rng default
x = randi(3,[20 1]);
thresh = 6;
while 1
xc = cumsum(x);
k = find(xc > thresh,1,'first');
if isempty(k)
% done
break
end
x(k) = -xc(k-1);
end
When you say set that row equal to zero, the cumsum on the next row will still be greater than 6. (something greater than 6 + 0 is still greater than 6). Thus you need to set the value to the negative of the previous value in order to reset the cumulative sum. If you need zeros at the end, remove the negatives.
% If you need to zero out those points
x(x<0) = 0
