How do I work with Rand for Random Number Generation in Rust?

Walkthrough

Rand is the de facto standard library for random number generation in Rust. It provides a unified interface for various random number generators (RNGs), distributions, and utilities for generating random values. Rand supports both cryptographically secure and non-secure random number generation, making it suitable for games, simulations, cryptography, and more.

Key concepts:

RNG (Random Number Generator) — The core trait for generating random bytes
ThreadRng — Thread-local RNG (fast, cryptographically secure)
StdRng — Standard RNG with reproducibility
Distributions — Different probability distributions (Uniform, Normal, etc.)
Seeding — Controlling RNG initialization for reproducibility

When to use Rand:

Games and simulations
Generating random data for testing
Password and token generation
Sampling from datasets
Cryptographic applications (with appropriate RNG)

When NOT to use Rand:

Deterministic algorithms (unless seeded)
Very performance-critical inner loops (consider optimized solutions)

Code Examples

Basic Random Numbers

use rand::Rng;
 
fn main() {
    let mut rng = rand::thread_rng();
    
    // Random integer in range [0, 100)
    let n: i32 = rng.gen_range(0..100);
    println!("Random number: {}", n);
    
    // Random float in range [0.0, 1.0)
    let f: f64 = rng.gen();
    println!("Random float: {}", f);
    
    // Random boolean
    let b: bool = rng.gen();
    println!("Random bool: {}", b);
}

Random Numbers in Ranges

use rand::Rng;
 
fn main() {
    let mut rng = rand::thread_rng();
    
    // Exclusive range [1, 10)
    let n1: i32 = rng.gen_range(1..10);
    println!("1-9: {}", n1);
    
    // Inclusive range [1, 10]
    let n2: i32 = rng.gen_range(1..=10);
    println!("1-10: {}", n2);
    
    // Float range
    let f: f64 = rng.gen_range(0.0..1.0);
    println!("Float: {}", f);
    
    // Character range
    let c: char = rng.gen_range('a'..='z');
    println!("Letter: {}", c);
}

Random Element from Collection

use rand::seq::SliceRandom;
use rand::thread_rng;
 
fn main() {
    let mut rng = thread_rng();
    
    let choices = vec!["apple", "banana", "cherry", "date", "elderberry"];
    
    // Choose a random element
    if let Some(fruit) = choices.choose(&mut rng) {
        println!("Random fruit: {}", fruit);
    }
    
    // Choose multiple random elements
    let sampled: Vec<_> = choices.choose_multiple(&mut rng, 3).collect();
    println!("Sampled: {:?}", sampled);
    
    // Shuffle the collection
    let mut numbers: Vec<i32> = (1..=10).collect();
    numbers.shuffle(&mut rng);
    println!("Shuffled: {:?}", numbers);
}

Random String Generation

use rand::Rng;
use rand::distributions::{Alphanumeric, DistString};
 
fn main() {
    let mut rng = rand::thread_rng();
    
    // Random alphanumeric string
    let s: String = Alphanumeric.sample_string(&mut rng, 16);
    println!("Random string: {}", s);
    
    // Random string from custom characters
    const CHARSET: &[u8] = b"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    
    let password: String = (0..20)
        .map(|_| {
            let idx = rng.gen_range(0..CHARSET.len());
            CHARSET[idx] as char
        })
        .collect();
    println!("Password: {}", password);
}

Seeding for Reproducibility

use rand::{Rng, SeedableRng};
use rand::rngs::StdRng;
 
fn main() {
    // Create RNG with fixed seed for reproducibility
    let seed: u64 = 42;
    let mut rng1 = StdRng::seed_from_u64(seed);
    let mut rng2 = StdRng::seed_from_u64(seed);
    
    // Both will produce the same sequence
    let n1: i32 = rng1.gen_range(0..100);
    let n2: i32 = rng2.gen_range(0..100);
    
    println!("rng1: {}, rng2: {}", n1, n2);
    assert_eq!(n1, n2);  // Always true
}

