MATLAB: Can’t the variable “sumt” in the for loop keep on adding numbers

MATLABvariable

function out = blur(m,w)
[r,c] = size(m);
out = ones(r,c)*255;
for kk=  1:c
    for ll = 1:r
        out(kk,ll) = finddims(m,kk,ll,w,r,c);
    end
end
        
end
function ave = finddims(mat,x,y,w,row,col)
sumt = 0;
cou =0;
i1= x-w;
i2 = x+w;
j1 = y-w;
j2 = y+w;
if i1<1
    i1 =1;
elseif i2>col
    i2 = col;
end
if j1<1
    j1 = 1;
elseif j2>row
    j2 = row;
end
for mm= i1:i2
    for nn = j1:j2
        sumt = sumt + mat(nn,mm); % sum doesn't keep on adding in the loop
        cou = cou+1;
    end
end
ave = uint8(sumt/cou);
end

Best Answer

"Why can't the variable "sumt" in my for loop keep on adding numbers?"
Because of the class of mat.
Most likely mat has class uint8 (if you are dealing with images), in which case you need to pay attention to what happens when you add a scalar double and a scalar integer:
scalar integer + scalar double => scalar integer
So although sumt starts of being double (i.e. 0), the first time that you do that sum the output is integer, which means sumt is integer, which means that it is limited to 255 (or the maximum for whatever integer class your data has).
The solution is simple: use double to convert the integer before adding:
sumt = sumt + double(mat(nn,mm));
%             ^^^^^^^          ^ you need this!
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