erastosthenesSieve.rb

def populate_array(n)
  arr = Array.new(n, true)
  arr[0] = false
  arr[1] = false
  arr
end

def print_array(a)
  count = 0
  while count < a.length
    puts "#{count} is prime" if a[count]
    count += 1
  end
end

def mark_factors(a, count)
  mult = 2
  while count * mult <= a.length
    a[count * mult] = false
    mult += 1
  end
  a
end

def sieve(a)
  count = 2
  while count <= Math.sqrt(a.length)
    a = mark_factors(a, count) if a[count]
    count += 1
  end
end

def main
  puts 'Give me the limit for your primes'
  n = Integer(gets.chomp)
  a = populate_array(n + 1)
  sieve(a)
  print_array(a)
end

main