From: Levin Alexander <levin@...>
Date: 2006-02-13T05:49:13+09:00
Subject: Re: Implanting a new method for a class

On 2/12/06, Jeppe Jakobsen <jeppe88@gmail.com> wrote:> Hi there, I have a question regarding how to implant a new method into the> class Array. I would like to write a ruby file that sums all of the digits> in an array. I have tried  this:>> class Array>     def Array.sum>         until (Array.empty?)>             arraySum += Array.pop>         end>         puts arraySum>     end> end>> Gave no error when I ran the code. My problem occur when i try to use this> method:>> require "Library/Array_sum.rb"> a = [1..100]> a.sum>> I get a NoMethodError on sum. Can anybody tell me what I did wrong here?> Thanks in advance :)
You are confusing classes and instances of classes.  You are definingmethods for the Array class, not for actual array instances.
  class A    def A.foo      puts "Class method of #{self.inspect}, instance of class #{self.class}"    end    def foo      puts "Instance method for #{self.inspect}, of class #{self.class}"    end  end
  A.foo # prints "Class method of A, instance of class Class"  a = A.new  a.foo  # prints "Instance method of <#A:0xb7ce9000> of class A"
You can implement sum like so:
  class Array    def sum      total = 0      # self refers to the current array instance      self.each { |elem| total += elem }      total    end
    # but you can also implement sum on the class Array itself.  But then    # you need to provide the array, that you want to know the sum of    #    def Array.sum arr      arr.inject(0) { |total, elem| total += elem }    end  end
  puts [1,2,3,4].sum  puts Array.sum([1,2,3,4,5])
be careful with "pop", it changes the Array!
  def sum!(arr)    total = 0    total += arr.pop until a.empty?    total  end
  arr = [5,4,3]  p arr.sum!   #=> 12  p arr           #=> []   arr is now empty!

-Levin