Prime from 1 to N

gistfile1.rb

# prime.rb
limit = Integer(ARGV.shift || 1000)
primes = []

#list all numbers
for i in 2 .. limit
  primes[i] = i
end


for i in 2 .. Math.sqrt(limit)
  next unless primes[i] #check if number already striked out or not
  (i*i).step(limit, i) do |j| #Numbers like 2i, 3i, 4i, etc. can't be primes, strike them out.
    primes[j] = nil
  end
end
puts primes.compact.join " " #All numbers that not striked out is prime numbers :)