smallest_arr_number.rb

class NumberFormatter
  def format(_number)
    raise NotImplementedError, "subclass should specify its formatting details"
  end
end

class NonPositiveFormatter < NumberFormatter
  def format(_number)
    "\n" # nothing to print
  end
end


class PositiveFormatter < NumberFormatter
  def format(number)
    "#{number}\n"
  end
end

class SmallestNumber
  def initialize(target:, formatter: PositiveFormatter.new)
    @target    = target
    @formatter = formatter
  end

  # Let the formatter do its job. Just ask it
  def to_s
    formatter.format(target)
  end

  private
  attr_reader :target, :formatter
end

class SmallestArrNumber
  # it was not requested a limit of the range that an element of the array should have
  # therefore I am adding the possibility to this be passed, otherwise default to 500.
  def initialize(array_size: 1, limit: 500)
    @target_arr = Array.new(array_size){ rand(1..limit) }
  end

  def call
    smallest_number         = fetch_smallest_number(target_arr)
    missing_smallest_number = smallest_number - 1
    formatter               = missing_smallest_number > 0 ? PositiveFormatter.new : NonPositiveFormatter.new
    puts SmallestNumber.new(target: missing_smallest_number, formatter: formatter)
  end

  # Sort array by small to greater value so that we have elements in order
  # and the smallest number on it will be at its first position
  def fetch_smallest_number(arr)
    # one could implement a better sorting algoritm here instead of ruby default algoritm for its sort method
    arr.sort.first
  end

  private
  attr_reader :target_arr
end


# loading it in your console
# i.e cd into folder of the script: cd folder
# irb -r ./script_file_name
# the console will be opened with the file already loaded.
# > SmallestArrNumber.new(array_size: 20).call

A simple form would be:

def small_number(n:, limit: 500)
  target_arr = Array.new(n){ rand(1..limit) }
  small_number = target_arr.sort.first
  non_included_small_number = small_number - 1
  result = non_included_small_number > 0 ? "#{non_included_small_number}\n" : "\n"
  puts result
end

small_rumber(5)