rand 0.4.6

Random number generators and other randomness functionality.


A Rust library for random number generators and other randomness functionality.

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Add this to your Cargo.toml:

rand = "0.4"

and this to your crate root:

extern crate rand;


Version 0.4was released in December 2017. It contains almost no breaking changes since the 0.3 series, but nevertheless contains some significant new code, including a new "external" entropy source (JitterRng) and no_std support.

Version 0.5 is in development and contains significant performance improvements for the ISAAC random number generators.


There is built-in support for a random number generator (RNG) associated with each thread stored in thread-local storage. This RNG can be accessed via thread_rng, or used implicitly via random. This RNG is normally randomly seeded from an operating-system source of randomness, e.g. /dev/urandom on Unix systems, and will automatically reseed itself from this source after generating 32 KiB of random data.

let tuple = rand::random::<(f64, char)>();
println!("{:?}", tuple)
use rand::Rng;

let mut rng = rand::thread_rng();
if rng.gen() { // random bool
    println!("i32: {}, u32: {}", rng.gen::<i32>(), rng.gen::<u32>())

It is also possible to use other RNG types, which have a similar interface. The following uses the "ChaCha" algorithm instead of the default.

use rand::{Rng, ChaChaRng};

let mut rng = rand::ChaChaRng::new_unseeded();
println!("i32: {}, u32: {}", rng.gen::<i32>(), rng.gen::<u32>())


By default, rand is built with all stable features available. The following optional features are available:

  • i128_support enables support for generating u128 and i128 values

  • nightly enables all unstable features (i128_support)

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

    -   `OsRng` is entirely 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 (user must provide entropy)
    -   `thread_rng`, `weak_rng` and `random` are all disabled
    -   exponential, normal and gamma type distributions are unavailable
        since `exp` and `log` functions are not provided in `core`
    -   any code requiring `Vec` or `Box`
  • alloc can be used instead of std to provide Vec and Box


Unfortunately, cargo test does not test everything. The following tests are recommended:

# Basic tests for rand and sub-crates
cargo test --all

# Test no_std support (build only since nearly all tests require std)
cargo build --all --no-default-features

# Test 128-bit support (requires nightly)
cargo test --all --features nightly

# Benchmarks (requires nightly)
cargo bench
# or just to test the benchmark code:
cargo test --benches


You can derive the Rand trait for your custom type via the #[derive(Rand)] directive. To use this first add this to your Cargo.toml:

rand = "0.4"
rand_derive = "0.3"

Next in your crate:

extern crate rand;
extern crate rand_derive;

#[derive(Rand, Debug)]
struct MyStruct {
    a: i32,
    b: u32,

fn main() {
    println!("{:?}", rand::random::<MyStruct>());


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

See LICENSE-APACHE, and LICENSE-MIT for details.