File: sieve.rb

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# sieve of Eratosthenes
max = Integer(ARGV.shift || 100)
sieve = []
for i in 2 .. max
  sieve[i] = i
end

for i in 2 .. Math.sqrt(max)
  next unless sieve[i]
  (i*i).step(max, i) do |j|
    sieve[j] = nil
  end
end
puts sieve.compact.join ", "