Published at Dec 19 2018
·
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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.

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

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.

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.

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

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

```
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(10000), 104743);
}
```

```
// https://en.wikipedia.org/wiki/Primality_test#Pseudocode
fn is_prime(n: u32) -> bool {
if n <= 3 {
return n > 1;
} else if n % 2 == 0 || n % 3 == 0 {
return false;
}
let mut i = 5;
while i * i <= n {
if n % i == 0 || n % (i + 2) == 0 {
return false;
}
i += 6;
}
true
}
pub fn nth(n: u32) -> u32 {
let n = (n + 1) as usize;
let mut ans = vec![];
let mut i = 0;
loop {
if is_prime(i) {
ans.push(i);
}
i += 1;
if ans.len() == n {
return *ans.last().unwrap();
}
}
}
```

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

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