def prime(n)
  return [] if n < 2
  return [2] if n == 2
  result = [2]
  3.upto(n) do |i|
    r = result.dup
    r.each_with_index do |ele,index|
      break if i.gcd(ele) != 1
      result << i if i.gcd(ele) == 1 && index == r.size - 1
    end
  end
  result
end

上面的方法是返回一个数组（给出一个n，然后返回n以内的所有质数）

然后如果想简单的话。。。。用下面的

require 'prime'

def prime(n)
  Prime.take_while { |p| p <= n }
end

Prime的方法如下：

Prime.methods
[:instance, :method_added, :each, :prime?, :int_from_prime_division, :prime_division, :to_a, :entries, :sort, :sort_by, :grep, :count, :find, :detect, :find_index, :find_all, :select, :reject, :collect, :map, :flat_map, :collect_concat, :inject, :reduce, :partition, :group_by, :first, :all?, :any?, :one?, :none?, :min, :max, :minmax, :min_by, :max_by, :minmax_by, :member?, :include?, :each_with_index, :reverse_each, :each_entry, :each_slice, :each_cons, :each_with_object, :zip, :take, :take_while, :drop, :drop_while, :cycle, :chunk, :slice_before, :lazy, :allocate, :new, :superclass, :freeze, :===, :==, :<=>, :<, :<=, :>, :>=, :to_s, :inspect, :included_modules, :name, :ancestors, :instance_methods, :public_instance_methods, :protected_instance_methods, :private_instance_methods, :constants, :const_get, :const_set, :const_defined?, :const_missing, :class_variables, :remove_class_variable, :class_variable_get, :class_variable_set, :class_variable_defined?, :public_constant, :private_constant, :module_exec, :class_exec, :module_eval, :class_eval, :method_defined?, :public_method_defined?, :private_method_defined?, :protected_method_defined?, :public_class_method, :private_class_method, :autoload, :autoload?, :instance_method, :public_instance_method, :nil?, :=~, :!~, :eql?, :hash, :class, :singleton_class, :clone, :dup, :taint, :tainted?, :untaint, :untrust, :untrusted?, :trust, :frozen?, :methods, :singleton_methods, :protected_methods, :private_methods, :public_methods, :instance_variables, :instance_variable_get, :instance_variable_set, :instance_variable_defined?, :remove_instance_variable, :instance_of?, :kind_of?, :is_a?, :tap, :send, :public_send, :respond_to?, :extend, :display, :method, :public_method, :define_singleton_method, :object_id, :to_enum, :enum_for, :equal?, :!, :!=, :instance_eval, :instance_exec, :__send__, :__id__]