From: Phrogz <phrogz@...>
Date: 2007-10-18T00:30:06+09:00
Subject: Re: 2 dimensional array

On Oct 17, 4:53 am, Shuaib Zahda <shuaib.za...@gmail.com> wrote:
> I want to declare 2-dimensional arrays it has 6 columns but uknow number
> of columns.
> i tried this way
> array = [][] # did not work
> array = [[],[]] # it works but only for two elements.

There is no such thing as a 2-dimensional array in Ruby (not in the
core, anyhow). Using core objects, you can instead create an array of
arrays (as you have done above).

Here's an example of some code that automatically creates rows with 6
nil values each time you ask for a row:

  class SixColArray < Array
    def initialize( num_rows=0 )
      # Simply asking for a row ensures it and all lesser rows exist
      self[num_rows-1] if num_rows > 0
    end
    def []( row_number )
      unless row = super
        # Create a new row with 6 'column' arrays
        row = self[ row_number ] = Array.new(6)
        # Ensure lower row numbers exist by asking for them
recursively
        self[ row_number - 1 ] if row_number > 0
      end
      row
    end
  end

  require 'pp'
  six_col = SixColArray.new( 4 )
  six_col[ 0 ][ 3 ] = 42
  six_col[ 3 ][ 1 ] = 15
  pp six_col
  #=> [[nil, nil, nil, 42, nil, nil],
  #=>  [nil, nil, nil, nil, nil, nil],
  #=>  [nil, nil, nil, nil, nil, nil],
  #=>  [nil, 15, nil, nil, nil, nil]]

  # A new row is created on the fly
  six_col[ 4 ][ 5 ] = 99
  pp six_col
  #=> [[nil, nil, nil, 42, nil, nil],
  #=>  [nil, nil, nil, nil, nil, nil],
  #=>  [nil, nil, nil, nil, nil, nil],
  #=>  [nil, 15, nil, nil, nil, nil],
  #=>  [nil, nil, nil, nil, nil, 99]]

  # There is no limit on the number of columns in a row
  # ...or the type of values for each slot
  six_col[ 2 ][ 8 ] = 'ow'
  pp six_col
  #=> [[nil, nil, nil, 42, nil, nil],
  #=>  [nil, nil, nil, nil, nil, nil],
  #=>  [nil, nil, nil, nil, nil, nil, nil, nil, "ow"],
  #=>  [nil, 15, nil, nil, nil, nil],
  #=>  [nil, nil, nil, nil, nil, 99]]

If you want something more robust (for example ensuring that you can't
create new columns on the fly), look for true matrix classes. (One
exists in the standard library; see http://ruby-doc.org/stdlib/libdoc/matrix/rdoc/index.html
for more information. Also see the NArray class.)

> another question I believe the answer is no but i want to confirm
> can we have multi-dimensional hash? :)

Of course, they are called hashes of hashes. For example:

  people = {
    :gavin => {
      :age => 34,
      :sex => :male
    },
    :lisa  => {
      :age => 33,
      :sex => female
    }
  }

  p people[ :lisa ][ :age ]
  #=> 33

Here's a Hash that automatically creates new hashes for each key you
try to access (specifically 1-level deep for a '2-dimensional' hash):

  people = Hash.new{ |me,key| me[ key ] = {} }
  people[ :gavin ][ :age ] = 34
  people[ :gavin ][ :sex ] = :male
  people[ :fido ][ :species ] = :dog
  p people
  #=> {:gavin=>{:age=>34, :sex=>:male}, :fido=>{:species=>:dog}}

As you can see, again, there is no constraint on what sort of keys are
allowed at any level.

I hope this helps you see how:
a) The core classes of Ruby do not include a lot of very specialized
cases,
b) You can write any classes you want (and someone probably already
has)
c) If you don't require the program to ensure you don't go 'out of
bounds', you can use all sorts of nested objects to create what you
want.