#265 Picker - The Rust Way

Using a lotto picker example to demonstrate random numbers and set operations the rust way

Notes

Picking numbers for a lottery is a simple demonstration of two language features:

random number generation
producing a unique set

This is a little demonstration of how to do this the rust way.

Random Number Generation

There are a bunch of options for random number generation in rust, including:

rand
fastrand
oorandom - also includes a good “buyers guide” to the alternatives

I’m just going to go with rand for this example - it is the most full-featured but also most complex to use.

The simplest and most direct method of sampling from a range is the gen_range function. It returns a random value in the given range, and is what I’ll use here.

The rand crate provides other options, such as using a Uniform distiribution.

Unique Sets

We need to pick a unique set of numbers i.e. no repeats. There are many ways of doing this. Since we are picking from a continuous series, we can simplify things by just ensuring that the results set does is unique i.e. does not contain an repeating elements.

This is good application for the HashSet collection type.

The Example

See lib.rs for the core routine:

use rand::Rng;
use std::collections::HashSet;

pub fn pick(number_count: usize, upper_limit: usize) -> HashSet<usize> {
    let mut rng = rand::thread_rng();
    let mut choices: HashSet<usize> = HashSet::new();

    while choices.len() < number_count {
        choices.insert(rng.gen_range(1..=upper_limit));
    };
    choices
}

This returns a HashSet, so the main.rs routine uses itertools to sort and join as a comma-separated string:

println!("{}", lpickr::pick(number_count, upper_limit).iter().sorted().format(", "));

Building the Example

Just the standard way with cargo:

cargo new lpickr
cd lpickr
# add code
cargo build
cargo test
cargo run

Using the Example

The lpickr program supports a simple command line to pick “n” numbers from 1 to a max number “m”

Call for instructions:

$ cargo run ?
# or after compilation:
$ target/debug/lpickr ?
Usage: lpickr <number_to_pick> <max>

Some sample runs:

$ target/debug/lpickr 3 10
4, 5, 7
$ target/debug/lpickr 3 10
4, 7, 9
$ target/debug/lpickr 3 10
1, 3, 8
$ target/debug/lpickr 3 10
3, 4, 7
$ target/debug/lpickr 7 49
14, 19, 25, 27, 35, 36, 40

Running the Tests

Some basic tests are included in test_lpickr.py

$ cargo test
   Compiling lpickr v0.1.0
    Finished test [unoptimized + debuginfo] target(s) in 0.99s
     Running target/debug/deps/lpickr-91a691446e285203

running 4 tests
test tests::test_fails_if_ask_for_more_numbers_than_available ... ok
test tests::test_empty_if_ask_for_zero ... ok
test tests::test_returns_expected_number_of_numbers ... ok
test tests::test_returns_all_numbers_within_range ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running target/debug/deps/lpickr-707b18c635ac93c3

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests lpickr

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Project Notes