Struct Standard

pub struct Standard;

A generic random value distribution, implemented for many primitive types. Usually generates values with a numerically uniform distribution, and with a range appropriate to the type.

Provided implementations

Assuming the provided Rng is well-behaved, these implementations generate values with the following ranges and distributions:

• Integers (i32, u32, isize, usize, etc.): Uniformly distributed over all values of the type.
• char: Uniformly distributed over all Unicode scalar values, i.e. all code points in the range 0...0x10_FFFF, except for the range 0xD800...0xDFFF (the surrogate code points). This includes unassigned/reserved code points.
• bool: Generates false or true, each with probability 0.5.
• Floating point types (f32 and f64): Uniformly distributed in the half-open range [0, 1). See notes below.
• Wrapping integers (Wrapping<T>), besides the type identical to their normal integer variants.

The Standard distribution also supports generation of the following compound types where all component types are supported:

• Tuples (up to 12 elements): each element is generated sequentially.
• Arrays (up to 32 elements): each element is generated sequentially; see also [Rng::fill] which supports arbitrary array length for integer and float types and tends to be faster for u32 and smaller types. When using rustc ≥ 1.51, enable the min_const_gen feature to support arrays larger than 32 elements. Note that [Rng::fill] and Standard’s array support are not equivalent: the former is optimised for integer types (using fewer RNG calls for element types smaller than the RNG word size), while the latter supports any element type supported by Standard.
• Option<T> first generates a bool, and if true generates and returns Some(value) where value: T, otherwise returning None.

Custom implementations

The Standard distribution may be implemented for user types as follows:

use rand::Rng;
use rand::distributions::{Distribution, Standard};

struct MyF32 {
    x: f32,
}

impl Distribution<MyF32> for Standard {
    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> MyF32 {
        MyF32 { x: rng.gen() }
    }
}

Example usage

use rand::prelude::*;
use rand::distributions::Standard;

let val: f32 = StdRng::from_entropy().sample(Standard);
println!("f32 from [0, 1): {}", val);

Floating point implementation

The floating point implementations for Standard generate a random value in the half-open interval [0, 1), i.e. including 0 but not 1.

All values that can be generated are of the form n * ε/2. For f32 the 24 most significant random bits of a u32 are used and for f64 the 53 most significant bits of a u64 are used. The conversion uses the multiplicative method: (rng.gen::<$uty>() >> N) as $ty * (ε/2).

See also: [Open01] which samples from (0, 1), [OpenClosed01] which samples from (0, 1] and Rng::gen_range(0..1) which also samples from [0, 1). Note that Open01 uses transmute-based methods which yield 1 bit less precision but may perform faster on some architectures (on modern Intel CPUs all methods have approximately equal performance).

Trait Implementations

impl Clone for Standard

fn clone(&self) -> Standard

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Standard

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Copy for Standard