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Lotto Picker - The Rust Way

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


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.

See also:

Random Number Generation

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

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 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 {

This returns a HashSet, so the 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

$ 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

Credits and References

About LCK#236 rust
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This page is a web-friendly rendering of my project notes shared in the LittleCodingKata GitHub repository.

LittleCodingKata is my collection of programming exercises, research and code toys broadly spanning things that relate to programming and software development (languages, frameworks and tools).

These range from the trivial to the complex and serious. Many are inspired by existing work and I'll note credits and references where applicable. The focus is quite scattered, as I variously work on things new and important in the moment, or go back to revisit things from the past.

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