The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.

Find the sum of all the primes below two million.



In [21]:

    
def is_prime?(num)
  return true if num == 2
  return false if num % 2 == 0
  return (2..Math.sqrt(num).ceil).all?{|i| num % i != 0}
end

is_prime? 104693









    Out[21]:





true



In [20]:

    
is_prime? 2









    Out[20]:





false



In [19]:

    
primes = []

(2..2000000).lazy.each do |i| 
  primes << i if is_prime?(i)
end

primes.inject(&:+)









    Out[19]:





142913828920



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