Forum: Ruby Implanting a new method for a class

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Jeppe J. (Guest)
on 2006-02-12 22:21
(Received via mailing list)
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 :)
Levin A. (Guest)
on 2006-02-12 22:49
(Received via mailing list)
On 2/12/06, Jeppe J. <removed_email_address@domain.invalid> wrote:
>     end
> 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 = 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
Keith L. (Guest)
on 2006-02-12 22:55
Jeppe J. 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 :)

Jeppe,
You have a number of problems here. First, if what you are really trying
to do is to sum the array, rather than just learn how to add methods to
a class, then all you have to do is

a = [1,2,3]
a.inject { |v,n| v+n }
=> 6

Second, you created your function as a class method rather than an
instance method by calling it Array.sum. All you need is
def sum
...
end

This may be why you are getting the MethodNotFound, since the instance
of array "a" does not have this method - the class Array does.

Finally, a = [1..100] will give you an array with the first value being
the range [1..100], not an array with all the values from 1 to 100,
[1,2,3...].

Two suggestions: learn to use irb to see what happens when you call
methods, and (2) pick up Programming Ruby by Dave T. if you have not
already done so. It explains all of this in detail.

Hope this helps,
Keith
James B. (Guest)
on 2006-02-12 22:57
(Received via mailing list)
Jeppe J. wrote:
> ...
> I get a NoMethodError on sum. Can anybody tell me what I did wrong here?
> Thanks in advance :)
>

You've defined a class method (e.g. Array.sum)

You need to do this:


  class Array
      def sum
         until (Array.empty?)
  ...


--
James B.

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Jeppe J. (Guest)
on 2006-02-12 23:05
(Received via mailing list)
Thank you, for sorting things out for me :)

2006/2/12, Levin A. <removed_email_address@domain.invalid>:
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