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POMini sudoku solver in Prolog stops partway through
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  1. POMini sudoku solver in Prolog stops partway through
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    copied!<p>I'm working through 'Seven Languages in Seven Weeks', and I'm just trying to get an example from the book working. It solves a mini sudoku grid (4x4).</p> <p>The author is using gprolog, but I am using swi-prolog (I couldn't get gprolog to work on my VM for whatever reason, but swi-prolog worked first try).</p> <p>I am running Ubuntu 10.04 in VirtualBox 4.0.4 r70112 (hopefully that's not too relevant!)</p> <p>Here is the code in my prolog file:</p> <pre><code>:- use_module(library(clpfd)). valid([]). valid([Head|Tail]) :- all_different(Head), % in the book, this is 'fd_all_different' valid(Tail). % beginning of sudoku rule itself sudoku(Puzzle, Solution) :- Solution = Puzzle, Puzzle = [S11, S12, S13, S14, S21, S22, S23, S24, S31, S32, S33, S34, S41, S42, S43, S44], Puzzle ins 1..4, % in the book, this is 'fd_domain' Row1 = [S11, S12, S13, S14], Row2 = [S21, S22, S23, S24], Row3 = [S31, S32, S33, S34], Row4 = [S41, S42, S43, S44], Col1 = [S11, S21, S31, S41], Col2 = [S12, S22, S32, S42], Col3 = [S13, S23, S33, S43], Col4 = [S14, S24, S34, S44], Square1 = [S11, S12, S21, S22], Square2 = [S13, S14, S23, S24], Square3 = [S31, S32, S41, S42], Square4 = [S33, S34, S43, S44], valid([Row1, Row2, Row3, Row4, Col1, Col2, Col3, Col4, Square1, Square2, Square3, Square4]). </code></pre> <p>The only parts that I (intentionally) changed were: </p> <ul> <li>adding <code>use_module(library(clpfd)).</code> at the top</li> <li>changing <code>fd_all_different(Head),</code> to <code>all_different(Head),</code></li> <li>changing <code>fd_domain(Puzzle, 1, 4),</code> to <code>Puzzle ins 1..4,</code></li> </ul> <p>Here is the invocation from swipl</p> <pre><code>?- sudoku([_, _, 2, 3, _, _, _, _, _, _, _, _, 3, 4, _, _], Solution). Solution = [4, 1, 2, 3, 2, 3, 4, 1, 1|...] ; false. </code></pre> <p>The solution is correct right up until it cuts off, at which point prolog seems to determine that there is no solution. But there is:</p> <pre><code>4 1 2 3 2 3 4 1 1 2 3 4 3 4 1 2 </code></pre> <p>I've scoured the code looking for a typo or misplaced column, but have been unable to find the source of this. Any ideas?</p>
 

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