Crate fastrand

source ·
Expand description

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 mut 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. See the WebAssembly Notes section for more information.

WebAssembly Notes

For non-WASI WASM targets, there is additional sublety to consider when utilizing the global RNG. By default, std targets will use entropy sources in the standard library to seed the global RNG. However, these sources are not available by default on WASM targets outside of WASI.

If the js feature is enabled, this crate will assume that it is running in a JavaScript environment. At this point, the getrandom crate will be used in order to access the available entropy sources and seed the global RNG. If the js feature is not enabled, the global RNG will use a predefined seed.


  • A random number generator.


  • Generates a random char in ranges a-z and A-Z.
  • Generates a random char in ranges a-z, A-Z and 0-9.
  • boolstd
    Generates a random bool.
  • charstd
    Generates a random char in the given range.
  • Choose an item from an iterator at random.
  • Collects amount values at random from the iterator into a vector.
  • Generates a random digit in the given base.
  • f32std
    Generates a random f32 in range 0..1.
  • f64std
    Generates a random f64 in range 0..1.
  • Gives back current seed that is being held by the thread-local generator.
  • i8std
    Generates a random i8 in the given range.
  • i16std
    Generates a random i16 in the given range.
  • i32std
    Generates a random i32 in the given range.
  • i64std
    Generates a random i64 in the given range.
  • i128std
    Generates a random i128 in the given range.
  • Generates a random isize in the given range.
  • Generates a random char in range a-z.
  • seedstd
    Initializes the thread-local generator with the given seed.
  • Shuffles a slice randomly.
  • u8std
    Generates a random u8 in the given range.
  • u16std
    Generates a random u16 in the given range.
  • u32std
    Generates a random u32 in the given range.
  • u64std
    Generates a random u64 in the given range.
  • u128std
    Generates a random u128 in the given range.
  • Generates a random char in range A-Z.
  • Generates a random usize in the given range.