import same variable without overwrite

I try to import one single point from 20 different mat file and save those point into a mat file without overwrite it. However my code keep overwritting it and only display the last value. Here is my code

for k = 1:20
  myfilename = sprintf('Image%d.mat', k);
  a = load(myfilename);
  newimg = (a.img -273);
  % The point of plants
  i = 100;
  j = 84;
  % Take the average of itself and all four of its neighboring 
  % values average each point in array  
  newimg(i,j) = 0.2*(newimg(i+1,j) + newimg(i-1,j) + newimg(i,j+1) + newimg(i,j-1) + newimg(i,j))
  val = newimg(i,j)
  newa = length(k);
  save('newa.mat','-append','val');

end

[Stephen23]

Rather than save your variable to the .mat-file on each iteration, simply construct an array with those values, and then save the array once after the loop, something like this:

N = 20;
mat = NaN(1,N); % preallocate
for k = 1:N;
    ...
    mat(k) = val;
end
save(..., 'mat')

Answer 1

[Azzi Abdelmalek]

You are using the same variable name. If you want to append your new data, you have to store them in a new variable name. But its better if you create a cell array then each update of your data, the size of your cell array will increase

For example

for k=1:3
  a{k}=randi(4,4);
  save('file','a','-append')
end

[Stephen23]

There is no need to use a cell array, a simple numeric array will work perfectly, and is simpler. Some preallcoation would be nice though!

[Azzi Abdelmalek]

That depends on the size of the new data

[Ki Lung Chan]

thank you! I got this