Snippet

project_euler/7.rb

Code:

# Project Euler Problem 7: 10001st Prime

# https://projecteuler.net/problem=7

# By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, 
# we can see that the 6th prime is 13.
# What is the 10,001st prime number?

def sieve(max)
    primes = (0..max).to_a
    primes[0] = primes[1] = nil
    counter = 0
    primes.each do |prime|
        next unless prime 
        break if prime ** 2 > max
        counter += 1
        (prime ** 2).step(max, prime) { |m| primes[m] = nil }
    end
    return primes.compact
end

def solve(n)
    return sieve(2 * n * Math.log(n))[n]
end

puts solve(6)
puts solve(10_001)

Output:

17
104759