Generate Random Values

Generate random numbers

Generates random numbers with help of random-number generator rand::Rng obtained via rand::rng. Each thread has an initialized generator. Integers are uniformly distributed over the range of the type, and floating point numbers are uniformly distributed from 0 up to but not including 1.

use rand::Rng;

fn main() {
    let mut rng = rand::thread_rng();
    let random_number: u32 = rng.gen();
    println!("Random number: {}", random_number);
}

Generate random numbers within a range

Generates a random value within half-open [0, 10) range (not including 10) with [Rng::gen_range].

use rand::Rng;

fn main() {
    let mut rng = rand::rng();
    println!("Integer: {}", rng.gen_range(0..10));
    println!("Float: {}", rng.gen_range(0.0..10.0));
}

[Uniform] can obtain values with uniform distribution. This has the same effect, but may be faster when repeatedly generating numbers in the same range.

use rand::Rng;
use rand_distr::{Distribution, Uniform};

fn main() {
    let mut rng = rand::rng();
    let die = Uniform::new_inclusive(1, 6)
        .expect("Failed to create uniform distribution: invalid range");

    loop {
        let throw = die.sample(&mut rng);
        println!("Roll the die: {}", throw);
        if throw == 6 {
            break;
        }
    }
}

Generate random numbers with given distribution

By default, random numbers in the rand crate have uniform distribution. The rand_distr crate provides other kinds of distributions. To use them, you instantiate a distribution, then sample from that distribution using Distribution::sample with help of a random-number generator rand::Rng.

The distributions available are documented here. An example using the Normal distribution is shown below.

use rand::Rng;
use rand_distr::{Distribution, LogNormal, Normal};

fn main() {
    let mut rng = rand::rng();
    let normal = Normal::new(2.0, 3.0)
        .expect("Failed to create normal distribution");
    let log_normal = LogNormal::new(1.0, 0.5)
        .expect("Failed to create log-normal distribution");

    let v = normal.sample(&mut rng);
    println!("{} is from a N(2, 9) distribution", v);
    let v = log_normal.sample(&mut rng);
    println!("{} is from an ln N(1, 0.25) distribution", v);
}

Generate random values of a custom type

Randomly generates a tuple (i32, bool, f64) and variable of user defined type Point. Implements the [Distribution] trait on type Point for [Standard] in order to allow random generation.

use rand::Rng;

#[derive(Debug)]
struct Point {
    x: i32,
    y: i32,
}

impl Point {
    fn random<R: Rng>(rng: &mut R) -> Self {
        Point {
            x: rng.random(),
            y: rng.random(),
        }
    }
}

fn main() {
    let mut rng = rand::rng();
    let rand_tuple = rng.random::<(i32, bool, f64)>();
    let rand_point = Point::random(&mut rng);
    println!("Random tuple: {:?}", rand_tuple);
    println!("Random Point: {:?}", rand_point);
}

Create random passwords from a set of alphanumeric characters

Randomly generates a string of given length ASCII characters in the range A-Z, a-z, 0-9, with Alphanumeric sample.

use rand::Rng;

fn main() {
    const CHARSET: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZ\
                            abcdefghijklmnopqrstuvwxyz\
                            0123456789";
    const PASSWORD_LEN: usize = 30;
    let mut rng = rand::rng();

    let password: String = (0..PASSWORD_LEN)
        .map(|_| {
            let idx = rng.random_range(0..CHARSET.len());
            CHARSET[idx] as char
        })
        .collect();

    println!("{}", password);
}

Create random passwords from a set of user-defined characters

Randomly generates a string of given length ASCII characters with custom user-defined bytestring, with gen_range.

use rand::Rng;

const CHARSET: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZ\
                        abcdefghijklmnopqrstuvwxyz\
                        0123456789)(*&^%$#@!~";
const PASSWORD_LEN: usize = 30;

fn main() {
    let mut rng = rand::rng();

    let password: String = (0..PASSWORD_LEN)
        .map(|_| {
            let idx = rng.gen_range(0..CHARSET.len());
            char::from(CHARSET[idx])
        })
        .collect();

    println!("{:?}", password);
}