rand 0.5.6

Random number generators and other randomness functionality.


Build Status Build Status Latest version Documentation Minimum rustc version

A Rust library for random number generation.

Rand provides utilities to generate random numbers, to convert them to useful types and distributions, and some randomness-related algorithms.

The core random number generation traits of Rand live in the rand_core crate; this crate is most useful when implementing RNGs.

API reference: master branch, by release.


Add this to your Cargo.toml:

rand = "0.5"

and this to your crate root:

extern crate rand;

use rand::prelude::*;

fn main() {
  // basic usage with random():
  let x: u8 = random();
  println!("{}", x);

  let y = random::<f64>();
  println!("{}", y);

  if random() { // generates a boolean

  // normal usage needs both an RNG and a function to generate the appropriate
  // type, range, distribution, etc.
  let mut rng = thread_rng();
  if rng.gen() { // random bool
      let x: f64 = rng.gen(); // random number in range [0, 1)
      println!("x is: {}", x);
      let ch = rng.gen::<char>(); // Sometimes you need type annotation
      println!("char is: {}", ch);
      println!("Number from 0 to 9: {}", rng.gen_range(0, 10));


The Rand crate provides:

  • A convenient to use default RNG, thread_rng: an automatically seeded, crypto-grade generator stored in thread-local memory.
  • Pseudo-random number generators: StdRng, SmallRng, prng module.
  • Functionality for seeding PRNGs: the FromEntropy trait, and as sources of external randomness EntropyRng, OsRng and JitterRng.
  • Most content from rand_core (re-exported): base random number generator traits and error-reporting types.
  • 'Distributions' producing many different types of random values:
    • A Standard distribution for integers, floats, and derived types including tuples, arrays and Option
    • Unbiased sampling from specified Uniform ranges.
    • Sampling from exponential/normal/gamma distributions.
    • Sampling from binomial/poisson distributions.
    • gen_bool aka Bernoulli distribution.
  • seq-uence related functionality:
    • Sampling a subset of elements.
    • Randomly shuffling a list.


Version 0.5 is the latest version and contains many breaking changes. See the Upgrade Guide for guidance on updating from previous versions.

Version 0.4 was released in December 2017. It contains almost no breaking changes since the 0.3 series.

For more details, see the changelog.

Rust version requirements

The 0.5 release of Rand requires Rustc version 1.22 or greater. Rand 0.4 and 0.3 (since approx. June 2017) require Rustc version 1.15 or greater. Subsets of the Rand code may work with older Rust versions, but this is not supported.

Travis CI always has a build with a pinned version of Rustc matching the oldest supported Rust release. The current policy is that this can be updated in any Rand release if required, but the change must be noted in the changelog.

Crate Features

Rand is built with only the std feature enabled by default. The following optional features are available:

  • alloc can be used instead of std to provide Vec and Box.
  • i128_support enables support for generating u128 and i128 values.
  • log enables some logging via the log crate.
  • nightly enables all unstable features (i128_support).
  • serde1 enables serialization for some types, via Serde version 1.
  • stdweb enables support for OsRng on wasm-unknown-unknown via stdweb combined with cargo-web.

no_std mode is activated by setting default-features = false; this removes functionality depending on std:

  • thread_rng(), and random() are not available, as they require thread-local storage and an entropy source.
  • OsRng and EntropyRng are unavailable.
  • JitterRng code is still present, but a nanosecond timer must be provided via JitterRng::new_with_timer
  • Since no external entropy is available, it is not possible to create generators with fresh seeds using the FromEntropy trait (user must provide a seed).
  • Exponential, normal and gamma type distributions are unavailable since exp and log functions are not provided in core.
  • The seq-uence module is unavailable, as it requires Vec.


Rand is distributed under the terms of both the MIT license and the Apache License (Version 2.0).