From: Perry Smith <pedz@...>
Date: 2007-08-02T13:43:05+09:00
Subject: Re: change method Array.new? (Was: Unexpected behavior with

Armin Armbruster wrote:
> unknown wrote:
>> Hi --
>> 
>> 
>> Think of it as a rite of passage :-)  Sort of like realizing that:
>> 
>>    array.each {|a| puts a }
>> 
>> can be written as:
>> 
>>    puts array
>> 
>> or that ^ and $ operate per line rather than per string in regular
>> expressions.
>> 
>> 
>> David
> 
> I have no problem with Array.new(size, obj), in fact I think it is a 
> syntactically nice form. What I have a problem with is that it behaves 
> differently, depending what the object is.
> 
>> a = Array.new(3,0)
> => [0, 0, 0]
>> a[1] = 1
> => 1
>> a
> => [0, 1, 0]
> 
>> b = Array.new(3,[])
> => [[], [], []]
>> b[1] << 1
> => [1]
>> b
> => [[1], [1], [1]]
> 
>> c = [[],[],[]]
> => [[], [], []]
>> c[1] << 1
> => [1]
>> c
> => [[], [1], []]
> 
> 
> Why can't b behave like a in this case

This seems really obvious to me and it must seem really non-obvious to 
you.  Interesting (I think).

The second form creates an array.  Each element in that array is a 
pointer (this is how I think of it at least) to the same array -- lets 
call it b.  You put something into b, and you will see it from each of 
the pointers to b.

In the first example, if you could put a tag on the 1 (somehow) or, like 
in lisp, if you could put a property on it.  Then it too would show up 
in all three views of that object.

Oh, how about this:

In the b example, instead of:

b[1] << 1

(which adds an element to b), you need to replace the second element:
irb(main):003:0> b = Array.new(3, [])
=> [[], [], []]
irb(main):004:0> b[1] = [ 1 ]
=> [1]
irb(main):005:0> b
=> [[], [1], []]
irb(main):006:0>

Does any of that help?

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