From: Daniel Finnie <dan@...>
Date: 2008-05-08T03:41:16+09:00
Subject: Re: get method in Array subclass: where's it defined?

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Hi,

* is the splat operator.  It means that all of the arguments to the method
will go in the args variable.  None of the def [] part affects this.

If you call [] with the arguments 1, 2, and 3, then args will be an array
with the values 1, 2, and 3.
If you call [] with the arguments 1 and 3, then args will be an array with
the values 1 and 3.
If you call [] with one argument, then args will be an array containing only
that argument.
If you call [] with no arguments, then args will be an empty array.

This behavior is useful because you don't know how many arguments [] was
called with.

So, when you do args.length == 2, you make sure that the [] method was
called with two arguments.  When you do args[0].is_a? Integer, you make sure
that the first argument given was an integer, and likewise with the 2nd
args[1].is_a? integer.

Conversely, when you do get(*args), you use the splat operator in reverse.
Instead of taking arguments and putting them into an array, you take an
array and put use its elements as arguments like so:

def max(a, b)
  if a > b
    a
  else
    b
  end
end

max(2, 3) #=> 3
max(3, 2) #=> 3
an_array = [3, 2]
max(an_array) #=> Error because you only gave one argument, the array
max(*an_array) #=> 3 because the array was split into arguments.

On another note (sorry if this confuses you), a more idomatic way to write
args[0].is_a? Integer and args[1].is_a? Integer is this:
args.all? {|arg| arg.is_a? Integer }

Dan

On 5/7/08, RichardOnRails <RichardDummyMailbox58407@uscomputergurus.com>
wrote:
>
> On May 7, 4:06 am, Albert Schlef <albertsch...@gmail.com> wrote:
> > RichardOnRails wrote:
> > > m = Matrix[ [10,20,30], [40,50,60], [70,80,90] ];
> > > puts m [0] [1].inspect  #  20
> > > puts m [2] [0].inspect  #  70
> > > puts m[1].inspect  #  [40, 50, 60]
> > [...]
> > > my adaptation of Daniel's suggestion, including sub-
> > > classing, works with no definition of get anywhere.  And the code runs
> > > without any visible definition of get
> >
> > Your code "works with no definition of get anywhere" because it doesn't
> > utilize this 'get'. When you do "puts m [0] [1]" you aren't triggering
> > your 'get'. That's because "m [0] [1]" is the plain-vanila array
> > indexing; it uses one-dimensional indexing. It's exactly like doing "row
> > = m[0]; row[1]". Multi-dimentional array indexing means: "m[0,1]". and
> > if you tried that you'd trigger the call to 'get', and since it's
> > absence, Ruby would complain.
> >
> > Implementiong your 'get' is no big deal. For a start, replace that
> > "get(*args)" with "self[args[0]][args[1]]".
> > --
> > Posted viahttp://www.ruby-forum.com/.
>
> Hi Albert,
>
> As you obviously recognized that I was totally out at sea on this
> thing.  The instrumented version, which I added in my reply to Phroz,
> confirms your assessment that "get" was never invoked.
>
> While I'm grateful for that revelation,  I am ever more grateful for
> your additional, thorough analysis of the situation.  I will employ
> them to good effect.
>
> What's really at the root of my problems with this thing is my
> ignorance about the def [] * args.
>
> It appears that what's being defined is "args".  I concluded this by
> changing its name and drawing an "undefined local variable or method
> `args'" error message for the next line. To me, that seems weird.  It
> looks like C or C++ with type info before the name of a variable being
> defined.
>
> I can't find "def [] *" on the net. Can you shed any light on this?
>
> Best wishes,
> Richard
>
>
>
>
>
>

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