nrew=0;

[i1,j1]=find(dir_srand);

[Ne, aux]=size(i1);
clear aux;

ntry=2*Ne;

for i=1:ntry;
   e1=1+floor(Ne*rand);
   e2=1+floor(Ne*rand);
   v1=i1(e1);
   v2=j1(e1);
   v3=i1(e2);
   v4=j1(e2);
   
   if (v1~=v3)&(v1~=v4)&(v2~=v4)&(v2~=v3);

         if (dir_srand(v1,v4)==0)&(dir_srand(v3,v2)==0);
            dir_srand(v1,v4)=dir_srand(v1,v2);
            dir_srand(v3,v2)=dir_srand(v3,v4);
            
            dir_srand(v1,v2)=0;
            dir_srand(v3,v4)=0;
            
            nrew=nrew+1;
            
            i1(e1)=v1;
            j1(e1)=v4;
            i1(e2)=v3;
            j1(e2)=v2;
            
         end;
    end;
end;


srand=dir_srand;