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Trait rand::Rng

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pub trait Rng: RngCore {
    fn gen<T>(&mut self) -> T
    where
        Standard: Distribution<T>,
    { ... }
    fn gen_range<T: SampleUniform, B1, B2>(&mut self, low: B1, high: B2) -> T
    where
        B1: SampleBorrow<T> + Sized,
        B2: SampleBorrow<T> + Sized,
    { ... }
    fn sample<T, D: Distribution<T>>(&mut self, distr: D) -> T { ... }
    fn sample_iter<T, D>(self, distr: D) -> DistIter<D, Self, T>ⓘNotable traits for DistIter<D, R, T>impl<D, R, T> Iterator for DistIter<D, R, T> where
    D: Distribution<T>,
    R: Rng,     type Item = T;
    where
        D: Distribution<T>,
        Self: Sized,
    { ... }
    fn fill<T: AsByteSliceMut + ?Sized>(&mut self, dest: &mut T) { ... }
    fn try_fill<T: AsByteSliceMut + ?Sized>(
        &mut self, 
        dest: &mut T
    ) -> Result<(), Error> { ... }
    fn gen_bool(&mut self, p: f64) -> bool { ... }
    fn gen_ratio(&mut self, numerator: u32, denominator: u32) -> bool { ... }
}
Expand description

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 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

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.

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

Generate a random value in the range [low, high), i.e. inclusive of low and exclusive of high.

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.

Panics

Panics if low >= high.

Example
use rand::{thread_rng, Rng};

let mut rng = thread_rng();
let n: u32 = rng.gen_range(0, 10);
println!("{}", n);
let m: f64 = rng.gen_range(-40.0f64, 1.3e5f64);
println!("{}", m);

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));

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 rng = thread_rng();

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

// String:
let s: String = rng.sample_iter(Alphanumeric).take(7).collect();

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

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

Fill dest entirely with random bytes (uniform value distribution), where dest is any type supporting AsByteSliceMut, namely slices and arrays over primitive integer types (i8, i16, u32, etc.).

On big-endian platforms this performs byte-swapping to ensure portability of results from reproducible generators.

This uses fill_bytes internally which may handle some RNG errors implicitly (e.g. waiting if the OS generator is not ready), but panics on other errors. See also try_fill which returns errors.

Example
use rand::{thread_rng, Rng};

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

Fill dest entirely with random bytes (uniform value distribution), where dest is any type supporting AsByteSliceMut, namely slices and arrays over primitive integer types (i8, i16, u32, etc.).

On big-endian platforms this performs byte-swapping to ensure portability of results from reproducible generators.

This is identical to fill except that it uses try_fill_bytes internally and forwards RNG errors.

Example
use rand::{thread_rng, Rng};

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

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.

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));

Implementors