I want to find whether the selected elements of a matrix are adjacent or not?

For e,g

A=[0 1 1 0;
   0 1 0 0]

i have selected A(1,2), A(1,3), A(2,2) for use , i want to know that which pair is adjacent to other ?

[Image Analyst]

If you have

A=[0 1 1 1;
   0 1 0 0]

Do you consider the top right 1 adjacent to the bottom left 1? They are connected but not immediate (1 pixel away) neighbors.

[Amir]

i want to know through their indices i.e A(1,2) A(1,3) are adjacent while A(1,2) and A(2,2) are also adjacent , so if I have these matrix indices, i want to check which index position is adjacent to other ?

[Image Analyst]

That's not what I asked. I want to know if you consider A(2,2) "adjacent" to A(1,3) because there is a path of 1's connecting them. If so, you can simply use bwlabel.

Answer 1

[Image Analyst]

If you have two locations: row1, col1 and row2, col2, then they are immediately adjacent (1 layer away) if

if abs(row1-row2) <= 1 && abs(col1-col2) <= 1
  % Adjacent
else
  % Not adjacent
end

Answer 2

[Matt J]

How about computing an adjacency matrix.

    [i,j]=find(A);

    AdjacencyMatrix = xor( abs(bsxfun(@minus,i,i.'))==1 , abs(bsxfun(@minus,j,j.'))==1 )

Don't know if you want to allow diagonal adjacency. The above assumes not.

[Amir]

Sorry , i don't need that ! i need an algorithm to find whether particular indices of a matrix are adjacent? For eg. how to know that A(1,2) and A(1,3) are horizontally adjacent and A(1,2) A(2,2) are vertically adjacent ??

[Matt J]

Yep. The adjacency matrix carries that information. Each row/column represents a point. If an entry is 1, it means the pair of points represented by that row/column are adjacent.