make a list of prime numbers using the sieve of Eratosthenes.

listprimes_sieve.rb

#!/opt/local/bin/ruby1.9 -w
# -*- coding: utf-8 -*-
# listprimes_sieve.rb: make a list of prime numbers using the sieve of Eratosthenes

start = ARGV.shift.to_i
start = 2 if start < 2
range = ARGV.shift.to_i

def sieve(start, range)
  limit = start + range
  max = Math::sqrt(limit).to_i
  s = Array.new(limit, true)
  s[0] = s[1] = false
  i = step = 2
  while i <= max
    j = i + step
    while j < limit
      s[j] = false
      j += step
    end
    i += 1
    until s[i] or i > max
      i += 1
    end
    step = i
  end
  primes = []
  (start...limit).each do |n|
    primes.push(n) if s[n]
  end
  return primes
end

t0 = Time.now
sieve(start, range).each do |p|
  print "#{p}\n"
end
elapsed = Time.now - t0
print "Elapsed: #{elapsed}\n"