Arrays in C

An array is a linear data structure used to store multiple values of the same data type in a single variable.

Instead of creating separate variables for similar data, we can store all the values inside an array.

Arrays store elements in contiguous memory locations, which allows fast and efficient access to elements using their index.

Why Do We Need Arrays?

Imagine you want to store marks of 100 students.

Without arrays:

int mark1;
int mark2;
int mark3;
...
int mark100;

Managing so many variables becomes difficult.

Using an array:

int marks[100];

All values can be stored in a single variable.

Example of Array

#include <stdio.h>

int main()
{
    int arr[] = {2, 4, 8, 12, 16, 18};

    int n = sizeof(arr) / sizeof(arr[0]);

    for(int i = 0; i < n; i++)
    {
        printf("%d ", arr[i]);
    }

    return 0;
}

Output

2 4 8 12 16 18

Characteristics of Arrays

• Arrays store elements of the same data type.

• Elements are stored in contiguous memory locations.

• Array size is fixed after declaration.

• Elements can be accessed directly using an index.

• Indexing starts from 0.

Creating an Array

Creating an array involves two steps:

1. Array Declaration

2. Array Initialization

Array Declaration

Array declaration specifies:

• Data type

• Array name

• Number of elements

Syntax

data_type array_name[size];

Example

int marks[5];

This creates an array named marks that can store 5 integer values.

Array Initialization

Initialization means assigning values to array elements.

Example

int marks[5] = {10, 20, 30, 40, 50};

Declaration and Initialization Together

int marks[] = {10, 20, 30, 40, 50};

In this case, the compiler automatically determines the size of the array.

Partial Initialization

We can initialize only some elements.

int marks[5] = {10, 20};

Remaining elements automatically become 0.

The array will contain:

10 20 0 0 0

Accessing Array Elements

Array elements are accessed using their index number.

Syntax

array_name[index]

Example

#include <stdio.h>

int main()
{
    int arr[5] = {2, 4, 8, 12, 16};

    printf("%d\n", arr[2]);
    printf("%d\n", arr[4]);
    printf("%d\n", arr[0]);

    return 0;
}

Output

8
16
2

Important Note

Array indexing starts from 0.

For an array of size 5:

Index : 0 1 2 3 4
Value : 2 4 8 12 16

• First element → index 0

• Last element → index 4

Updating Array Elements

Array elements can be modified using the assignment operator.

Example

#include <stdio.h>

int main()
{
    int arr[5] = {2, 4, 8, 12, 16};

    arr[0] = 1;

    printf("%d", arr[0]);

    return 0;
}

Output

1

Array Traversal

Array traversal means visiting every element of an array one by one.

Loops are commonly used for traversal.

Traversing an Array

#include <stdio.h>

int main()
{
    int arr[5] = {2, 4, 8, 12, 16};

    for(int i = 0; i < 5; i++)
    {
        printf("%d ", arr[i]);
    }

    return 0;
}

Output

2 4 8 12 16

Traversing an Array in Reverse Order

#include <stdio.h>

int main()
{
    int arr[5] = {2, 4, 8, 12, 16};

    for(int i = 4; i >= 0; i--)
    {
        printf("%d ", arr[i]);
    }

    return 0;
}

Output

16 12 8 4 2

Size of an Array

The size of an array represents the total number of elements it can store.

C does not automatically provide the length of an array, but we can calculate it using the sizeof() operator.

Formula

sizeof(array) / sizeof(array[0])

Example

#include <stdio.h>

int main()
{
    int arr[5] = {2, 4, 8, 12, 16};

    int size = sizeof(arr) / sizeof(arr[0]);

    printf("%d", size);

    return 0;
}

Output

5

How It Works

Suppose:

int arr[5];

Each integer occupies 4 bytes.

Therefore:

sizeof(arr) = 20 bytes
sizeof(arr[0]) = 4 bytes

So:

20 / 4 = 5

Hence, the array contains 5 elements.

Advantages of Arrays

• Store multiple values using a single variable.

• Easy to access elements using index numbers.

• Reduce the number of variables in a program.

• Efficient memory usage.

• Easy traversal using loops.

Limitations of Arrays

• Array size is fixed after declaration.

• Only elements of the same data type can be stored.

• Inserting and deleting elements can be costly.

• Memory may be wasted if the array size is larger than required.

Arrays and Functions

Arrays can be passed to functions as arguments.

Example

void display(int arr[], int size)
{
    for(int i = 0; i < size; i++)
    {
        printf("%d ", arr[i]);
    }
}

This allows functions to process multiple values efficiently.

Multidimensional Arrays

C also supports arrays with more than one dimension.

