Create two-dimensional arrays and access sub-arrays in Ruby

I wonder if there's a possibility to create a two dimensional array and to quickly access any horizontal or vertical sub array in it?

I believe we can access a horizontal sub array in the following case:

x = Array.new(10) { Array.new(20) }

x[6][3..8] = 'something'

But as far as I understand, we cannot access it like this:

x[3..8][6]

How can I avoid or hack this limit?

There are some problems with 2 dimensional Arrays the way you implement them.

a= [[1,2],[3,4]]
a[0][2]= 5 # works
a[2][0]= 6 # error

Hash as Array

I prefer to use Hashes for multi dimensional Arrays

a= Hash.new
a[[1,2]]= 23
a[[5,6]]= 42

This has the advantage, that you don't have to manually create columns or rows. Inserting into hashes is almost O(1), so there is no drawback here, as long as your Hash does not become too big.

You can even set a default value for all not specified elements

a= Hash.new(0)

So now about how to get subarrays

(3..5).to_a.product([2]).collect { |index| a[index] }
[2].product((3..5).to_a).collect { |index| a[index] }

(a..b).to_a runs in O(n). Retrieving an element from an Hash is almost O(1), so the collect runs in almost O(n). There is no way to make it faster than O(n), as copying n elements always is O(n).

Hashes can have problems when they are getting too big. So I would think twice about implementing a multidimensional Array like this, if I knew my amount of data is getting big.

rows, cols = x,y  # your values
grid = Array.new(rows) { Array.new(cols) }

As for accessing elements, this article is pretty good for step by step way to encapsulate an array in the way you want:

How to ruby array

You didn't state your actual goal, but maybe this can help:

require 'matrix'  # bundled with Ruby
m = Matrix[
 [1, 2, 3],
 [4, 5, 6]
]

m.column(0) # ==> Vector[1, 4]

(and Vectors acts like arrays)

or, using a similar notation as you desire:

m.minor(0..1, 2..2) # => Matrix[[3], [6]]

Here's a 3D array case

class Array3D
   def initialize(d1,d2,d3)
    @data = Array.new(d1) { Array.new(d2) { Array.new(d3) } }
   end

  def [](x, y, z)
    @data[x][y][z]
  end

  def []=(x, y, z, value)
    @data[x][y][z] = value
  end
end

You can access subsections of each array just like any other Ruby array. @data[0..2][3..5][8..10] = 0 etc

x.transpose[6][3..8] or x[3..8].map {|r| r [6]} would give what you want.

Example:

a = [ [1,  2,  3,  4,  5],
      [6,  7,  8,  9,  10],
      [11, 12, 13, 14, 15],
      [21, 22, 23, 24, 25]
    ]
  
#a[1..2][2]  -> [8,13]
puts a.transpose[2][1..2].inspect   # [8,13]
puts a[1..2].map {|r| r[2]}.inspect  # [8,13]

I'm quite sure this can be very simple

2.0.0p247 :032 > list = Array.new(5)

 => [nil, nil, nil, nil, nil] 

2.0.0p247 :033 > list.map!{ |x| x = [0] }

 => [[0], [0], [0], [0], [0]] 

2.0.0p247 :034 > list[0][0]
 
  => 0

a = Array.new(Array.new(4))

0.upto(a.length-1) do |i|
  0.upto(a.length-1) do |j|
    a[i[j]] = 1
  end
end

0.upto(a.length-1) do |i|
  0.upto(a.length-1) do |j|
    print a[i[j]] = 1 #It's not a[i][j], but a[i[j]]
  end
  puts "\n"
end

Here is the simple version

 #one
 a = [[0]*10]*10

 #two
row, col = 10, 10
a = [[0]*row]*col

Here is an easy way to create a "2D" array.

2.1.1 :004 > m=Array.new(3,Array.new(3,true))

=> [[true, true, true], [true, true, true], [true, true, true]]