Random Tuples and Arrays

use rand::Rng;
 
fn main() {
    let mut rng = rand::thread_rng();
    
    // Random tuple
    let point: (f64, f64) = rng.gen();
    println!("Random point: ({}, {})", point.0, point.1);
    
    // Random array
    let arr: [u8; 8] = rng.gen();
    println!("Random bytes: {:02x?}", arr);
    
    // Fill existing array
    let mut buffer = [0u8; 32];
    rng.fill(&mut buffer);
    println!("Buffer: {:02x?}", buffer);
}

Probability Distributions

use rand::distributions::{Distribution, Uniform, Normal, Alphanumeric};
use rand::thread_rng;
 
fn main() {
    let mut rng = thread_rng();
    
    // Uniform distribution (default for gen_range)
    let uniform = Uniform::from(0..100);
    let n: i32 = uniform.sample(&mut rng);
    println!("Uniform: {}", n);
    
    // Normal (Gaussian) distribution
    let normal = Normal::new(0.0, 1.0).unwrap();  // mean=0, std_dev=1
    let gaussian: f64 = normal.sample(&mut rng);
    println!("Gaussian: {}", gaussian);
    
    // Sample multiple values
    let values: Vec<f64> = normal.sample_iter(&mut rng).take(5).collect();
    println!("Multiple: {:?}", values);
}

Weighted Random Selection

use rand::seq::IteratorRandom;
use rand::thread_rng;
 
fn main() {
    let mut rng = thread_rng();
    
    let items = vec![('a', 1), ('b', 2), ('c', 3), ('d', 4)];
    
    // Weighted selection (simplified approach)
    let total_weight: i32 = items.iter().map(|(_, w)| w).sum();
    let choice = rng.gen_range(1..=total_weight);
    
    let mut cumulative = 0;
    for (item, weight) in items {
        cumulative += weight;
        if choice <= cumulative {
            println!("Selected: {}", item);
            break;
        }
    }
}

Bernoulli Distribution

use rand::distributions::{Distribution, Bernoulli};
use rand::thread_rng;
 
fn main() {
    let mut rng = thread_rng();
    
    // 70% chance of success
    let bernoulli = Bernoulli::new(0.7).unwrap();
    
    let mut successes = 0;
    for _ in 0..100 {
        if bernoulli.sample(&mut rng) {
            successes += 1;
        }
    }
    
    println!("Successes: {}/100", successes);
}

Cryptographically Secure RNG

use rand::rngs::OsRng;
use rand::RngCore;
 
fn main() {
    let mut rng = OsRng;
    
    // Cryptographically secure random bytes
    let mut key = [0u8; 32];
    rng.fill_bytes(&mut key);
    
    println!("Secure key: {:02x?}", key);
    
    // Random u64
    let secure_u64 = rng.next_u64();
    println!("Secure u64: {}", secure_u64);
}

UUID Generation

use rand::Rng;
 
fn generate_uuid_v4() -> String {
    let mut rng = rand::thread_rng();
    
    let mut bytes = [0u8; 16];
    rng.fill(&mut bytes);
    
    // Set version to 4 (random)
    bytes[6] = (bytes[6] & 0x0f) | 0x40;
    // Set variant to RFC4122
    bytes[8] = (bytes[8] & 0x3f) | 0x80;
    
    format!(
        "{:02x}{:02x}{:02x}{:02x}-{:02x}{:02x}-{:02x}{:02x}-{:02x}{:02x}-{:02x}{:02x}{:02x}{:02x}{:02x}{:02x}",
        bytes[0], bytes[1], bytes[2], bytes[3],
        bytes[4], bytes[5],
        bytes[6], bytes[7],
        bytes[8], bytes[9],
        bytes[10], bytes[11], bytes[12], bytes[13], bytes[14], bytes[15]
    )
}
 
fn main() {
    println!("UUID: {}", generate_uuid_v4());
}

Custom Random Type

use rand::Rng;
use rand::distributions::{Distribution, Standard};
 
