How do I generate a vector of random numbers in a range?

TL;DR:

use rand::{distributions::Uniform, Rng}; // 0.8.0

fn main() {
    let range = Uniform::from(0..20);
    let values: Vec<u64> = rand::thread_rng().sample_iter(&range).take(100).collect();
    println!("{:?}", values);
}

It's important to use rand::distributions::uniform::Uniform instead of simply performing the modulo of a uniform random number. See Why do people say there is modulo bias when using a random number generator? for more details.

Since we are generating multiple numbers from a range, it's more performant to create the Uniform once and reuse it. Creating the Uniform does some computation to avoid sampling bias.

We can use Rng::sample_iter to create an iterator of random values and then take some number of them, collecting into a Vec. collect will even make use of Iterator::size_hint to allocate exactly the right number of elements.

---

If you only needed a single random number in the range, you could use the shortcut Rng::gen_range:

use rand::Rng; // 0.8.0

fn main() {
    let mut rng = rand::thread_rng();
    let value: u64 = rng.gen_range(0..20);
}

If you needed a vector of random values without limiting to a range, you can use the Standard distribution:

use rand::{distributions::Standard, Rng}; // 0.8.0

fn main() {
    let values: Vec<u64> = rand::thread_rng().sample_iter(Standard).take(100).collect();
    println!("{:?}", values);
}

There are a few main pieces that you need here. First, how to create a vector of 100 calculated items? The easiest way is to create a range of 100 and map over those items. For instance you could do:

let vals: Vec<u64> = (0..100).map(|v| v + 1000).collect();
// [1000, 1001, 1002, 1003, 1004, 1005, 1006, 1007, ...

Splitting this up:

1. 0..100 creates an iterator for 0 through 99
2. .map processes each item in the iterator when the iterator is processed.
3. .collect() takes an iterator and converts it into any type that implements FromIterator which in your case is Vec.

Expanding on this for your random values, you can adjust the .map function to generate a random value from 0 to 20 using the rand crate's gen_range function to create a numeric value within a given range.

use rand::Rng; // 0.6.5

fn main() {
    let mut rng = rand::thread_rng();

    let vals: Vec<u64> = (0..100).map(|_| rng.gen_range(0, 20)).collect();

    println!("{:?}", vals);
}

(On the Playground)

You should also consider using the rand::distributions::Uniform type to create the range up front, which is is more efficient than calling gen_range multiple times, then pull samples from it 100 times:

use rand::{distributions::Uniform, Rng}; // 0.6.5

fn main() {
    let mut rng = rand::thread_rng();
    let range = Uniform::new(0, 20);

    let vals: Vec<u64> = (0..100).map(|_| rng.sample(&range)).collect();

    println!("{:?}", vals);
}

(On the Playground)