In a recent project it would of been handy to write some ruby code which looked like:When searching through the ruby libraries there wasn’t any indication it existed. Wanting to avoid a class method – a quick google turned up this StackOverflow post – Calculate percentage in ruby. It presents a nice solution to the problem (one I should have thought of!). Create a class called Numeric and add the method within it:Now a number object has a method called percent_of which takes it self and another number as an argument to calculate the percentage. How Does It Work? There are two tricks going on here. The self KeywordOn line 4 the first number in the percent of equation is not passed to the method. Instead the the method refers to the value of its own object. In the example above this is a Numeric object with the value 10. So the value 10 is used. Extending a ClassMore interestingly the Numeric class is part of the standard ruby library. So how come it can be extended? Ruby has a great feature – to add to an existing class just create it within your codebase and add to it. In contrast other languages might need to use the this keyword (or similar) on a object passed to the method (see: C# Extension Methods). Here this is used to create an elegant result. Within the codebase just add a numeric.rb file with a Numeric class in it. Then the the percent_of method can be defined. Ruby will treat it as part of the original class. Pretty neat. You can find another example of this at the blog Ruby Fleebie.