Testing whether a number is prime

This lesson covers testing for a prime number in Julia, with the aim of understanding a basic mathematical algorithm built on a loop.

Whether a number is prime — divisible only by 1 and itself — can be settled by testing divisibility from 2 up to its square root. If nothing divides it, it is prime.

The sample code loops over the range for i in 2:isqrt(n) and concludes the number is not prime the moment it finds an i where n % i == 0. Julia provides isqrt() specifically for an integer square root.

A common early stumble is the difference between sqrt() and isqrt(). isqrt() returns an integer, which makes it convenient to use straight in a range.

It is an important mathematical property that underlies cryptography, and there is a more advanced technique — the Sieve of Eratosthenes — for finding many primes efficiently at once.

📖 Reference code

function is_prime(n)
    if n < 2
        return false
    end
    for i in 2:isqrt(n)
        if n % i == 0
            return false
        end
    end
    return true
end

println(is_prime(17))
println(is_prime(18))

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run Julia directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).