Finding the ratio of numbers in an array

I access my array as:

data{1, i}(:,2)

So it is in the second column of data{1,i}

I have data in columns 3 and 4 already. I would like to make a column 5. I want column 5 to be the value in column 2 divided by the value in column 2 but 30 cells up. This means the first 30 values cannot be defined since we cannot compare to 30 cells prior. I keep trying this but I either get an array of zeros, or a single value. Can anyone help?

For instance, this does not work:

    for j =31:1:1000
       rois=data{1, i}(j,2)/data{1, i}(j-30,2) 
    end

[Prasobhkumar P. P.]

Why step size is 100?shouldnt be 1?

[Benjamin Cowen]

Yeah, it should be 1, sorry about that. I tried 1 at first and it got hung, so i assumed i was doing something wrong

Answer 1

[Rik]

rois=data{1, i}(31:end,2)./data{1, i}(1:(end-30),2);

In your current loop you are overwriting your result every iteration, you should have used indexing instead, skip the loop entirely.