sieve of eratosthenes

require 'prime'
primes = Array.new
counter = ARGV[0].to_i
counter = 30 if counter == 0
(2..counter).to_a.each { |x|
  if Prime.prime? x then
    primes << x
  end
}

x = primes
y = primes

print '     '
x.each {|i| print "%-3d  " % i}
print "\n     "
x.each {|i| print '---- '}
print "\n"

y.each do |j|
   print "%-3d| " % j
   x.each {|i| print "%-3d  " % (i*j)}
   print "\n"
end