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Ruby Define Method

define_method builds a method while the program runs. It takes a name and a block, and the block becomes the method body.

Three readers from one loop

One loop visits three names in an array. Each name becomes a method that answers with its own word.

class Basket
  [:apple, :pear, :plum].each do |name|
    define_method name do
      "fresh #{name}"
    end
  end
end
basket = Basket.new
puts basket.apple
puts basket.pear
puts basket.plum

Generated methods with an argument

A generated method can declare a parameter in its block. The caller passes a value just like a written method.

class Greeter
  [:hello, :goodbye].each do |word|
    define_method word do |name|
      "#{word}, #{name}"
    end
  end
end
greeter = Greeter.new
puts greeter.hello "Ana"
puts greeter.goodbye "Ana"

Methods from a hash

A hash can hold the names and the values together. Each pair becomes a method that answers with its stored value.

class Prices
  PRICES = { apple: 3, pear: 5 }
  PRICES.each do |name, price|
    define_method name do
      price
    end
  end
end
prices = Prices.new
puts prices.apple
puts prices.pear

Example

class Shirt
  [:small, :medium, :large].each do |size|
    define_method size do
      "Size #{size}"
    end
  end
end
shirt = Shirt.new
puts shirt.small
puts shirt.large

Key points

  • define_method takes a name and a block.
  • The block becomes the body of the new method.
  • A loop can build several similar methods in one pass.
  • A generated method can take arguments like any other method.
  • The names can come from an array or a hash.

In this example, one loop builds three methods on the Shirt class. Each name in the array becomes a method. Each method answers with its own size word.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

What does define_method need to build a method?