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 10001st prime number?

Solution

#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 10001st prime number?

 

primes = [2,3]

 

while primes.length < 10001 do

    candidate = primes[primes.length – 1] + 2;

    isPrime = false

    while !isPrime do

        isPrime = true

        for prime in primes

            if candidate % prime == 0 then

                isPrime = false

                break

            end

        end

        if !isPrime then

            candidate += 2

        else

            primes.push(candidate)

        end

    end

end

 

puts primes[primes.length–1]

 

Discussion

This is pretty much a straight port of the C# solution.  It turns out to be about an order of magnitude slower than the C# version (IronRuby ~11 seconds, C# ~ 1 second).  It still comes up with the correct answer, and it’s well within the 1 minute rule.

If you have any questions, leave a comment, find me on Twitter (@azzlsoft) or email (rich@azzlsoft.com).