Ruby练习三

> Here are some hints and things to notice to get you started:
1. The first thing to notice is that if we define attr_accessor_with_history in class Class, we can use it as in the snippet above. This is because in Ruby a class is simply an object of class Class.
2. The second thing to notice is that Ruby provides a method class_eval that takes a string and evaluates it in the context of the current class, that is, the class from which you're calling attr_accessor_with_history.  This string will need to contain a method definition that implements a setter-with-history for the desired attribute attr_name.
3. bar_history should always return an Array of elements, even if no values have been assigned yet.
4. Don't forget that the very first time the attribute receives a value, its history array will have to be initialized.
5. Don't forget that instance variables are referred to as @bar within getters and setters.
6. Although the existing attr_accessor can handle multiple arguments (e.g. attr_accessor :foo, :bar), your version just needs to handle a single argument.  However, it should be able to track multiple instance variables per class, with any legal class names or variable names, so it should work if used this way:
7. History of instance variables should be maintained separately for each object instance. that is, if you do:

> ```ruby
class SomeOtherClass attr_accessor_with_history :foo
    attr_accessor_with_history :bar
end

> then the last line should just return [nil,4], rather than [nil,1,2,4]. Here is some skeleton code:

> ```ruby
class Class
 def attr_accessor_with_history(attr_name)
   attr_name = attr_name.to_s   # make sure it's a string
   attr_reader attr_name        # create the attribute's getter
   attr_reader attr_name+"_history" # create bar_history getter
   class_eval "your code here, use %Q for multiline strings"
 end
end

####My Example Code

```ruby
class Class
    def attr_accessor_with_history(attr_name)
        attr_name = attr_name.to_s
        attr_reader attr_name
        attr_reader attr_name + "_history"

        #our setter code here
        class_eval %Q{
            def #{attr_name}=(attr_name)
                @#{attr_name} = attr_name

                unless @#{attr_name + "_history"}
                    @#{attr_name + "_history"} = []
                    @#{attr_name + "_history"} << nil
                end
                @#{attr_name + "_history"} << attr_name
            end
        }
    end
end

class Test
    attr_accessor_with_history :sample
end

t = Test.new
t.sample = "test"
t.sample = 1
t.sample = :abc
print t.sample_history

####My Example Code
```ruby
class Numeric
    @@currencies = {'dollar' => 1.000,'yen' => 0.013, 'euro' => 1.292, 'rupee' => 0.019}
    
    def method_missing(method_id)
        sigular_currency = method_id.to_s.gsub(/s$/,'')
        if @@currencies.has_key?(sigular_currency)
            self * @@currencies[sigular_currency]
        else
            super
        end
    end

    def in(currency)
        sigular_currency = currency.to_s.gsub(/s$/,'')
        self / @@currencies[sigular_currency]       
    end

end

puts 5.dollars.in(:euros)
puts 10.euros.in(:rupees)