One-Dimensional Array in C

A one-dimensional array stores elements in a single sequence.

int numbers[5];

This array can store five integer values.

Index:   0    1    2    3    4
Value:  [ ]  [ ]  [ ]  [ ]  [ ]

The syntax for declaring a one-dimensional array is:

data_type array_name[size];

Example:

int marks[5];

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

A one-dimensional array uses one index to identify each element.

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

The indexes are:

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

A one-dimensional array can be initialized when it is declared.

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

The values are stored in the array from left to right.

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

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

The compiler determines that the array contains five elements.

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

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

Output:

10

The index 0 accesses the first element.

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

printf("%d", numbers[3]);

Output:

40

Because the array has four elements, the last valid index is 3.

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

numbers[1] = 200;

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

Output:

200

An array element can be modified using its index.

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

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

Output:

10 20 30 40 50

A for loop is commonly used to traverse a one-dimensional array.

int numbers[5];

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

Each input value is stored at a different array 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[] = {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);

Output:

Average = 30.00

int numbers[] = {25, 60, 15, 90, 40};

int largest = numbers[0];

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

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

Output:

Largest = 90

int numbers[] = {25, 60, 15, 90, 40};

int smallest = numbers[0];

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

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

Output:

Smallest = 15

A loop can be used to search for a value in a one-dimensional array.

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

for (int i = 0; i < 5; i++)
{
    if (numbers[i] == search)
    {
        found = 1;
        break;
    }
}

if (found)
{
    printf("Element found");
}
else
{
    printf("Element not found");
}

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 checks whether each element is divisible by 2.

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

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

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

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

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

Output:

50 40 30 20 10

The array is traversed from the last index to the first index.

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

for (int i = 0; i < 5; i++)
{
    destination[i] = source[i];
}

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

Each element is copied individually using a loop.

int a[] = {10, 20, 30};
int b[] = {10, 20, 30};
int same = 1;

for (int i = 0; i < 3; i++)
{
    if (a[i] != b[i])
    {
        same = 0;
        break;
    }
}

if (same)
{
    printf("Arrays are equal");
}
else
{
    printf("Arrays are different");
}

Elements are compared one by one.

When the array is available in its original scope, the number of elements can be calculated using sizeof.

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

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

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

Output:

Size = 5

#include <stdio.h>

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

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

    display(numbers, 4);

    return 0;
}

The array and its element count are passed to the function.

#include <stdio.h>

int arraySum(int numbers[], int size)
{
    int sum = 0;

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

    return sum;
}

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

    int result = arraySum(numbers, 4);

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

    return 0;
}

#include <stdio.h>

void update(int numbers[], int size)
{
    for (int i = 0; i < size; i++)
    {
        numbers[i] *= 2;
    }
}

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

    update(numbers, 3);

    for (int i = 0; i < 3; i++)
    {
        printf("%d ", numbers[i]);
    }

    return 0;
}

Output:

20 40 60

Always use indexes within the valid range of the array.

int numbers[5];

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

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

For an array of five elements, the valid indexes are 0 to 4. Accessing outside this range causes undefined behavior.

#include <stdio.h>

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

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

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

    printf("Enter number to search: ");
    scanf("%d", &search);

    for (int i = 0; i < 5; i++)
    {
        if (numbers[i] == search)
        {
            found = 1;
            break;
        }
    }

    if (found)
    {
        printf("Element found");
    }
    else
    {
        printf("Element not found");
    }

    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];
    int smallest = numbers[0];

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

        if (numbers[i] < smallest)
        {
            smallest = numbers[i];
        }
    }

    printf("Largest = %d\n", largest);
    printf("Smallest = %d", smallest);

    return 0;
}

• Forgetting that indexing starts from zero.
• Using an index greater than or equal to the array size.
• Using the wrong loop limit.
• Reading uninitialized local array elements.
• Forgetting to pass the array size to a function when it is needed.
• Confusing an array's total byte size with its number of elements.

int numbers[5];

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

Using the correct loop limit prevents accidental out-of-bounds access.

Practice the following programs:

1. Declare and display a one-dimensional integer array.
2. Take five numbers from the user and display them.
3. Find the sum of all elements.
4. Find the average of all elements.
5. Find the largest element.
6. Find the smallest element.
7. Count even numbers.
8. Count odd numbers.
9. Search for an element.
10. Reverse the array while displaying its elements.
11. Copy one array into another.
12. Compare two arrays element by element.

Practice each problem using loops and array indexes.