array are stored in these memory locations. The important thing about arrays is that array elements are always stored in consecutive memory locations. We can verify this fact by printing the memory addresses of the elements. (Just like every person has a street address, every location in the memory has a memory address, usually a number, by which it can be uniquely identified.)

#include<stdio.h>

int main(void)
{
     int a[] = {1,2,3,4,5,6,7,8,9,10};
     int i;

     /* Print the Addresses */
     for (i = 0; i < 10; i++)
     {
          printf("\nAddress of a[%d] : %p", i, (void *) &a[i]);
     }

     return 0;
}

OUTPUT

Address of a[0] : ffe2
Address of a[1] : ffe4
Address of a[2] : ffe6
Address of a[3] : ffe8
Address of a[4] : ffea
Address of a[5] : ffec
Address of a[6] : ffee
Address of a[7] : fff0
Address of a[8] : fff2
Address of a[9] : fff4

As we can see from the output, the elements are stored at ffe2, ffe4, ffe6, etc. You might be wondering why the numbers are not consecutive. The reason is very simple. The size of the int data type in C is at least 2 bytes (depending on the implementation). In this example it is 2 bytes wide, so a[0] will be stored at ffe2, ffe3, a[1] will be stored at ffe4, ffe5, a[2] will be stored at ffe6, ffe7 and so on.

If instead we declared an array of floats (which usually take around 4 bytes each), we will find a[0] will be stored at ffe2, ffe3, ffe4, ffe5, a[1] will be stored at ffe6, ffe7, ffe8, ffe9 and so on. Note that you may see entirely different numbers that represent the address locations.

As you can see, the elements are consecutive. This concludes the lesson on arrays.