From: "David A. Black" <dblack@...>
Date: 2008-05-02T21:24:05+09:00
Subject: Re: Beginner Question on Ruby

Hi --

On Fri, 2 May 2008, Arun Kumar wrote:

> On Fri, May 2, 2008 at 5:27 PM, David A. Black <dblack@rubypal.com> wrote:
>
>> Hi --
>>
>>
>> On Fri, 2 May 2008, Arun Kumar wrote:
>>
>> Hi all,
>>>
>>> I am beginning to learn ruby and cant seem to reason this behaviour. I
>>> expect since empty or nill is the array, arr*n shouldnt change it.
>>>
>>> Why is this behaviour so?
>>>
>>> Code:
>>>
>>> num_arr = Array.new
>>> puts "Num Alpha: #{num_arr} and size: #{num_arr.size}"
>>> puts "Num Alpha: #{num_arr << num_arr} and size: #{num_arr.size}"
>>> num_arr.insert(-1, num_arr << num_arr)
>>> puts "Num Alpha: #{num_arr} and size: #{num_arr.size}"
>>>
>>> Output:
>>>
>>> Num Alpha:  and size: 0
>>> Num Alpha: [...] and size: 1
>>> Num Alpha: [...][...][...][...][...][...][...][...][...] and size: 3
>>>
>>
>> You're inserting an array into itself. Ruby represents that kind of
>> recursive inclusion with dots.
>>
>> irb(main):005:0> a = []
>> => []
>> irb(main):006:0> a << a
>> => [[...]]
>>
> Hello David,
>
> Thanks for the answer.
>
> But would you know why it is so? Is there something syntactic or semantic
> issue that I am making,
>
> the reason i ask this is...
>
> n = 10
> x += n
> does a perfect x = nil + n => n initialization, which is good and
> explainable.

You can't add 10 to nil. You'll get an error if you try to do that,
because nil has no + method.

> In the array case however,
> One would expect [atleast i am :) ] the elements of the source array [if
> any] be appended to the target. Now why is the reference going in making it
> a recursive :?
>
> sorry if this question is lame or bozo but i feel a clear behaviour enables
> clear understanding or would require even more clearer reasoning.

You've using the << method:

  num_arr = Array.new
  num_arr << num_arr

which appends the object num_arr (not its elements, but it, itself) to
the object num_arr.

If you want to add the elements of an array to another array, you can
use concat:

irb(main):001:0> a = [1,2,3]
=> [1, 2, 3]
irb(main):002:0> a.concat(a)
=> [1, 2, 3, 1, 2, 3]

It's all perfectly reasonable -- you just have to know what the
methods you're using actually do :-)


David

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