Rust Practice Exercises

Prime Numbers in Rust

Test whether a number is prime and list the primes up to 30 with filter.

Beginner
2 min read
1 runnable example

What you'll learn

Trial division
Stopping at the square root
filter on a range

Run code →

2 to 7

2prime

3prime

42 x 2

5prime

62 x 3

7prime

Only divisible by 1 and itself

A prime number is larger than 1 and can only be divided by 1 and itself. Write a function that tests one number, then use it to filter a range.

It is enough to try divisors up to the square root of n, which is what divisor * divisor <= n checks. filter keeps the numbers for which is_prime returns true and collect gathers them into a vector.

Key points

Numbers below 2 are not prime.
Divisors up to the square root are enough.
filter keeps the numbers that pass the test.

Runnable Rust example

Rust

Run code →

main.rs

fn is_prime(n: u32) -> bool {
    if n < 2 {
        return false;
    }
    let mut divisor = 2;
    while divisor * divisor <= n {
        if n % divisor == 0 {
            return false;
        }
        divisor += 1;
    }
    true
}

fn main() {
    let primes: Vec<u32> = (1..=30).filter(|&n| is_prime(n)).collect();
    println!("{:?}", primes);
    println!("97 is prime: {}", is_prime(97));
}

Get a Run button for this example Result of running the code

[2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
97 is prime: true

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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