From: Robert Klemme <shortcutter@...>
Date: 2008-09-21T17:47:00+09:00
Subject: Re: Question about multi-demension hash initialization

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