From: Orion Hunter <orion2480@...>
Date: 2003-05-01T00:22:47+09:00
Subject: Arrays

I am trying to create an array of certain length, with a default value of 
Array.new (such that I will have an array of arrays).  I cannot use the 
"default" parameter setting Array.new( 4, Array.new ) because I end up with 
an array populated with references to the same array.  For example:

    irb> a = Array.new( 4, Array.new )
    irb> [ [], [], [], [] ]
    irb> a[0][0] = 1
    irb> a
    irb> [ [1], [1], [1], [1] ]

After some thought, this result was not surprising.  The interpreter 
evaluates the Array.new parameter before passing, thus it ends up passing a 
reference, and this single reference gets populated into each element, 
instead of a new array into each.

My next approach was to create the array, with a dafault length, and iterate 
over it assigning a new array to each element:


    my_arr = Array.new( 4 )
    my_arr.each{ |i|
        i = Array.new
    }

    my_arr.each { |j|
        puts j.type
    }

I get the following output:
    NilClass
    NilClass
    NilClass
    NilClass
    [nil, nil, nil, nil]

I was finally able to achieve the desired result by using array.each_index:

    my_arr.each_index{ |m|
        my_arr[m] = Array.new
    }

    my_arr.each{ |n|
        puts n.type
    }

Output:
    Array
    Array
    Array
    Array
    [ [], [], [], [] ]


Conclusion: I understand why I can't pass Array.new as the default value in 
Array.new.  However, I do _not_ understand why the first iterative approach 
does not work.  I obviously know how to acquire the desired result, but that 
doesn't quench my curiousity as to why the other does not work.  Can I 
bother someone for some enlightenment?

Matt


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