Index exceeds the number of array elements - Error message

So I am getting an error where I believe n1 is larger than the array after n becomes n = 3, which is giving the error in ma. Premise of the code is to take 1 2, 3 4 5, 6 7 8 9, etc. Pretty sure I have to change the length but not sure how to go about it. What should I do? Here is the code:

clear all

mdata = [1 2 3 4 5 6 7 8 9];
n1 = 1;
k = 1;

for n = 1:length(mdata)
    ma(n) = (1/(k + 1)) * sum(mdata(n1:n1 + k));
    n1 = ((n1 + k) + 1);
    k = k + 1;
    std_ma = std(mdata);
end

Error message:

Index exceeds the number of array elements (9).

Error in untitled22 (line 11)

ma(n) = (1/(k + 1)) * sum(mdata(n1:n1 + k));

Answer 1

[Voss]

Here's one way to determine the number of iterations required:

clear all

mdata = [1 2 3 4 5 6 7 8 9];
n1 = 1;
k = 1;

N = numel(mdata);
n_iter = find(cumsum(2:N) <= N,1,'last')

n_iter = 3

for n = 1:n_iter
    ma(n) = (1/(k + 1)) * sum(mdata(n1:n1 + k));
    n1 = ((n1 + k) + 1);
    k = k + 1;
    std_ma = std(mdata);
end

disp(ma)

    1.5000    4.0000    7.5000

[zen]

Thank you so much, it worked!

[Voss]

You're welcome!