From: Ruby Student <ruby.student@...>
Date: 2008-09-21T19:57:13+09:00
Subject: Re: Question about multi-demension hash initialization

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On Sun, Sep 21, 2008 at 4:47 AM, Robert Klemme
<shortcutter@googlemail.com>wrote:

> On 20.09.2008 21:30, Ruby Student wrote:
>
>> On Sat, Sep 20, 2008 at 10:47 AM, Robert Klemme
>> <shortcutter@googlemail.com>wrote:
>>
>>> On 19.09.2008 20:21, Ruby Student wrote:
>>>
>> First, thank you for your help and sample code.
>>
>
> You're welcome.
>
>  What I need to do is to pass an integer compose of minimum 2 digits and
>> maximum of 8 and count the how many time a digit appears.
>>
>
> That's easy.
>
> irb(main):006:0> i = rand 100000000
> => 3976173
> irb(main):007:0> h = Hash.new 0
> => {}
> irb(main):008:0> i.to_s(10).scan(/\d/) {|dg| h[dg] += 1}
> => "3976173"
> irb(main):009:0> h
> => {"6"=>1, "7"=>2, "9"=>1, "1"=>1, "3"=>2}
> irb(main):010:0>
>
>  For example, I have a multidimensional array (9x9), which I then take each
>> row and each column at a time.
>>
>
> This is a _two_ dimensional Array.
>
>  For instance, the following are two rows extracted from the 9x9 array:
>>
>> r1 = [345, 8, 459, 125679, 123, 45679, 379, 679, 79]
>> r8 = [9, 4, 6789, 356, 1, 357, 2356, 257, 67]
>>
>> So, for row r1:
>> 1s = 2, 2s = 2, 3s = 3, 4s = 3, 5s = 4, 6s = 3, 7s = 5, 8s = 1, 9s = 6
>>
>> The same reasoning follows for r8.
>> So the 9x9 hash represents the 9x9 array for Sudoku, but instead of
>> representing a Sudoku element, the key = the number while the value is the
>> count (frequency) of that key.
>> I am not sure if I am explaining myself clearly.
>>
>
> Me, too. ;-)  Given the fact that you are dealing with Sudoku it is a
> complete mystery to me why you want to count "digits".  It seems, a Sudoku
> can have _numbers_ in all locations and you want to make sure that no number
> appears twice in a row and in a column.  Basically there are many ways to do
> this, one would be a bit set (you can use an integer for that, see at end).
>
>  I need this information in order to successfully implement one of the
>> Sudoku
>> rules, the so called x-wing rule, which explanation can be found at:
>> http://www.sudokuoftheday.com/pages/techniques-8.php
>>
>> I can find my way around with arrays, but with hashes I am not familiar at
>> all.
>>
>
> Since you are indexing by number (or more precisely: digit) anyway what do
> you gain by using a Hash?
>
> Cheers
>
>        robert
>
>
> A half baked example to illustrate what I mean with the bit set check:
>
> #!/bin/env ruby
>
> require 'pp'
>
> class Sudoku
>  def initialize(size)
>    raise ArgumentError unless 2 <= size
>    @size = size
>    @dat = Array.new(size * size)
>    @rows = Array.new(size, 0)
>    @cols = Array.new(size, 0)
>  end
>
>  def [](x,y)
>    @dat[pos(x,y)]
>  end
>
>  def []=(x,y,v)
>    raise "Not allowed" if @rows[x][v] == 1 || @cols[y][v] == 1
>    @dat[pos(x,y)] = v
>    @rows[x] |= 1 << v
>    @cols[y] |= 1 << v
>  end
>
>  def to_s
>    s = ""
>    @size.times do |i|
>      s << "[" << @dat[i * @size, @size].join(", ") << "]\n"
>    end
>    s
>  end
>
> private
>  def pos(x,y)
>    raise ArgumentError unless [x,y].all? {|a| Integer === a}
>    x * @size + y
>  end
> end
>
> s = Sudoku.new 9
>
> s[2,0] = 1
> puts s
>
> s[1,0] = 1
>
> Robert,
Again, thanks a bunch.
You have given me enough material for a good few days to digest.
I'll play and learn from your sample code.

Thanks again,

Ruby Student

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