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Re: Sudoku

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On 03/27/2013 01:44 AM, Eric Parry wrote:
> I downloaded the following program from somewhere

It'd be good to show where you found it, and credit the apparent author. 
  Bill Barksdale posted this in 2008 at:
 
http://stackoverflow.com/questions/201461/shortest-sudoku-solver-in-python-how-does-it-work

I don't know if there are older ones somewhere, but I didn't find any. 
I did find places that quoted his code without attribution.

Another thing worth pointing out is that it's only valid for Python 2.x 
  (naturally, since I don't think Python 3 was out at that point)

using a link from Wikipedia and inserted the “most difficult Sudoku 
puzzle ever” string into it and ran it. It worked fine and solved the 
puzzle in about 4 seconds. However I cannot understand how it works. It 
seems to go backwards and forwards at random. Can anyone explain how it 
works in simple terms?
> Eric.
>
>
> def same_row(i,j): return (i/9 == j/9)
> def same_col(i,j): return (i-j) % 9 == 0
> def same_block(i,j): return (i/27 == j/27 and i%9/3 == j%9/3)
>
> def r(a):
>    i = a.find('0')
>    if i == -1:
>      print a
>      exit(a)
>
>    excluded_numbers = set()
>    for j in range(81):
>      if same_row(i,j) or same_col(i,j) or same_block(i,j):
>        excluded_numbers.add(a[j])
>
>    for m in '123456789':
>      if m not in excluded_numbers:
>        # At this point, m is not excluded by any row, column, or block, so let's place it and recurse
>        r(a[:i]+m+a[i+1:])
>
> r('800000000003600000070090200050007000000045700000100030001000068008500010090000400')
> Sudoku solver where the puzzle is an 81 character string representing the puzzle read left-to-right, top-to-bottom, and 0 is a blank.
>  
>


-- 
DaveA

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Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-26 22:44 -0700
  Re: Sudoku Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2013-03-27 08:58 +0100
    Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-27 20:00 -0700
      Re: Sudoku Dave Angel <davea@davea.name> - 2013-03-28 00:36 -0400
        Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-28 15:11 -0700
          Re: Sudoku Dave Angel <davea@davea.name> - 2013-03-28 19:28 -0400
            Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-28 22:07 -0700
            Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-28 22:07 -0700
          Re: Sudoku Chris Angelico <rosuav@gmail.com> - 2013-03-29 09:45 +1100
            Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-29 14:47 -0700
              Re: Sudoku Dave Angel <davea@davea.name> - 2013-03-29 18:11 -0400
                Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-30 15:06 -0700
                Re: Sudoku Dave Angel <davea@davea.name> - 2013-03-30 19:15 -0400
                Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-31 15:03 -0700
                Re: Sudoku Dave Angel <davea@davea.name> - 2013-03-31 18:34 -0400
                Re: Sudoku Arnaud Delobelle <arnodel@gmail.com> - 2013-03-31 23:59 +0100
                Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-31 15:03 -0700
                Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-31 15:27 -0700
                Re: Sudoku Chris Angelico <rosuav@gmail.com> - 2013-04-01 09:35 +1100
                Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-31 20:58 -0700
                Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-31 15:27 -0700
                Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-30 15:06 -0700
            Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-29 14:47 -0700
        Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-28 15:11 -0700
  Re: Sudoku Damien Wyart <damien.wyart@free.fr> - 2013-03-27 09:49 +0100
  Re: Sudoku Dave Angel <davea@davea.name> - 2013-03-27 05:38 -0400
  Re: Sudoku Eric Parry <joan4eric@gmail.com> - 2013-03-27 19:49 -0700

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