Surrounding elements in an array affect and change an element over a loop.

Okay for some context. Im a uni student, im getting a kick in my rear side. I don't really fully understand what i'm doing in matlabs, so a through explanation if possible would be greatly appreciated.

My task is: The living system is represented as a 2D array of any size, predators, prey and bare ground have the following encoding • Predator → 2 • Prey → 1 • Bare Ground → 0. I need to consider the surrounding elements around any element and if it meets a condition say a predator is in range of a prey, the matrix changes value which is called a generation. This loop occurs for however many generations that the user inputs. If im on the edge of an array, it will loop round and use those elements as the surrounding ones so say a 6x6 matrix where im on Prey(1,6), in the x direction the element surrounding the prey to the right is (1,1). My question is how do I even start a loop which checks every element and sees what surrounds it, I have no idea how to approach this, info im finding online explains the concept on a more basic level of say how loops work, but its not helping.

ngen=7
%G=0
%neighbour=0

%M=[1 2 0;]
M = [0 ,0 ,1 ,0 ,1 ,0 ,1 ,1 ,0 ,0;1 ,1 ,1 ,0 ,0 ,1 ,0 ,0 ,0 ,0;0 ,0 ,0 ,0 ,2 ,0 ,0 ,0 ,0 ,1;1 ,0 ,1 ,0 ,0 ,0 ,1 ,0 ,0 ,0;0 ,1 ,0 ,1 ,1 ,0 ,0 ,0 ,0 ,1;1 ,0 ,0 ,0 ,0 ,2 ,0 ,1 ,0 ,1;1 ,0 ,1 ,1 ,1 ,1 ,0 ,0 ,1 ,1;0 ,1 ,0 ,1 ,1 ,1 ,2, 1 ,1 ,0;1 ,0 ,1 ,1 ,0 ,1 ,1 ,1 ,1 ,0;0 ,0 ,1 ,1 ,1 ,0 ,0 ,1 ,1 ,0];

[nRow,nCol] = size(M);
if nRow == nCol
    
else 
    disp("Invalid square matrice")
    M=zeros(1,length(M));
    return;
end

for i=1:length(M)
    if M(i)~=0 && M(i)~=1 && M(i)~=2
        disp('ERROR: wrong inputs')
        M=zeros(1,length(M));
        
        return;
    end
end


for i=length(M)
    if M(i) < 0
        M=zeros(1,length(M));
        return;
    end
end


if ngen <=0
    disp("wrong input of generations")
    M=zeros(1,length(M));
    return;
end

%M=G;
neighbour=zeros(1,3);

%for g=1:ngen

%end

here is my code, the top parts are just testing for some parameter to be met, otherwise I get M=0, but im unsure where to go from here.

[KSSV]

Read about knnsearch.

[Dominik Just]

Thank you!

Answer 1

[Drishan Poovaya]

Hi,

Essentially you want to check all elements in the neighbourhooud of a particualr element but you want the search to wrap around as well. You can do this by some careful application of for loops and using the mod function

% M is a 10x10 array, and for each element we will search a 3x3 grid around
% it
% For example, in the grid below, if the element to be searched is marked
% as x, all elements marked as y will be considered as it's neighbours

%     y     y     1     0     1     0     1     y     y     x
%     y     y     1     0     0     1     0     y     y     y
%     0     0     0     0     2     0     0     y     y     y
%     1     0     1     0     0     0     1     0     0     0
%     0     1     0     1     1     0     0     0     0     1
%     1     0     0     0     0     2     0     1     0     1
%     1     0     1     1     1     1     0     0     1     1
%     0     1     0     1     1     1     2     1     1     0
%     y     y     1     1     0     1     1     y     y     y
%     y     y     1     1     1     0     0     y     y     y

for g=1:ngen
    for i = 1:nRow
        for j = 1:nRow
            %loop over each element
            %element is M(i,j)
            for ii = i-1:i+1
                for jj = j-1:j+1
                    % Checks neighbours accounting for wrap around
                    
                    idxI = mod(ii,nRow);
                    idxJ = mod(jj,nRow);
                    if idxI == 0
                        idxI = nRow;
                    end
                    if idxJ == 0
                        idxJ = nRow;
                    end
                    %Do something with M(idxI,idxJ)
                end
            end
        end
    end
end