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## to Prime Factors in the Rust Track

Published at Dec 20 2018 · 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
``````

## 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!);
}

#[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]);
}``````
``````pub fn factors(n: u64) -> Vec<u64> {
let mut ans: Vec<u64> = Vec::new();
let mut x = n;
let mut last_prime = 2;
for i in 1..=((n / 2) + 1) {
if is_prime(i, last_prime) {
last_prime = i;
while x % i == 0 {
ans.push(i);
x = x / i;
}
}
}
ans
}

// https://en.wikipedia.org/wiki/Primality_test#Pseudocode
fn is_prime(n: u64, start: u64) -> bool {
if n <= 3 {
return n > 1;
} else if n % 2 == 0 || n % 3 == 0 {
return false;
}

let mut i = start;
while i * i <= n {
if n % i == 0 || n % (i + 2) == 0 {
return false;
}
i += 6;
}
true
}``````