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to Prime Factors in the Rust Track

Published at Oct 19 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

Compute the prime factors of a given natural number.

A prime number is only evenly divisible by itself and 1.

Note that 1 is not a prime number.

Example

What are the prime factors of 60?

  • Our first divisor is 2. 2 goes into 60, leaving 30.
  • 2 goes into 30, leaving 15.
    • 2 doesn't go cleanly into 15. So let's move on to our next divisor, 3.
  • 3 goes cleanly into 15, leaving 5.
    • 3 does not go cleanly into 5. The next possible factor is 4.
    • 4 does not go cleanly into 5. The next possible factor is 5.
  • 5 does go cleanly into 5.
  • We're left only with 1, so now, we're done.

Our successful divisors in that computation represent the list of prime factors of 60: 2, 2, 3, and 5.

You can check this yourself:

  • 2 * 2 * 3 * 5
  • = 4 * 15
  • = 60
  • Success!

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 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

The Prime Factors Kata by Uncle Bob http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata

Submitting Incomplete Solutions

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

prime-factors.rs

use prime_factors::factors;

#[test]
fn test_no_factors() {
    assert_eq!(factors(1), vec![]);
}

#[test]
#[ignore]
fn test_prime_number() {
    assert_eq!(factors(2), vec![2]);
}

#[test]
#[ignore]
fn test_square_of_a_prime() {
    assert_eq!(factors(9), vec![3, 3]);
}

#[test]
#[ignore]
fn test_cube_of_a_prime() {
    assert_eq!(factors(8), vec![2, 2, 2]);
}

#[test]
#[ignore]
fn test_product_of_primes_and_non_primes() {
    assert_eq!(factors(12), vec![2, 2, 3]);
}

#[test]
#[ignore]
fn test_product_of_primes() {
    assert_eq!(factors(901255), vec![5, 17, 23, 461]);
}

#[test]
#[ignore]
fn test_factors_include_large_prime() {
    assert_eq!(factors(93819012551), vec![11, 9539, 894119]);
}
// https://en.wikipedia.org/wiki/Trial_division
// stole check from
// https://github.com/rust-rosetta/rust-rosetta/blob/master/tasks/primality-by-trial-division/src/main.rs
fn is_prime(number: i32) -> bool {
    if number % 2 == 0 && number != 2 || number == 1 {
        return false;
    }

    let limit = (number as f32).sqrt() as i32 + 1;

    // We test if the number is divisible by any odd number up to the limit
    (3..limit).step_by(2).all(|x| number % x != 0)
}
pub fn factors(n: u64) -> Vec<u64> {
    let mut factored = n;
    let mut factor = 2;
    let mut factors = vec![];
    loop {
        if factored % factor == 0 && is_prime(factor as i32) {
            factors.push(factor);
            factored = factored / factor;
        } else if factored > 1 {
            factor += 1;
        } else {
            break;
        }
    }

    factors
}

Community comments

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efx's Reflection

I outsourced prime detection but it was fun applying the looping steps for factorization.
I also learned 1 is not a prime number.