#[derive(Debug)]
enum Coin {
    Heads,
    Tails,
}
 
impl Distribution<Coin> for Standard {
    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> Coin {
        if rng.gen_bool(0.5) {
            Coin::Heads
        } else {
            Coin::Tails
        }
    }
}
 
fn main() {
    let mut rng = rand::thread_rng();
    
    // Now we can use gen() for Coin
    let flip: Coin = rng.gen();
    println!("Flipped: {:?}", flip);
    
    // Multiple flips
    let flips: Vec<Coin> = (0..5).map(|_| rng.gen()).collect();
    println!("Flips: {:?}", flips);
}

Shuffling and Permutations

use rand::seq::SliceRandom;
use rand::thread_rng;
 
fn main() {
    let mut rng = thread_rng();
    
    // Full shuffle
    let mut deck: Vec<&str> = vec![
        "A♠", "2♠", "3♠", "4♠", "5♠", "6♠", "7♠", "8♠", "9♠", "10♠", "J♠", "Q♠", "K♠",
        "A♥", "2♥", "3♥", "4♥", "5♥", "6♥", "7♥", "8♥", "9♥", "10♥", "J♥", "Q♥", "K♥",
    ];
    
    deck.shuffle(&mut rng);
    println!("First 5 cards: {:?}", &deck[..5]);
    
    // Partial shuffle
    let mut numbers: Vec<i32> = (1..=100).collect();
    numbers.partial_shuffle(&mut rng, 5);
    println!("First 5 after partial shuffle: {:?}", &numbers[..5]);
}

Random Sampling Without Replacement

use rand::seq::IteratorRandom;
use rand::thread_rng;
 
fn main() {
    let mut rng = thread_rng();
    
    // Sample from iterator without replacement
    let sample: Vec<i32> = (1..=1000)
        .choose_multiple(&mut rng, 5);
    
    println!("Sampled 5 from 1000: {:?}", sample);
    
    // Sample from array
    let items = vec!["a", "b", "c", "d", "e", "f", "g", "h"];
    let chosen: Vec<_> = items.iter().choose_multiple(&mut rng, 3);
    println!("Chosen 3: {:?}", chosen);
}

Random Index Generation

use rand::seq::index;
use rand::thread_rng;
 
fn main() {
    let mut rng = thread_rng();
    
    // Get random indices from a range
    let indices: Vec<usize> = index::sample(&mut rng, 100, 5).into_iter().collect();
    println!("Random indices from 0-99: {:?}", indices);
    
    // Get all indices in random order
    let shuffled: Vec<usize> = index::shuffle(&mut rng, 10).into_vec();
    println!("Shuffled 0-9: {:?}", shuffled);
}

Dice Rolling Simulation

use rand::Rng;
use rand::distributions::{Distribution, Uniform};
 
fn roll_die(sides: u32) -> u32 {
    let mut rng = rand::thread_rng();
    rng.gen_range(1..=sides)
}
 
fn roll_dice(count: u32, sides: u32) -> Vec<u32> {
    let mut rng = rand::thread_rng();
    let die = Uniform::from(1..=sides);
    (0..count).map(|_| die.sample(&mut rng)).collect()
}
 
fn main() {
    let mut rng = rand::thread_rng();
    
    // Roll a d20
    let d20 = roll_die(20);
    println!("d20: {}", d20);
    
    // Roll 2d6
    let two_d6 = roll_dice(2, 6);
    let sum: u32 = two_d6.iter().sum();
    println!("2d6: {:?} = {}", two_d6, sum);
    
    // Roll 4d6, drop lowest (D&D stat generation)
    let mut rolls = roll_dice(4, 6);
    rolls.sort();
    let stat: u32 = rolls[1..].iter().sum();
    println!("4d6 drop lowest: {:?} = {}", rolls, stat);
}

