Why Does the Standard Iterator Range Use a Half-Open Interval [begin, end)?

Understanding the Standard Iterator Range Convention [begin, end)

The Standard defines an iterator range as [begin, end), where end() represents one past the actual end. This differs from the intuitive notion of a range ending at the end. The rationale behind this design decision warrants examination.

Dijkstra's Argument

Edsger Dijkstra, known for his contributions to computer science, provided compelling arguments for the half-open convention:

• Simple Range Size Determination: The size of the range can be conveniently calculated as end - begin. This simplifies computations.
• Naturality in Degeneration: As ranges degenerate into empty ones, including the lower bound (begin) feels more natural. Additionally, excluding the lower bound would necessitate the existence of a "one-before-the-beginning" sentinel value, which complicates the conception.

Benefits in Algorithm Design

The [begin, end) convention offers advantages in algorithm design when dealing with multiple nested or iterated range-based constructions:

• Natural Chaining: The ranges can be chained naturally without the need for corrections or offsets.
• Readable Iteration Loops: The standard iteration loop (for (it = begin; it != end; it)) runs end - begin times, providing a concise and readable structure.
• Handling Empty Ranges: Dealing with empty ranges is simplified as both ends are not inclusive.

Starting at Zero

The convention of starting at zero further enhances the simplicity of the design:

• Natural Representation: When given a range of N elements, 0 represents the "beginning," allowing for a range representation of [0, N) without any adjustments.

Conclusion

The half-open range convention [begin, end) is a deliberate design choice aimed at simplifying algorithm design, enabling intuitive operations when working with ranges, and ensuring efficient and clear code. This design principle has become an essential foundation of modern programming.

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