From: Albert Schlef <albertschlef@...>
Date: 2008-05-08T03:46:15+09:00
Subject: Re: get method in Array subclass: where's it defined?

Daniel Finnie was faster than me ;-) but I'll my answer nevertheless; it 
duplicates some of his.

RichardOnRails wrote:
>
> [...] is my ignorance about the def [] * args.

There are two _unrelated_ features in that syntax: the '*' and the '[]'.

Let's start with a simple method that accept one argument:

def simple(one)
  puts one
end

Now, let's extend it to accept two arguments:

def simple(one, two)
  puts one, two
end

But what if we wanted this method to accept _any_ number of arguments? 
We would use the '*' syntax, that tells Ruby to lumb all arguments into 
an array:

def simple(*all)
  all.each { |i|
    puts i
  }
end

We can invoke this method thus:

simple 4
simple
simple 9, 4, "blah"
simple -3, 5.6

======

That's all for the '*'. Now, for the '[]':

let's look at this expression:

m[4]

The 'm' object would return the 4th element. It has to have some method 
that responds to this request. Ruby would invoke this method whenever 
the '[]' syntax is used. But what would be the name of this method? The 
answer is simple: [].

So we define a '[]' method that would be invoked whenever we do 
"some_object [ some_thing ]". Here it is:

class Matrix
  def [] (one, two, three)
    puts "hello #{one}"
    puts "hello #{two}"
    puts "hello #{three}"
  end
end

Now, it we did:

m = Matrix.new
m[100, 77, "box"]

We'd see:

hello 100
hello 77
hello box

We could use the '*' syntax to allow for _any_ number of arguments, not 
just three:

class Matrix
  def [] (*args)
    args.each { |a|
      puts "hello #{a}"
    }
  end
end

m = Matrix.new
m["how", "nice", "to", "have", 13, "chocolates"]

Note that "def [] *args" is a synonym for "def [] (*args)". Parentheses 
aren't mandatory in Ruby.
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