Storing the positions of neighbouring elements that have specific values.

A = zeros(5,5);
A(3,4)=1;
A(4,1)=1

I have a matrix of zeros with two 1's in postions (3,4) and (4,1). Starting with the 1 at (3,4) I want to store all the positions of neighbouring elements that are eqaul to 0. I want to do this as my next step is to choose one of the neighbours that equal 0, change that to a 1, and then start the whole process again, starting at any element that equals 1.

Answer 1

[dpb]

>> i0=3; j0=4;
>> [i,j]=find(~A(i0-1:i0+1,j0-1:j0+1));
>> [i,j]
ans =
          1.00          1.00
          2.00          1.00
          3.00          1.00
          1.00          2.00
          3.00          2.00
          1.00          3.00
          2.00          3.00
          3.00          3.00
>> 

You'll have to restrict search to either include only interior initial points or modify to account for location on boundary in computing range vectors.

[Phillip Smith]

Okay that works but the indices are all relative to the initial point, I would like to have the postions relative to the whole matrix. Since it starts at 3,4 the output should give me the positions 2,3 2,4 ,2,5 3,3 3,5 4,3 4,4 4,5 right?

[dpb]

Just add the initial position offset less one for one-based counting (twice)...

>> [i+i0,j+j0]-2
ans =
          2.00          3.00
          3.00          3.00
          4.00          3.00
          2.00          4.00
          4.00          4.00
          2.00          5.00
          3.00          5.00
          4.00          5.00
>> 

[Phillip Smith]

I did this and has worked perfectly

[r,c]=find(~D(i:i+2,j:j+2));
          E=[r,c];
          n=length(E);
          for k=1:n
              for x=1:2
                  if E(k,x)==1
                     E(k,x)=-1;
                  elseif E(k,x)==2
                     E(k,x)=0;
                  elseif E(k,x)==3
                     E(k,x)=+1;
                  end
                  
              end
          end
          Y= E+[i j]; 
          

where D is just the matrix that i am using and i,j is the initial point!!