> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # Swift Half-Open Range The `..<` operator is the half-open range operator in Swift. It constructs a mathematical range that includes its lower bound but strictly excludes its upper bound. ## Syntax Forms The operator functions in two distinct arities: binary (two-sided) and prefix (one-sided). ```swift theme={"dark"} // Binary (Two-sided) let lowerBound = 0 let upperBound = 5 let twoSidedRange = lowerBound..` structure. It requires both a lower and an upper bound. * **Prefix Form:** Evaluates to a `PartialRangeUpTo` structure. It requires only an upper bound, implicitly extending infinitely downward. ## Evaluation Rules 1. **Bound Relationship:** For the binary form, the `lowerBound` must be less than or equal to the `upperBound` (`lowerBound <= upperBound`). A runtime crash occurs if the lower bound exceeds the upper bound. 2. **Empty Ranges:** If `lowerBound` is exactly equal to `upperBound`, the resulting `Range` is empty. 3. **Exclusivity:** The `upperBound` is never a member of the resulting range. Evaluating `range.contains(upperBound)` will always return `false`. 4. **Postfix Restriction:** Unlike the closed range operator (`...`), the half-open range operator (`..<`) cannot be used as a postfix operator. Swift does not support a half-open `PartialRangeFrom` because a range extending infinitely upward has no defined upper bound to exclude.

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