From: Josh Cheek <josh.cheek@...>
Date: 2009-09-07T16:15:46+09:00
Subject: Re: Two Dimensional Arrays in Ruby?

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On Sun, Sep 6, 2009 at 10:11 PM, Mason Kelsey <masonkelsey@gmail.com> wrote:

> No doubt this question has been asked before but I cannot find it and it is
> not documented in three Ruby books I've checked, which is very strange as
> two dimensional arrays are a common need in programming.
>
> How does one define and use a two dimensional array in Ruby?
>
> I tried
> anarray = [[1, 2, 3], [4, 5, 6] [7, 8, 9]]
> with
> puts anarray[1][1]
> does not deliver the 5 that I would expect but instead, provides (in SciTE)
> the error message on the line defining the array:
> >ruby test_2_dimensional_array.rb
> test_2_dimensional_array.rb:1:in `[]': wrong number of arguments (3 for 2)
> (ArgumentError)
>  from test_2_dimensional_array.rb:1
> >Exit code: 1
>
> So what is the secret for working with two dimensional arrays in Ruby?  And
> is it documented somewhere?
>
> Thanks in advance!
>

Hi, Mason

In Ruby, Arrays are objects, they only have one dimension. But they can hold
anything, including other arrays, so they can behave as multi dimensional
arrays. To get your desired output, you would so something like this

#a two dimensional array
x = Array.new(3){ Array.new(3) }
p x # => [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]

#a two dimensional array with your desired output
x = Array.new(3){|i| Array.new(3){|j| 3*i+j+1 } }
x # => [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

In this example, we say Array.new(3) which says to make an array with three
indexes. Then we pass it a block, which determines how to initialize each
index. The block accepts the variable i, which is the index being assigned,
so it will have values 0, 1, and 2. Then for each of these indexes, we want
it to be an Array, which gets us our second dimension. So in our block we
assign another Array.new(3), to say an array with three indexes. We pass
this Array the block {|j| 3*i+j+1} so j will have values 0, 1, 2 and i will
have values 0, 1, 2. Then we multiply i by 3, because each second dimension
has three indexes, and we add 1 because we want to begin counting at 1
instead of zero.

So this says "create an array with three indexes, where each index is an
array with three indexes, whose values are 3 times the index of the 1st
dimension, plus the second dimension, plus 1"

Hope that helps.

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