problem-010.rb

#!/usr/bin/env ruby

# Find the sum of all the primes below two million.

# Function to check if a number is prime, using the algorithm provided in the solution to problem 7

def prime?(number)
  if number == 1
    return false
  elsif number < 4
    return true
  elsif number%2 == 0
    return false
  elsif number < 9
    return true
  elsif number%3 == 0
    return false
  else
    r = (Math.sqrt(number)).floor
    f = 5
    while f<=r
      if number%f == 0
        return false
      elsif number%(f+2) == 0
        return false
      end
      f += 6
    end
    return true
  end
end

sum_of_primes = 0
(2..2000000).each do |n|
  if prime?(n)
    sum_of_primes += n
  end
end

puts "Answer: " + sum_of_primes.to_s