Labeling objects in an image using nearest neighbors

I am trying to label all the objects in an image to analyze them and find their centroid. I am trying to do this without using the built in functions in matlab. I am trying to use the four nearest neighbors approach. Here is my code so far:

clc; clear all; close all;

SS = imread('star_sky.png');

SS2 = 0.2989 *SS(:,:,1)+0.5870*SS(:,:,2)+0.1140*SS(:,:,3);

threshold = 100;
SS3 = SS2 >= threshold;

[rows, cols] = size(SS3);

for M = 2 : R-1
    for N = 2 : C-1
        for k = 2:12
            if SS4{M,N}=1
                if (SS4(M-1, N)=1 && SS4(M, N-1)=0)
                   SS4(M,N)==SS4(M-1, N);
                end
                if (SS4(M-1, N)==0 && SS4(M, N-1)==1)
                   SS4(M,N)==SS4(M,N-1);
                end
                if (SS4(M-1,N)==1 && SS4(M, N-1)==1)
                   SS4(M,N)==SS4(M-1,N);
                end
                if (SS4(M-1, N)==0 && SS4(M, N-1)==0)
                   SS4(M,N) == k;
                   k;
                end
            end
        end
    end
end

Could someone help with why my code is not labeling the objects? Thanks!