From: Keith Lancaster <klancaster1957@...>
Date: 2006-02-13T05:56:09+09:00
Subject: Re: Implanting a new method for a class

Jeppe Jakobsen 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 Thomas if you have not 
already done so. It explains all of this in detail.

Hope this helps,
Keith



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