Array bounds define the valid index range of an array. For an array containing n elements, valid indices range from 0 to n - 1.

#include <stdio.h>

int main(void)
{
    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;
}

Example Output #

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

Explanation #

The declaration:

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

creates an array with five elements. Its valid indices are 0, 1, 2, 3, and 4.

The loop condition:

i < 5

ensures that the index remains within the valid range.

Valid and Invalid Array Indices #

C does not automatically check array indices at runtime. An expression such as numbers[5] accesses beyond the array’s bounds and causes undefined behavior.

For an array whose size is known at the point of declaration, a loop can use the element count to avoid hardcoding the bound:

size_t count = sizeof(numbers) / sizeof(numbers[0]);

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

This calculation works here because numbers is an actual array, not a pointer parameter.