For loop Averaging Values Sequentially

Hi,

I'm working on analyzing some data that requires me to find the average of two numbers listed sequentially in a column in an 4000x9 array, where the 9th column needs populating via a script. Every 200 rows the data repeat the values in the 1st column. I want to average the values of a row in the first column and the value beneath it (row+1) and put it in the 9th column automatically, and then every time the values start to repeat, to just halve the value of that row. This is what I wrote:

kbo_array(:,9) = zeros;
kbo_array(1,9) = kbo_array(1,1)/2;
for i = kbo_array(2,1):kbo_array(4000,1),
    if kbo_array(i,1) > kbo_array(i-1,1)
        kbo_array(i,9) = (kbo_array(i,1) + kbo_array(i+1,1))/2;
    else
        kbo_array(i,9) = i/2;
    end
end

But, I get the error: "Index in position 1 is invalid. Array indices must be positive integers or logical values." I've tried some variants on the inputs but none work. I know how to do this in Excel, but I should learn how to do this properly.

I've searched this site to figure out for/while loops and IF statements, but I'm a MatLab newbie and I don't fully understand the terminology and code in the answers, or why the solutions work. Can someone please explain what I'm doing wrong and offer a suggestion on how to fix it? Many thanks in advance!