How can I create a random array between a range of two other arrays?

Question

I need to generate an array of random 20 bytes between a given range of arrays. Since arrays are comparable in Rust, this works:

let low = [0u8; 20];
let high = [2u8; 20];
assert_eq!(true, low < high);
assert_eq!(false, low > high);
assert_eq!(true, low == [0u8; 20]);

For these bounds:

let low: [u8; 20] = [98, 0, 1, 0, 2, 6, 99, 3, 0, 5, 23, 3, 5, 6, 11, 8, 0, 2, 0, 17];
let high: [u8; 20] = [99, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1];

These would be a valid result:

[98, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[99, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

These are not:

[98, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[99, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2]

I want to do something like:

use rand::prelude::*;

fn main() {
    let low = [0u8; 20];
    let high = [2u8; 20];
    let value = rand::thread_rng().gen_range(low, high);
    println!("{:?}", value);
}

but I get following error:

error[E0277]: the trait bound `[u8; 20]: rand::distributions::uniform::SampleUniform` is not satisfied
 --> src\main.rs:6:36
  |
6 |     let value = rand::thread_rng().gen_range(low, high);
  |                                    ^^^^^^^^^ the trait `rand::distributions::uniform::SampleUniform` is not implemented for `[u8; 20]`

I tried implementing SampleUniform and UniformSampler without much success. Is there a simple way to implement this?

What do you mean with "between given range of arrays"? Do the arrays represent a number? For the rand crate, each array is just a sequence of bytes. There is no such concept as the range of values between two arrays — aochagavia
– aochagavia, Commented May 26, 2019 at 13:35
If you can use a Vec instead of an array, I would just do: let value: Vec<u8> = iter::repeat_with(|| rand::thread_rng().gen_range(low, high)).take(20).collect(); — Peter Hall
– Peter Hall, Commented May 26, 2019 at 13:49
@aochagavia - The bounds are just two 160 bit number (big endian). I want to get a random value between these bounds. For example [1u8;20] would be a valid result between [0u8;20] and [2u8;20]. — Gurwinder Singh
– Gurwinder Singh, Commented May 26, 2019 at 13:58
I think you need to implement the various rand traits. However, you won't be able to do that for [u8; 20] because that is not defined in your own crate and neither are the traits. Instead you'd have to create a newtype. e.g. struct U8x20([u8; 20]) and implement for that. — Peter Hall
– Peter Hall, Commented May 26, 2019 at 14:30

Shepmaster · Accepted Answer · 2019-05-29 12:26:32Z

1

If you want to treat the byte arrays as big integers, use the num-bigint crate with the rand feature enabled:

use bigint::{ToBigInt, RandBigInt};

let low = -10000.to_bigint().unwrap();
let high = 10000.to_bigint().unwrap();
let b = rng.gen_bigint_range(&low, &high);

You could also use unsigned integers instead of signed. There are methods to convert to and from big endian byte arrays:

5 Comments

Gurwinder Singh Over a year ago

I tried it but getting no method named ``gen_biguint_range`` found for type ``rand::rngs::thread::ThreadRng`` in the current scope

Peter Kolloch Over a year ago

does it recommend using a certain trait after the message?

Peter Kolloch Over a year ago

use bigint::{ToBigInt, RandBigInt};

Gurwinder Singh Over a year ago

Tried that.. it doesn't work. Also, doesn't recommend any traits. It keeps saying RandBigInt is not in the root. I am using rand 0.6.

Peter Kolloch Over a year ago

You also need to enable the "rand" feature

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