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

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.

Answer 1

[Walter Roberson]

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))

[HYZ]

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]

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

Answer 2

[Akira Agata]

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

>>

[Ahmad Bayhaqi]

Hi @Akira Agata , I am just wondering, how to implement your code in the 3d-matrix?

I have data with 3d matrix (x,y, and time) then, the threshold has also the same dimension.

Each points has different threshold. I also want to take the values above threshold with 5 consecutive days.

I have tried the bwareafilt, but it doesnt worl,

do you have idea?

Thank you

[Akira Agata]

Hi @Ahmad Bayhaqi san,

Could you provide a small example? Unfortunately, it's not clear for me what your "3d-matrix" looks like. Is that a matrix with 3 dimentions (i.e L-by-M-by-N matrix)? Or N-by-3 matrix (column1~3 represents Time, X and Y, respectively) ?

[Ahmad Bayhaqi]

Hi Mr @Akira Agata,

Thank you for your reply,

I have temperature data with a=76x80,60266 and b=76x80x60266. ; 76=lon, 78=lat and 60266 is time

So, Each point has different thershold. I want to take values 'a' that are above 'b' threshold with the 5 consecutive days. However, I want to do it for my all regions.

Then, do yu have idea about this? I always get stuck for this.

Your help is greatly appreciated.

Thank you

[Akira Agata]

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}

[Ahmad Bayhaqi]

Hi Mr @Akira Agata , thank you so much for you help.

but I still get an error like this

"Index in position 1 is invalid. Array indices must be positive integers or logical values.

Error in Untitled2 (line 33)

idx0 = bwareafilt(squeeze(idx(lat,lon,:)),[5,Inf]);"

do you know why?

Thank you

[Akira Agata]

Hi @Ahmad Bayhaqi san,

Let me try to clarify the situation. Did you face this error when running exactly the same sample code I have posted yesterday? Or after modifying the code?

[Ahmad Bayhaqi]

Hi Mr @Akira Agata

I just changed the sample code with my data.

Everything is OK until it reaches the 'idx0'.

I used the real coordinate in meshgrid for lat and lon with the dimension lat=80x1 double and lon=76xdouble.

But then, when I changed the meshgrid using lat=1:80 and and lon 1:76, It works for the entire loop and I get the result.

What I have to do to get the result with the real corrdinate value?

Big thanks.

I am so sorry for the inconvenience

[Akira Agata]

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]

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

Answer 3

[KSSV]

N(N>4)

[HYZ]

N is the vector I want after thresholding and the values must be consecutive for at least 5 times.