MATLAB: How to find 4 or more consecutive zeros and replace these zero’s

countmatrixreplace consecutive

I have a large matrix and need to loop through it and find where there are instances of 4 or more 0's and replace these 0's with 2's. For example, in the below image I need to replace the 4 or more consecutive 0's (red) with 2's but the other 0 further down is fine. The image on the right is what I'm expecting.
I can count the number of times 0 appears but am stuck on how to alter these 0's. This is what I have so far:
for i = 1:length(x)
    y = x(:,end);
    if y(i) == 0
        count = count + 1;
    else if count >= 4
            lastIndex = i - 1;
            %change 0's in the block of 4 to 2's
            %this is the bit I'm stuck on            
            count = 0; %reset count
        end
        count = 0;
    end
end
Any help would be greatly appreciated.

Best Answer

Here is one way to do it, which does not involve looping over the whole vector (only over each run of zero)
transitions = diff([0; x == 0; 0]); %find where the array goes from non-zero to zero and vice versa
runstarts = find(transitions == 1);
runends = find(transitions == -1); %one past the end
runlengths = runends - runstarts;
%keep only those runs of length 4 or more:
runstarts(runlengths < 4) = [];
runends(runlengths < 4) = [];
%expand each run into a list indices:
indices = arrayfun(@(s, e) s:e-1, runstarts, runends, 'UniformOutput', false);
indices = [indices{:}];  %concatenate the list of indices into one vector
x(indices) = 2 %replace the indices with 2
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