---
title: RandomNumberGenerator
framework: swift
role: symbol
role_heading: Protocol
path: swift/randomnumbergenerator
---

# RandomNumberGenerator

A type that provides uniformly distributed random data.

## Declaration

```swift
protocol RandomNumberGenerator
```

## Overview

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<G: RandomNumberGenerator>(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()](swift/randomnumbergenerator/next().md)
- [next(upperBound:)](swift/randomnumbergenerator/next(upperbound:).md)

## Relationships

### Conforming Types

- [SystemRandomNumberGenerator](swift/systemrandomnumbergenerator.md)

## See Also

### Random Number Generators

- [SystemRandomNumberGenerator](swift/systemrandomnumbergenerator.md)