fastrand 2.0.2

A simple and fast random number generator


Build License Cargo Documentation

A simple and fast random number generator.

The implementation uses Wyrand, a simple and fast generator but not cryptographically secure.


Flip a coin:

if fastrand::bool() {
} else {

Generate a random i32:

let num = fastrand::i32(..);

Choose a random element in an array:

let v = vec![1, 2, 3, 4, 5];
let i = fastrand::usize(..v.len());
let elem = v[i];

Sample values from an array with O(n) complexity (n is the length of array):

fastrand::choose_multiple(vec![1, 4, 5].iter(), 2);
fastrand::choose_multiple(0..20, 12);

Shuffle an array:

let mut v = vec![1, 2, 3, 4, 5];
fastrand::shuffle(&mut v);

Generate a random Vec or String:

use std::iter::repeat_with;

let v: Vec<i32> = repeat_with(|| fastrand::i32(..)).take(10).collect();
let s: String = repeat_with(fastrand::alphanumeric).take(10).collect();

To get reproducible results on every run, initialize the generator with a seed:

// Pick an arbitrary number as seed.

// Now this prints the same number on every run:
println!("{}", fastrand::u32(..));

To be more efficient, create a new Rng instance instead of using the thread-local generator:

use std::iter::repeat_with;

let rng = fastrand::Rng::new();
let mut bytes: Vec<u8> = repeat_with(|| rng.u8(..)).take(10_000).collect();

This crate aims to expose a core set of useful randomness primitives. For more niche algorithms, consider using the fastrand-contrib crate alongside this one.


  • std (enabled by default): Enables the std library. This is required for the global generator and global entropy. Without this feature, [Rng] can only be instantiated using the with_seed method.
  • js: Assumes that WebAssembly targets are being run in a JavaScript environment.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.