Arrays in C

An array stores multiple values of the same type under one variable name.

int marks[5];

The array marks can store five integer values.

Suppose we want to store the marks of five students. Without an array, we could create five separate variables.

int mark1;
int mark2;
int mark3;
int mark4;
int mark5;

An array makes this easier:

int marks[5];

The basic syntax for declaring an array is:

data_type array_name[size];

Example:

int numbers[5];

• int is the data type.
• numbers is the array name.
• 5 is the number of elements.

int numbers[5];

This creates an array capable of storing five integer elements.

The indexes are:

0  1  2  3  4

Each element of an array has an index. The first index is always 0.

int numbers[5];

numbers[0] = 10;
numbers[1] = 20;
numbers[2] = 30;
numbers[3] = 40;
numbers[4] = 50;

The last index of an array with five elements is 4.

An array can be initialized when it is declared.

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

The values are assigned to the array elements from left to right.

When an array is initialized with values, its size can be omitted.

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

The compiler determines the number of elements from the initializer. Here, the array contains four elements.

int numbers[] = {10, 20, 30};

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

Output:

10

The expression numbers[0] accesses the first element.

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

printf("%d\n", numbers[0]);
printf("%d\n", numbers[1]);
printf("%d\n", numbers[2]);
printf("%d\n", numbers[3]);

Output:

10
20
30
40

int numbers[] = {10, 20, 30};

numbers[1] = 100;

printf("%d", numbers[1]);

Output:

100

Array elements can be changed using their indexes.

Traversing means visiting each element of an array, usually with a loop.

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

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

The loop accesses every element from index 0 to 4.

#include <stdio.h>

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

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

    return 0;
}

A loop can be used to take input for each array element.

int numbers[5];

for (int i = 0; i < 5; i++)
{
    scanf("%d", &numbers[i]);
}

Each input value is stored at the corresponding index.

#include <stdio.h>

int main()
{
    int numbers[5];

    printf("Enter 5 numbers:\n");

    for (int i = 0; i < 5; i++)
    {
        scanf("%d", &numbers[i]);
    }

    printf("Array elements:\n");

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

    return 0;
}

int numbers[] = {10, 20, 30, 40, 50};
int sum = 0;

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

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

Output:

Sum = 150

int numbers[] = {25, 10, 45, 30, 20};
int largest = numbers[0];

for (int i = 1; i < 5; i++)
{
    if (numbers[i] > largest)
    {
        largest = numbers[i];
    }
}

printf("Largest = %d", largest);

The first element is used as the initial largest value.

int numbers[] = {25, 10, 45, 30, 20};
int smallest = numbers[0];

for (int i = 1; i < 5; i++)
{
    if (numbers[i] < smallest)
    {
        smallest = numbers[i];
    }
}

printf("Smallest = %d", smallest);

int numbers[] = {10, 15, 20, 25, 30};
int count = 0;

for (int i = 0; i < 5; i++)
{
    if (numbers[i] % 2 == 0)
    {
        count++;
    }
}

printf("Even numbers = %d", count);

The modulo operator is used to check whether each number is even.

int numbers[] = {10, 20, 30, 40, 50};
int sum = 0;

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

float average = sum / 5.0f;

printf("Average = %.2f", average);

Using 5.0f makes the division floating-point.

Arrays can store other data types such as float.

float prices[4] = {
    10.5f,
    20.5f,
    30.5f,
    40.5f
};

for (int i = 0; i < 4; i++)
{
    printf("%.2f\n", prices[i]);
}

A character array stores characters.

char letters[5] = {'A', 'B', 'C', 'D', 'E'};

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

Character arrays are also used to represent strings when a terminating null character is included.

For an array declared in the same scope, sizeof can be used to calculate the total number of bytes occupied by the array.

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

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

printf("Elements = %d", size);

The result is 5.

An array can be passed to a function for processing.

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

int main()
{
    int numbers[] = {10, 20, 30};

    display(numbers, 3);

    return 0;
}

The size is commonly passed separately.

void change(int numbers[])
{
    numbers[0] = 100;
}

int main()
{
    int numbers[] = {10, 20, 30};

    change(numbers);

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

    return 0;
}

Output:

100

When an array is passed to a function, the function can modify its elements through the parameter.

Never access an index outside the valid range of an array.

int numbers[5];

numbers[0] = 10;
numbers[4] = 50;

/* Invalid access */
numbers[5] = 60;

For an array of five elements, valid indexes are only 0 through 4. C does not perform automatic bounds checking.

If an array is initialized with fewer values than its specified size, the remaining elements are initialized to zero.

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

The elements are effectively:

10 20 0 0 0

#include <stdio.h>

int main()
{
    int numbers[5];
    int sum = 0;

    printf("Enter 5 numbers:\n");

    for (int i = 0; i < 5; i++)
    {
        scanf("%d", &numbers[i]);
    }

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

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

    return 0;
}

#include <stdio.h>

int main()
{
    int numbers[5];

    printf("Enter 5 numbers:\n");

    for (int i = 0; i < 5; i++)
    {
        scanf("%d", &numbers[i]);
    }

    int largest = numbers[0];

    for (int i = 1; i < 5; i++)
    {
        if (numbers[i] > largest)
        {
            largest = numbers[i];
        }
    }

    printf("Largest = %d", largest);

    return 0;
}

• Forgetting that array indexes start from 0.
• Accessing an index outside the array's valid range.
• Using the wrong array size in a loop.
• Forgetting to initialize an array when necessary.
• Using an uninitialized local array and assuming its values are zero.
• Forgetting to pass the array size when a function needs it.

int numbers[5];

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

For a local array like this, the elements are not automatically initialized to zero.

Practice the following programs:

1. Store and display five numbers using an array.
2. Take five numbers from the user and display them.
3. Find the sum of array elements.
4. Find the average of array elements.
5. Find the largest element.
6. Find the smallest element.
7. Count even numbers in an array.
8. Count odd numbers in an array.
9. Search for a specific value in an array.
10. Pass an array to a function and display its elements.

Practice using loops with arrays because loops are commonly used to traverse array elements.