Example:

int matrix[3][3];

Multidimensional arrays are commonly used for:

• Matrices

• Tables

• Grids

• Game Boards

Summary

• An array is a collection of elements of the same data type.

• Arrays store data in contiguous memory locations.

• Array indexing starts from 0.

• Arrays support fast random access using indexes.

• Elements can be accessed and updated using square brackets [].

• Loops are commonly used for array traversal.

• The size of an array can be calculated using the sizeof() operator.

• Arrays help manage large amounts of similar data efficiently.

Practice Array Problems

1. Find Maximum Value in an Array

#include <stdio.h>

int main()
{
    int arr[] = {10, 25, 8, 45, 30};
    int max = arr[0];

    int size = sizeof(arr) / sizeof(arr[0]);

    for(int i = 1; i < size; i++)
    {
        if(arr[i] > max)
        {
            max = arr[i];
        }
    }

    printf("Maximum Element = %d", max);

    return 0;
}

Output

Maximum Element = 45

2. Calculate Sum of Array Elements

#include <stdio.h>

int main()
{
    int arr[] = {10, 20, 30, 40, 50};
    int sum = 0;

    int size = sizeof(arr) / sizeof(arr[0]);

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

    printf("Sum = %d", sum);

    return 0;
}

Output

Sum = 150

3. Reverse Array in C

Program

#include <stdio.h>

int main()
{
    int arr[] = {10, 20, 30, 40, 50};

    int size = sizeof(arr) / sizeof(arr[0]);

    printf("Reversed Array: ");

    for(int i = size - 1; i >= 0; i--)
    {
        printf("%d ", arr[i]);
    }

    return 0;
}

Output

Reversed Array: 50 40 30 20 10

4. Insert an Element in an Array

Program

#include <stdio.h>

int main()
{
    int arr[10] = {10, 20, 30, 40, 50};
    int size = 5;

    int pos = 2;
    int value = 25;

    for(int i = size; i > pos; i--)
    {
        arr[i] = arr[i - 1];
    }

    arr[pos] = value;
    size++;

    for(int i = 0; i < size; i++)
    {
        printf("%d ", arr[i]);
    }

    return 0;
}

Output

10 20 25 30 40 50

5. Delete an Element in an Array

Program

#include <stdio.h>

int main()
{
    int arr[] = {10, 20, 30, 40, 50};
    int size = 5;

    int pos = 2;

    for(int i = pos; i < size - 1; i++)
    {
        arr[i] = arr[i + 1];
    }

    size--;

    for(int i = 0; i < size; i++)
    {
        printf("%d ", arr[i]);
    }

    return 0;
}

Output

10 20 40 50

6. Array Rotation (Left Rotation by One Position)

Program

#include <stdio.h>

int main()
{
    int arr[] = {10, 20, 30, 40, 50};

    int size = sizeof(arr) / sizeof(arr[0]);

    int first = arr[0];

    for(int i = 0; i < size - 1; i++)
    {
        arr[i] = arr[i + 1];
    }

    arr[size - 1] = first;

    for(int i = 0; i < size; i++)
    {
        printf("%d ", arr[i]);
    }

    return 0;
}

Output

20 30 40 50 10

7. Return an Array from a Function

Program

#include <stdio.h>

int* getArray()
{
    static int arr[] = {10, 20, 30, 40, 50};
    return arr;
}

int main()
{
    int *ptr = getArray();

    for(int i = 0; i < 5; i++)
    {
        printf("%d ", ptr[i]);
    }

    return 0;
}

Output

10 20 30 40 50

8. Sort an Array in Ascending Order

Program

#include <stdio.h>

int main()
{
    int arr[] = {50, 20, 40, 10, 30};

    int size = sizeof(arr) / sizeof(arr[0]);

    for(int i = 0; i < size - 1; i++)
    {
        for(int j = i + 1; j < size; j++)
        {
            if(arr[i] > arr[j])
            {
                int temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
        }
    }

    printf("Sorted Array: ");

    for(int i = 0; i < size; i++)
    {
        printf("%d ", arr[i]);
    }

    return 0;
}

Output

Sorted Array: 10 20 30 40 50

9. Find the Pair with a Given Sum in Array

Program

#include <stdio.h>

int main()
{
    int arr[] = {2, 7, 11, 15, 5};

    int target = 9;

    int size = sizeof(arr) / sizeof(arr[0]);

    for(int i = 0; i < size; i++)
    {
        for(int j = i + 1; j < size; j++)
        {
            if(arr[i] + arr[j] == target)
            {
                printf("Pair Found: %d + %d = %d",
                       arr[i], arr[j], target);
            }
        }
    }

    return 0;
}

Output

Pair Found: 2 + 7 = 9