MATLAB: Doesn’t this code have a recursion error

Question

There is a link to a Matlab sudoku solver here: Matlab Sudoku Solver (shown below). I wrote my own solver (shown below) and I get "recursion error" when it runs. It's basically the exact same thing with different variable names. Why does my code get a recursion error while the code in the link doesn't? Thanks!

Matlab Code

function X = sudoku(X) 
% SUDOKU  Solve Sudoku using recursive backtracking. 
%   sudoku(X), expects a 9-by-9 array X. 
 % Fill in all “singletons”. 
 % C is a cell array of candidate vectors for each cell. 
 % s is the first cell, if any, with one candidate. 
 % e is the first cell, if any, with no candidates. 
 [C,s,e] = candidates(X); 
 while ~isempty(s) && isempty(e) 
    X(s) = C{s}; 
    [C,s,e] = candidates(X); 
 end 
 % Return for impossible puzzles. 
 if ~isempty(e) 
    return 
 end 
 % Recursive backtracking. 
 if any(X(:) == 0) 
    Y = X; 
    z = find(X(:) == 0,1);    % The first unfilled cell. 
    for r = [C{z}]            % Iterate over candidates. 
       X = Y; 
       X(z) = r;              % Insert a tentative value. 
       X = sudoku(X);         % Recursive call. 
       if all(X(:) > 0)       % Found a solution. 
          return 
       end 
     end 
   end 
% ------------------------------ 
   function [C,s,e] = candidates(X) 
      C = cell(9,9); 
      tri = @(k) 3*ceil(k/3-1) + (1:3); 
      for j = 1:9 
         for i = 1:9 
            if X(i,j)==0 
               z = 1:9; 
               z(nonzeros(X(i,:))) = 0; 
               z(nonzeros(X(:,j))) = 0; 
               z(nonzeros(X(tri(i),tri(j)))) = 0; 
               C{i,j} = nonzeros(z)’; 
            end 
         end 
      end 
 L = cellfun(@length,C);   % Number of candidates. 
 s = find(X==0 & L==1,1); 
 e = find(X==0 & L==0,1); 
 end % candidates 
end % sudoku

My Code

function X=SD(X)
    % C is a cell array of candidate vectors for each cell.
    % s is the first cell, if any, with one candidate.
    % e is the first cell, if any, with no candidates.
    [C,s,e] = candidate(X);
    while ~isempty(s) && isempty(e)
        X(s) = C{s};
        [C,s,e] = candidate(X);
    end
      % Return for impossible puzzles.
      if ~isempty(e)
          return
      end
      % Recursive Backtracking
      if any(X(:) > 0)
          Y=X;
          [row,col]=find(X==0,1,'first');
          for k=1:length(C{col,row})
              X=Y;
              X(col,row)=C{col,row}(k);
              X=SD(X);
              if all(X(:)>0)
                  return
              end
          end
      end
      function [C,s,e]=candidate(X)    
          C=cell(9,9);
          for j=1:9
              for i=1:9
                  % remove numbers already in current row/column
                  p=1:9;
                  t1=nonzeros(X(j,:));
                  t2=nonzeros(X(:,i));
                  % remove numbers already in current minor matrix
                  t3=nonzeros(X(3*ceil(j/3)-2:3*ceil(j/3),3*ceil(i/3)-2:3*ceil(i/3)));
                  p(nonzeros([t1' t2' t3']))=0;
                  C{j,i}=nonzeros(p);
              end
          end
          L = cellfun(@length,C);   
          s = find(X==0 & L==1,1);
          e = find(X==0 & L==0,1);
      end
  end

Answer 1

You changed this:

 if any(X(:) == 0)

into this:

 if any(X(:) > 0)

Think about it.

PS: MATLAB includes a Comparison tool which is very useful to track down problems like this.