# RandomNumberGenerator A type that provides uniformly distributed random data. ``` protocol RandomNumberGenerator ``` ## Overview When you call methods that use random data, such as creating new random values or shuffling a collection, you can pass a `RandomNumberGenerator` type to be used as the source for randomness. When you don’t pass a generator, the default `SystemRandomNumberGenerator` type is used. When providing new APIs that use randomness, provide a version that accepts a generator conforming to the `RandomNumberGenerator` protocol as well as a version that uses the default system generator. For example, this `Weekday` enumeration provides static methods that return a random day of the week: ``` enum Weekday: CaseIterable { case sunday, monday, tuesday, wednesday, thursday, friday, saturday static func random(using generator: inout G) -> Weekday { return Weekday.allCases.randomElement(using: &generator)! } static func random() -> Weekday { var g = SystemRandomNumberGenerator() return Weekday.random(using: &g) } } ``` # Conforming to the RandomNumberGenerator Protocol A custom `RandomNumberGenerator` type can have different characteristics than the default `SystemRandomNumberGenerator` type. For example, a seedable generator can be used to generate a repeatable sequence of random values for testing purposes. To make a custom type conform to the `RandomNumberGenerator` protocol, implement the required `next()` method. Each call to `next()` must produce a uniform and independent random value. Types that conform to `RandomNumberGenerator` should specifically document the thread safety and quality of the generator. ## Topics ### Generating Random Binary Data [`next()`](/documentation/Swift/RandomNumberGenerator/next()) Returns a value from a uniform, independent distribution of binary data. [`next(upperBound:)`](/documentation/Swift/RandomNumberGenerator/next(upperBound:)) Returns a random value that is less than the given upper bound. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)