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to Nth Prime in the Rust Track

Published at Oct 20 2019 · 0 comments
Instructions
Test suite
Solution

Given a number n, determine what the nth prime is.

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

If your language provides methods in the standard library to deal with prime numbers, pretend they don't exist and implement them yourself.

Remember that while people commonly count with 1-based indexing (i.e. "the 6th prime is 13"), many programming languages, including Rust, use 0-based indexing (i.e. primes[5] == 13). Use 0-based indexing for your implementation.

Rust Installation

Refer to the exercism help page for Rust installation and learning resources.

Writing the Code

Execute the tests with:

$ cargo test

All but the first test have been ignored. After you get the first test to pass, open the tests source file which is located in the tests directory and remove the #[ignore] flag from the next test and get the tests to pass again. Each separate test is a function with #[test] flag above it. Continue, until you pass every test.

If you wish to run all ignored tests without editing the tests source file, use:

$ cargo test -- --ignored

To run a specific test, for example some_test, you can use:

$ cargo test some_test

If the specific test is ignored use:

$ cargo test some_test -- --ignored

To learn more about Rust tests refer to the online test documentation

Make sure to read the Modules chapter if you haven't already, it will help you with organizing your files.

Further improvements

After you have solved the exercise, please consider using the additional utilities, described in the installation guide, to further refine your final solution.

To format your solution, inside the solution directory use

cargo fmt

To see, if your solution contains some common ineffective use cases, inside the solution directory use

cargo clippy --all-targets

Submitting the solution

Generally you should submit all files in which you implemented your solution (src/lib.rs in most cases). If you are using any external crates, please consider submitting the Cargo.toml file. This will make the review process faster and clearer.

Feedback, Issues, Pull Requests

The exercism/rust repository on GitHub is the home for all of the Rust exercises. If you have feedback about an exercise, or want to help implement new exercises, head over there and create an issue. Members of the rust track team are happy to help!

If you want to know more about Exercism, take a look at the contribution guide.

Source

A variation on Problem 7 at Project Euler http://projecteuler.net/problem=7

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

nth-prime.rs

use nth_prime as np;

#[test]
fn test_first_prime() {
    assert_eq!(np::nth(0), 2);
}

#[test]
#[ignore]
fn test_second_prime() {
    assert_eq!(np::nth(1), 3);
}

#[test]
#[ignore]
fn test_sixth_prime() {
    assert_eq!(np::nth(5), 13);
}

#[test]
#[ignore]
fn test_big_prime() {
    assert_eq!(np::nth(10_000), 104_743);
}
pub fn nth(n: u32) -> u32 {
    let mut i = 1;
    for _ in 0..n + 1 {
        loop {
            i += 1;
            let prime = is_prime(i);
            if prime {
                break;
            }
        }
    }
    i
}

fn is_prime(i: u32) -> bool {
    if i == 2 {
        return true;
    }
    for j in 2..i {
        if i % j == 0 {
            return false;
        }
    }
    true
}

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