Check values under a certain threshold with an undefined window

Question

bfenst 2018-4-13

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回答： Image Analyst 2018-4-13

Hello all,

I am new to Matlab. I have a large vector called A (over 400000 values). I would like to find the maximum value of that vector then look for values that are 10 % around the maximum.

Here is an example:

A=[9 10 47 50 49 48 2 46];
mymax=max(A); % in this case mymax=50
mincondition=mymax*0.9; % mincondition=45

Then I should be able to get all values around A(4)=50 that are over mincondtion. The window of considered values stops when mincondition is no longer surpassed. In this case, the results would be:

res=[47 50 49 48];

Can anyone please help me?

Thank you.

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David Fletcher 2018-4-13

Why isn't the 46 (last element) included in res? (sorry I didn't really understand what you mean by 'the window of considered values stops when mincondition is no longer surpassed' - I am just assuming that you want everything from A that is over mincondition)

bfenst 2018-4-13

Thank you for checking my question. I want everything from A that is over mincondition AND that is consecutive to the max or the values that are over mincondition.

That is why 46 is not included in the res.

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Answer 1

David Fletcher 2018-4-13

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A=[9 10 47 50 49 48 2 46];
mymax=max(A); % in this case mymax=50
mincondition=mymax*0.9; % mincondition=45
indexer=A>mincondition
[start,stop]=regexp(char(indexer+48),'1+')
res=A(indexer(1:stop(1)))

It's probably worth checking it works thoroughly - I kept getting disturbed by people actually wanting me to do some work while I was writing it.

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bfenst 2018-4-13

Thank you very much, it is exactly what I needed. Impressive how a few lines of code do the deed, I was at 30 lines trying to achieve with no results.

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Answer 2

Image Analyst 2018-4-13

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https://ww2.mathworks.cn/matlabcentral/answers/394686-check-values-under-a-certain-threshold-with-an-undefined-window#answer_314964

在 MATLAB Online 中打开

Here's one way using morphological reconstruction:

A = [9 10 47 50 49 48 2 46];
mask = A >= 0.9 * max(A)
indexes = imreconstruct(A == max(A), mask) % Requires Image Processing Toolbox.
finalValues = A(indexes)