# How can I implement a rolling list ?

4 views (last 30 days)
Blake on 30 Sep 2012
I am reading data over the network in real time and I want to limit a dataset to 1000 points. For example when the array has filled 1000 points I want it to start at 1 again to save memory. I also want to ensure I keep the order so that when i plot the information it will plot oldest to newest and not in the way it was stored.
Is there any such feature in MATLAB?

Walter Roberson on 30 Sep 2012
You could also consider
if length(data) == 1000
data(1) = [];
end
data(end+1) = theNewDataValue;

Image Analyst on 30 Sep 2012
Something like this perhaps:
% Assign new data to the next element.
data(nextIndexToUse) = theNewDataValue;
% Increment to the next element to use.
nextIndexToUse = nextIndexToUse + 1;
% Reset back to 1 if it exceeds 1000
if nextIndexToUse > 1000
nextIndexToUse = 1;
end

Malcolm Lidierth on 30 Sep 2012
Edited: Malcolm Lidierth on 30 Sep 2012
or
Edited for unity-based indexing as per @Blake's remark below:
x=zeros(1,1000)
circindex=@(k)mod(k-1,numel(x))+1
x(circindex(100001))=1

Blake on 30 Sep 2012
Edited: Blake on 30 Sep 2012
I like the circular index idea. I will note this for later but unfortunately it doesnt arrange the data in an easy to plot format. It is difficult to differentiate from new and old points just by looking at the list. Also there is a time at the end of the array where the mod(1000,1000) will try to access x(0). But it implements a rolling list perfectly other than that. Thank you.
Walter's code implements it exactly how I wanted it. Thanks
Malcolm Lidierth on 30 Sep 2012
@Blake
I forgot the unity-base in MATLAB. Code corrected above. To extract data with N samples:
x([circindex(N):-1:1 end:-1:circindex(N)+1])
Extra code for N<=0 not included.

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