Testing with Deterministic RNG

use rand::{Rng, SeedableRng};
use rand::rngs::StdRng;
 
fn game_simulation(rng: &mut StdRng) -> i32 {
    // Simulate game with random elements
    let score: i32 = rng.gen_range(0..100);
    score
}
 
#[cfg(test)]
mod tests {
    use super::*;
    
    #[test]
    fn test_deterministic() {
        let mut rng1 = StdRng::seed_from_u64(42);
        let mut rng2 = StdRng::seed_from_u64(42);
        
        // Same seed = same results
        assert_eq!(rng1.gen::<u32>(), rng2.gen::<u32>());
    }
    
    #[test]
    fn test_game_simulation() {
        let mut rng = StdRng::seed_from_u64(12345);
        let score = game_simulation(&mut rng);
        
        // Deterministic score for testing
        assert!(score >= 0 && score < 100);
    }
}
 
fn main() {
    let mut rng = StdRng::seed_from_u64(42);
    let score = game_simulation(&mut rng);
    println!("Score: {}", score);
}

Performance: Thread-local vs StdRng

use rand::{Rng, SeedableRng};
use rand::rngs::StdRng;
use std::time::Instant;
 
fn benchmark_thread_rng(count: u32) -> u32 {
    let mut rng = rand::thread_rng();
    let mut sum = 0u32;
    for _ in 0..count {
        sum = sum.wrapping_add(rng.gen_range(0..1000));
    }
    sum
}
 
fn benchmark_std_rng(count: u32) -> u32 {
    let mut rng = StdRng::from_entropy();
    let mut sum = 0u32;
    for _ in 0..count {
        sum = sum.wrapping_add(rng.gen_range(0..1000));
    }
    sum
}
 
fn main() {
    let iterations = 1_000_000;
    
    let start = Instant::now();
    benchmark_thread_rng(iterations);
    let thread_duration = start.elapsed();
    
    let start = Instant::now();
    benchmark_std_rng(iterations);
    let std_duration = start.elapsed();
    
    println!("ThreadRng: {:?}", thread_duration);
    println!("StdRng: {:?}", std_duration);
}

Hex String Generation

use rand::Rng;
 
fn random_hex(length: usize) -> String {
    let mut rng = rand::thread_rng();
    
    (0..length)
        .map(|_| format!("{:02x}", rng.gen::<u8>()))
        .collect()
}
 
fn main() {
    println!("Hex: {}", random_hex(16));
}

Summary

Rand Key Imports:

use rand::Rng;
use rand::thread_rng;
use rand::seq::SliceRandom;
use rand::distributions::{Distribution, Uniform, Alphanumeric};

RNG Types:

Type	Description
`thread_rng()`	Thread-local, secure, convenient
`StdRng`	Reproducible, seedable
`OsRng`	Cryptographically secure from OS

Core Methods:

Method	Description
`gen()`	Generate random value
`gen_range(a..b)`	Random in range
`gen_bool(p)`	Boolean with probability
`fill(&mut buf)`	Fill buffer with random bytes

Distributions:

Distribution	Description
`Uniform`	Equal probability in range
`Normal`	Gaussian distribution
`Bernoulli`	Boolean with probability
`Alphanumeric`	Random alphanumeric chars

Sequence Operations:

Method	Description
`choose()`	Random element
`choose_multiple()`	Multiple random elements
`shuffle()`	Shuffle in place
`partial_shuffle()`	Partial shuffle

Seeding:

// From entropy (random seed)
let rng = StdRng::from_entropy();
 
// From specific seed (reproducible)
let rng = StdRng::seed_from_u64(42);

Key Points:

Use thread_rng() for most cases (fast, secure)
Use StdRng with seed for reproducibility
Use OsRng for cryptographic security
Distributions provide probability patterns
SliceRandom for collection operations
Alphanumeric for random strings
Seed for deterministic testing
gen_range(1..=10) for inclusive range