From: Jeppe Jakobsen <jeppe88@...>
Date: 2006-02-13T06:05:23+09:00
Subject: Re: Implanting a new method for a class

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Thank you, for sorting things out for me :)

2006/2/12, Levin Alexander <levin@grundeis.net>:
>
> 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 +=3D 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 =3D [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 defining
> methods 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 =3D A.new
>   a.foo  # prints "Instance method of <#A:0xb7ce9000> of class A"
>
> You can implement sum like so:
>
>   class Array
>     def sum
>       total =3D 0
>       # self refers to the current array instance
>       self.each { |elem| total +=3D 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 +=3D 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 =3D 0
>     total +=3D arr.pop until a.empty?
>     total
>   end
>
>   arr =3D [5,4,3]
>   p arr.sum!   #=3D> 12
>   p arr           #=3D> []   arr is now empty!
>
>
> -Levin
>



--
"winners never quit, quitters never win"

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