MATLAB: How to replace zeros with other value in a big matrix fast

matrix manipulationprogramming

Hi all, I have a programming efficiency question. I would like to replace zero value of a big matrix with mean value of it. I know how to do it.However, it takes forever. I do not know if I did it wrong or it actually takes so much time. I am wondering if there is a better way to do it for a big matrix as big as 3320*3320. Here is my code. Assume A is the 3320*3320 matrix. I wrote only
A(A==0)=mean(mean(A));
Please let me know if I did it wrong. If I did it right, can somebody suggest any faster way to do the same thing ? Thank you very much,

Best Answer

Don't double call these functions (MAX,MIN,SUM,MEAN,etc.). Learn to use the colon, it is your friend!
A(~A(:))=mean(A(:));
This will be somewhat faster. If you need even more speed, replace the call to mean with its definition.
A(~A(:)) = sum(A(:))/numel(A);
The thing is, with a very large array, it might just take some time!
Here is how I timed these, BTW. You can play around with other options as well! Run the function 3 times (the first time warms it up after a save). The time will print to the command window.
function [] = write_ones()
T1 = 0;  % Time two different approches
T2 = 0;
for ii = 1:5
    A = round(rand(3320))>.75;
      tic
      A(A==0)=mean(mean(A));
      T1 = T1 + toc;
      clear A
      A = round(rand(3320))>.75;  % A new A.
      tic
      A(~A(:)) = sum(A(:))/numel(A);
      T2 = T2 + toc;
  end
[T1 T2]
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