Find at least 5 consecutive values above a certain threshold in a vector?

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HYZ 2019-4-17

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https://ww2.mathworks.cn/matlabcentral/answers/456821-find-at-least-5-consecutive-values-above-a-certain-threshold-in-a-vector

评论： Ahmad Bayhaqi 2021-5-1

采纳的回答： Walter Roberson

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I have a vector

M = [3,3,5,7,8,8,9,7,4,3,2,1,7,6,5,2,2,2];

I want to find at least 5 consecutive values which are above the threshold, which is 4. So the vector will be

N = [5,7,8,8,9,7]

Could anyone help me with this? Thanks.

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Walter Roberson 2019-4-17

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https://ww2.mathworks.cn/matlabcentral/answers/456821-find-at-least-5-consecutive-values-above-a-certain-threshold-in-a-vector#answer_370975

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mask = M > 4;
starts = strfind([0 mask], [0 1 1 1 1 1]);
stops = strfind([mask 0], [1 1 1 1 1 0]);
N = M(starts(1) : stops(1))

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HYZ 2019-4-17

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How can I modify this code to get N = [5 7 8 8 9 7]? I guess the code you provided gave only the first and last value. 
>> mask = M > 4;
starts = strfind([0 mask], [0 1 1 1 1 1]);
stops = strfind([mask 0], [1 1 1 1 1 0]);
N = M(starts(1) : stops(1))
N =
     5     7

Walter Roberson 2019-4-17

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stops = strfind([mask 0], [1 1 1 1 1 0]) + 4;

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更多回答（2 个）

Akira Agata 2019-4-17

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https://ww2.mathworks.cn/matlabcentral/answers/456821-find-at-least-5-consecutive-values-above-a-certain-threshold-in-a-vector#answer_370977

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If there are 2 or more consective values above the threshold, the following is one possible solution.

(* The following needs Image Processing Toolbox)

M = [3,3,5,7,8,8,9,7,4,3,6,5,8,5,8,2,1,7,6,2,6,2];
idx = M > 4;
idx = bwareafilt(idx,[5,Inf]);
label = bwlabel(idx);
N = cell(max(label),1);
for kk = 1:max(label)
  N{kk} = M(label == kk);
end

Result:

>> N

N =

2×1 cell array

{1×6 double}

{1×5 double}

>> N{1}

ans =

5 7 8 8 9 7

>> N{2}

ans =

6 5 8 5 8

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Akira Agata 2021-4-8

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Hi @Ahmad Bayhaqi san,

OK. Then, you should first break down your 3-D problem into the 1-D problem which corresponds to the original answer.

The following is a small example and hope it help you!

% Sample data (3:lat, 3:lon, 50:days)
rng('default'); % for reproducability
a = rand(3,3,50);
b = rand(3,3,50)*0.4;
% Extract values which satisfies a > b for more than 5 consecutive days
idx = a > b;
[latGrid, lonGrid] = meshgrid(1:3,1:3);
tResult = table(latGrid(:),lonGrid(:),'VariableNames',{'lat','lon'});
tResult.extData = cell(height(tResult),1);
for kk1 = 1:height(tResult)
  % break down into 1-D problem and apply the original answer
  lat = tResult.lat(kk1);
  lon = tResult.lon(kk1);
  idx0 = bwareafilt(squeeze(idx(lat,lon,:)),[5,Inf]);
  a0 = squeeze(a(lat,lon,:));
  label = bwlabel(idx0);
  N = cell(max(label),1);
  for kk2 = 1:max(label)
    N{kk2} = a0(label == kk2);
  end
  tResult.extData{kk1} = N;
end

The result will be as follows:

>> tResult

tResult =

9×3 table

lat lon extData

___ ___ __________

1 1 {4×1 cell}

1 2 {2×1 cell}

1 3 {5×1 cell}

2 1 {4×1 cell}

2 2 {3×1 cell}

2 3 {4×1 cell}

3 1 {4×1 cell}

3 2 {4×1 cell}

3 3 {4×1 cell}

Akira Agata 2021-4-9

Hi @Ahmad Bayhaqi san,

OK. I believe the error was due to the difference between the 3D matrix in my sample code and the real data.

Is it possible to upload part of your real data and the modified code which generates the error?

Ahmad Bayhaqi 2021-5-1

Hi @Akira Agata

Thank you for your code. It works for me.

However, since my temperature data contains date. How the result of looping includes the datetime to define when the temperature is higher than threshold?

Thank you very much

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