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The range 0..3 contains 0, 1, and 2; its upper bound is excluded. The inclusive form 1..=3 also contains 3. Half-open ranges fit indexing because a length can be used directly as the excluded end. Inclusive ranges are useful when both endpoints belong to the domain.
Half-open ranges make boundaries compose cleanly. In 0..3, the first value is included and 3 is excluded, so there are exactly three integers. This mirrors a slice whose length is 3: valid positions are 0, 1, and 2, while the length itself is the first position after the slice. Two adjacent spans such as 0..3 and 3..6 meet without overlapping. That is why half-open intervals are common in indexing and iteration.
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