# How to detect isolated points in a vector

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CSCh 2023-9-21

Hi,
I have a vector (see attachment), and I would like to detect isolated points (marked with red cicles in the image). Isolated in that sense, that same values have a certain distance to these. I would need the indices.
Thank you
Chris
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Dyuman Joshi 2023-9-21
I have seen the image. It does not do a good job of explaining what you mean by "isolated points".
Provide a mathematical explaination of the criteria to use to determine which points are isolated points.

### 采纳的回答

Harald 2023-9-21

Hi,
I believe this code does what you ask for:
N = 500;
isolated = false(size(iidx));
L = length(iidx);
for k = 1:L
left = max(1,k-N):k-1;
right = k+1:min(L,k+N);
isolated(k) = ~any(iidx([left, right]) == iidx(k));
end
result = find(isolated);
plot(iidx, "*")
hold on
plot(result, iidx(isolated), "ro")
hold off
I don't see an obvious way to avoid the loop. If there are concerns about performance, I can think about it a bit more - or perhaps, others have an idea.
Note that it does not identify two of the points that you have circled. If you zoom in, you will see that there are two consecutive identical values there. My understanding is that you did not want to extract these, even though they look isolated on the first glance? If you wanted these, I do not fully understand the question yet.
Best wishes,
Harald
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CSCh 2023-9-22
Hi Harald, thank you so much.

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