Trait Rng

pub trait Rng: RngCore {
    // Provided methods
    fn gen<T>(&mut self) -> T
       where Standard: Distribution<T> { ... }
    fn gen_range<T, R>(&mut self, range: R) -> T
       where T: SampleUniform,
             R: SampleRange<T> { ... }
    fn sample<T, D>(&mut self, distr: D) -> T
       where D: Distribution<T> { ... }
    fn sample_iter<T, D>(self, distr: D) -> DistIter<D, Self, T>
       where D: Distribution<T>,
             Self: Sized { ... }
    fn fill<T>(&mut self, dest: &mut T)
       where T: Fill + ?Sized { ... }
    fn try_fill<T>(&mut self, dest: &mut T) -> Result<(), Error>
       where T: Fill + ?Sized { ... }
    fn gen_bool(&mut self, p: f64) -> bool { ... }
    fn gen_ratio(&mut self, numerator: u32, denominator: u32) -> bool { ... }
}

An automatically-implemented extension trait on RngCore providing high-level generic methods for sampling values and other convenience methods.

This is the primary trait to use when generating random values.

Generic usage

The basic pattern is fn foo<R: Rng + ?Sized>(rng: &mut R). Some things are worth noting here:

• Since Rng: RngCore and every RngCore implements Rng, it makes no difference whether we use R: Rng or R: RngCore.
• The + ?Sized un-bounding allows functions to be called directly on type-erased references; i.e. foo(r) where r: &mut dyn RngCore. Without this it would be necessary to write foo(&mut r).

An alternative pattern is possible: fn foo<R: Rng>(rng: R). This has some trade-offs. It allows the argument to be consumed directly without a &mut (which is how from_rng(thread_rng()) works); also it still works directly on references (including type-erased references). Unfortunately within the function foo it is not known whether rng is a reference type or not, hence many uses of rng require an extra reference, either explicitly (distr.sample(&mut rng)) or implicitly (rng.gen()); one may hope the optimiser can remove redundant references later.

Example:

use rand::Rng;

fn foo<R: Rng + ?Sized>(rng: &mut R) -> f32 {
    rng.gen()
}

Provided Methods

fn gen<T>(&mut self) -> T

where Standard: Distribution<T>,

Return a random value supporting the Standard distribution.

Example

use rand::{thread_rng, Rng};

let mut rng = thread_rng();
let x: u32 = rng.gen();
println!("{}", x);
println!("{:?}", rng.gen::<(f64, bool)>());

Arrays and tuples

The rng.gen() method is able to generate arrays (up to 32 elements) and tuples (up to 12 elements), so long as all element types can be generated. When using rustc ≥ 1.51, enable the min_const_gen feature to support arrays larger than 32 elements.

For arrays of integers, especially for those with small element types (< 64 bit), it will likely be faster to instead use Rng::fill.

use rand::{thread_rng, Rng};

let mut rng = thread_rng();
let tuple: (u8, i32, char) = rng.gen(); // arbitrary tuple support

let arr1: [f32; 32] = rng.gen();        // array construction
let mut arr2 = [0u8; 128];
rng.fill(&mut arr2);                    // array fill

fn gen_range<T, R>(&mut self, range: R) -> T

where T: SampleUniform, R: SampleRange<T>,

Generate a random value in the given range.

This function is optimised for the case that only a single sample is made from the given range. See also the Uniform distribution type which may be faster if sampling from the same range repeatedly.

Only gen_range(low..high) and gen_range(low..=high) are supported.

Panics

Panics if the range is empty.

Example

use rand::{thread_rng, Rng};

let mut rng = thread_rng();

// Exclusive range
let n: u32 = rng.gen_range(0..10);
println!("{}", n);
let m: f64 = rng.gen_range(-40.0..1.3e5);
println!("{}", m);

// Inclusive range
let n: u32 = rng.gen_range(0..=10);
println!("{}", n);

fn sample<T, D>(&mut self, distr: D) -> T

where D: Distribution<T>,

Sample a new value, using the given distribution.

Example

use rand::{thread_rng, Rng};
use rand::distributions::Uniform;

let mut rng = thread_rng();
let x = rng.sample(Uniform::new(10u32, 15));
// Type annotation requires two types, the type and distribution; the
// distribution can be inferred.
let y = rng.sample::<u16, _>(Uniform::new(10, 15));

fn sample_iter<T, D>(self, distr: D) -> DistIter<D, Self, T>

where D: Distribution<T>, Self: Sized,

Create an iterator that generates values using the given distribution.

Note that this function takes its arguments by value. This works since (&mut R): Rng where R: Rng and (&D): Distribution where D: Distribution, however borrowing is not automatic hence rng.sample_iter(...) may need to be replaced with (&mut rng).sample_iter(...).

Example

use rand::{thread_rng, Rng};
use rand::distributions::{Alphanumeric, Uniform, Standard};

let mut rng = thread_rng();

// Vec of 16 x f32:
let v: Vec<f32> = (&mut rng).sample_iter(Standard).take(16).collect();

// String:
let s: String = (&mut rng).sample_iter(Alphanumeric)
    .take(7)
    .map(char::from)
    .collect();

// Combined values
println!("{:?}", (&mut rng).sample_iter(Standard).take(5)
                             .collect::<Vec<(f64, bool)>>());

// Dice-rolling:
let die_range = Uniform::new_inclusive(1, 6);
let mut roll_die = (&mut rng).sample_iter(die_range);
while roll_die.next().unwrap() != 6 {
    println!("Not a 6; rolling again!");
}

fn fill<T>(&mut self, dest: &mut T)

where T: Fill + ?Sized,

Fill any type implementing Fill with random data

The distribution is expected to be uniform with portable results, but this cannot be guaranteed for third-party implementations.

This is identical to try_fill except that it panics on error.

Example

use rand::{thread_rng, Rng};

let mut arr = [0i8; 20];
thread_rng().fill(&mut arr[..]);

fn try_fill<T>(&mut self, dest: &mut T) -> Result<(), Error>

where T: Fill + ?Sized,

Fill any type implementing Fill with random data

The distribution is expected to be uniform with portable results, but this cannot be guaranteed for third-party implementations.

This is identical to fill except that it forwards errors.

Example

use rand::{thread_rng, Rng};

let mut arr = [0u64; 4];
thread_rng().try_fill(&mut arr[..])?;

fn gen_bool(&mut self, p: f64) -> bool

Return a bool with a probability p of being true.

See also the Bernoulli distribution, which may be faster if sampling from the same probability repeatedly.

Example

use rand::{thread_rng, Rng};

let mut rng = thread_rng();
println!("{}", rng.gen_bool(1.0 / 3.0));

Panics

If p < 0 or p > 1.

fn gen_ratio(&mut self, numerator: u32, denominator: u32) -> bool

Return a bool with a probability of numerator/denominator of being true. I.e. gen_ratio(2, 3) has chance of 2 in 3, or about 67%, of returning true. If numerator == denominator, then the returned value is guaranteed to be true. If numerator == 0, then the returned value is guaranteed to be false.

See also the Bernoulli distribution, which may be faster if sampling from the same numerator and denominator repeatedly.

Panics

If denominator == 0 or numerator > denominator.

Example

use rand::{thread_rng, Rng};

let mut rng = thread_rng();
println!("{}", rng.gen_ratio(2, 3));

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors

impl<R> Rng for R

where R: RngCore + ?Sized,