# easy indexing lon-loop versus loop

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stephen williams on 29 Mar 2021
Hello, I have what I think is an easy indexing question, that has me scratching my head. I set up a simple moving average as such:
% the MA depth
Ml=16;
% the input data
x=[ zeros(1,Ml) 1:10 9:-1:7 8:10 9:-1:7 8:10 9:-1:7 8:10 9:-1:7]; % sequence to average out
% storage for output
y= zeros(1,length(x)+Ml);
%index
n= [Ml+1:length(x)];
%moving average
y(n)= y(n-1) + x(n)/Ml - x(n-Ml)/Ml;
which gives the wrong value for y(n). If I implement as a loop, it works fine.
% storage for output
yl= zeros(1,length(x)+Ml);
for n= Ml+1:length(x)
yl(n)= yl(n-1) + x(n)/Ml - x(n-Ml)/Ml
end
what is wrong with the non-loop version?
stephen williams on 29 Mar 2021
This is a general indexing question. I thought if you preallocated the y(n) locations, I could calculate y(n) based on y(n-1) but that is not working right.
I could have made up some other difference equation.

Bob Thompson on 29 Mar 2021
I think the issue is with your loop definition. You define y1 to be length(x)+M1 long, but n is defined from M1+1 to length(x). I assume you were intending to do 1+M1 to length(x)+M1, but you will need to add the M1 to both sides of the colon.
% storage for output
yl= zeros(1,length(x)+Ml);
for n= Ml+1:length(x)
yl(n)= yl(n-1) + x(n)/Ml - x(n-Ml)/Ml
end
stephen williams on 1 Apr 2021
thanks Bob.
Yes this issue is beucase I am trying to do the calculations all at once, and that is the question. I have alternative ways to solve. I am trying to understand why this way does not work.
--Steve

R2021a

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