From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2012-10-03T00:57:01+09:00
Subject: Re: how to read arrary with an array

On Tue, Oct 2, 2012 at 5:35 PM, Richard D. <lists@ruby-forum.com> wrote:
> after some experimenting, I understand but still have a bit of fog.
> Basically, the idea was to see how data would be pulled from slight
> different arrays.  But I'm getting two completely different results.  I
> was expecting to be the same, except my "=====" break to tell the
> different outputs.
>
> program 1
> --------------------------------------------------------------------
> array1 = [['a','b'],[1, 2],['x','y']]
> array2 = ['a',[1,2],'x']
>
> array1.each do |ray|
>   test1 = ray[0]
>   puts test1
> end
> puts '======='
>
> array1.each do |ray|
>   test2 = ray[1]
>   puts test2
> end
> puts '======='
> array2.each do |ray2|
>   test3 = ray2[0]
>   puts test3
> end
> puts '====='
> array2.each do |ray2|
>   test4 = ray2[1]
>   puts test4
> end
> puts '====='
> puts array1[0].to_s + ' ' + array2[0].to_s
> puts
> puts array1[1].to_s + ' ' + array2[1].to_s
> puts
> puts array1[2].to_s + ' ' + array2[2].to_s
>
> OUTPUT
> a
> 1
> x
> ====
> b
> 2
> y
> ====
> a
> 1
> x
> ====
>
> 2
>
> =====
> ["a","b"] a
>
> [1, 2] [1, 2]
>
> ["x", "y"] x
>
> Program 2
> ------------------------------------------------------------------
> {same as above, excpet)
> array1.each do |ray|
>   test1 = ray[0]
>   test2 = ray[1]
>   puts test1
>   puts '****'
>   puts test2
> end
> puts '======='
> array2.each do |ray2|
>   test3 = ray2[0]
>   test4 = ray2[1]
>   puts test3
>   puts '****'
>   puts test4
> end
>
> OUTPUT
> a
> ****
> b
> 1
> ****
> 2
> x
> ****
> y
> =====
> a
> ****
>
> 1
> ****
> 2
> x
> ****
>
> ======
> ["a","b"] a
>
> [1, 2] [1, 2]
>
> ["x", "y"] x
>
> So I think I got what the arrays do.  I just don't think I'm
> understanding how things are being located.  What I"m thinking is this
> array1 = [['a','b'],[1, 2],['x','y']]
> array1: a is slot0-0, b is slot0-1, 1 is slot1-0, 2 is slot 1-1, x is
> slot2-0, and y is slot 2-1
> array2 = ['a',[1,2],'x']
> a is slot1-0-0, 1 is 1-0, 2 is 1-1, and x is 2-0
>
> I thought in both programs the ray(x)[0] would output all x-0 slots, and
> ray(x)[1] would out put all -1 slots

The reason is that your metaphor about arrays being like a 2x2 matrix
is wrong, specially in the cases where the contents of the array are
not arrays themselves. It's better if you think of it this way:

array1 = [['a','b'],[1, 2],['x','y']]

array1 is an array with three elements, with indexes 0, 1 and 2. At
index 0, there is an object, which happens to be another array. At
index 1 there is an object that happens to be another array. The same
for index 2. So:

first_array = array1[0]

first array is now ['a', 'b'], which is an array. At position 0 there
is a String object, so first_array[0]  => 'a'. At position 1 there's
another string, so first_array[1] => 'b'

The confusion might come because you can "chain" method calls like this:

array1[0][0] #=> 'a'

but in reality this are two different steps. First, method [] with
argument 0 is called on array1. This returns a different array. Then,
method [] with argument 0 is called on the returned array. This method
call returns a String, which is 'a'.

When you move to array2, though, your assumptions break, because:

array2 = ['a', [1,2], 'x']

array2[0] calls method [] on the array, with argument 0, which returns
the first object of the array. It happens that in this case, this
object is not an array, but the string 'a'. You have the object
directly when you do that. This is not a 2x2 array. It's just an array
in which the second element is itself an array, but that's not the
case for the first or third objects, which are just strings. So:

array2[0] => 'a', a String

If you do

array2[0][0], this calls method [] with argument 0 of the array,
returning a String. You then call method [] with argument 0 of the
String (yes, String also has a method [], which returns a section of
the string), in this case returning 'a', because you asked for the
first character of the String.

Later in the code, you do array2[0][1], which is thus calling the
method [] with argument 1 on the string 'a', which is asking the
string for the second character, which doesn't exist, and so you get
nil (empty when puts'ed).

You should play a little bit in irb to see the different steps.

Jesus.