2.2.13 Off-by-One Errors

Overview

An off-by-one error repeats or accesses one item too many or too few. With a three-element array, `i < Length` visits indexes 0, 1, and 2. Using `i <= Length` permits i to become 3 and tries an invalid access.

int[] seats = { 12, 14, 16 };
for (int i = 0; i <= seats.Length; i++)
{
    Console.WriteLine(seats[i]); // Incorrect bound
}

Inspect the Boundary

The first three passes display valid elements. On the fourth, i equals Length and seats[3] is outside the array. The repair is `i < seats.Length`. A different off-by-one error, starting at 1, would skip the first element.

The diagram intentionally depicts the faulty condition. Use it to locate the extra pass; do not copy that bound into a correct traversal.

Trace the Outcome

Trace i = 0, 1, 2, and 3, writing down whether seats[i] exists. Change the bound and repeat to verify the fourth access disappears.