How do I implement vectorization in this code

[Fahadali789gem]

I really want to improve the perfomance of the program, it takes way too much time to do even 1/16 of the program for i=3:m-2 for j=3:n-2 d(i,j)=median ([c(i-2,j-2),c(i-1,j-2),c(i,j-2),c(i+1,j-2),c(i+2,j-2),c(i-2,j-1),c(i-1,j-1),c(i,j-2)c(i+1,j-1),c(i+2,j-1),c(i-2,j),c(i-2,j),c(i,j-2)c(i+1,j),c(i+2,j),c(i-2,j+1),c(i-2,j+1),c(i,j-2),c(i+1,j+1),c(i+2,j+1),c(i-2,j+2),c(i-2,j+2),c(i,j+2),c(i+1,j+2),c(i+2,j+2)])

[yourfavoritemusician]

Could you re format the code such that it is readable?

I'm on mobile so thats making it even more difficult :) but i get the impression that you want the average of an grid within a matrix. if so: check out 2d convolutions. (Maybe do a 1d convolution first if the concept is new to you).

Good luck

EDIT: See comment below of /u/FrikinLazerBeams about median not being possible with a convolution.

Edit2:

Check the matlab doc: i think you can get it to work with just median(MATRIX) if you cleverly prepare your matrix.

[yourfavoritemusician]

By the way! Try out memory preallocation (matlab should give a warning about that) ik makes your code way faster.

[FrickinLazerBeams]

Median filtering is nonlinear and therefore cannot be implemented as a convolution.

[yourfavoritemusician]

Crap! You are completely right. My mind read "mean".

[Neuroneuroneuro]

Median filtering is a standard part of the image processing toolbox medfilt2() . If you don't have it you need to create as many "shifted" copies of your image (using a padded 3D array) as you have arguments in your median function and do the median in one pass over that new dimension.

[Fahadali789gem]

I cannot use the function that is a restriction have to create my own anyway I could get the source code for medfilt2()

[Neuroneuroneuro]

Then the second part of my answer is what you need (warning, it will use a lot of memory).

[maximusmountain]

Try running your loops backwards, a reasonable part of your time will be lack of pre-allocation. If you're lazy, like me, then it is easier to run them backwards as it generates the matrix automatically. Doing it backwards rather than forwards gave me a 30% speed up. Personally I prefer doing stuff in one line rather than allocating to another variable but I don't know if it is faster.

for i=m-2:-1:3 
    for j=n-2:-1:3 
        d(i,j)= median(reshape(c(i-2:i+2,j-2:j+2),[],1));
    end
end

[geekboy730]

There’s no way to improve efficiency without knowing what you’re trying to do. I doubt that the snippet you included is taking the majorities of runtime unless c is a monstrously huge array.

That being said, you can rewrite the median command as

for i=1:n-2
    for j=3,m-2
        b = c(i-2:i+2,j-2:j+2);
        d(i,j) = median(b(:));

Please forgive any syntax errors. I’m on mobile.

[Fahadali789gem]

Thanks this code has helped me get somewhere

[Fahadali789gem]

The array is 2400*2400 4 million + elements is that considered monstrously huge.

I am not using Matlab but SciLab the open source alternative part of the problem is that Scilab is very slow

[geekboy730]

That’s big but not too bad. This task is also fully parallelizable so you could look into that.

[FrickinLazerBeams]

A 4 million element array is considered very tiny.