how to do logical indexing for, for and if loops to create a new dataset every time the condition is met.

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arjun luther 2023-10-25

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https://ww2.mathworks.cn/matlabcentral/answers/2038586-how-to-do-logical-indexing-for-for-and-if-loops-to-create-a-new-dataset-every-time-the-condition-is

回答： SAI SRUJAN 2023-10-30

matlab2.mat

Hi, i am trying to learn the loops in matlab and stuck in a situtaion. I have a time series dataset and i would like to do logical indexing of the table based on variable called global solar irradiance. Every time the value is between 180 and 400, the loops should create a new table based on whenever the condition is met. This would mean i should end up with around 30 to 40 or even more subsets/ tables . I have no idea where to begin. I know i would require a for and if loops for this .

I tried using simple logical indexing like

% refmean= 728
lower_threshold= 180 %0.01 * refmean
upper_threshold = 400 %0.80* refmean
highligting_indices=find(T3_filtered_final.("Global Solar Irradiance")>=lower_threshold & T3_filtered_final.("Global Solar Irradiance")<=upper_threshold)
New_Table=T3_filtered_final(highligting_indices,:)

This new table will have all the time intervals where the value of global solar irradiance meets the condition, but i would like to make subsets that whenever the conditions are met , the program should output a new table where the first value is always something near to 180 and last value near to 400. The new tables could have 1 second time gap or maybe 10 minutes time gap.

How to proceed with this program

for i =1:height(T3_filtered_final)% loop through whole table
    if T3_filtered_final.("Global Solar Irradiance")(i)>=180 && T3_filtered_final.("Global Solar Irradiance")(i)<=400
    % how to proceed
    end
    
end

sample data is provided within.

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dpb 2023-10-26

编辑：dpb 2023-10-26

在 MATLAB Online 中打开

matlab2.mat

"... I know i would require a for and if loops for this ."

Not necessarily, no. MATrixLABoratory is named this for a reason and while looping is sometimes necessary, vectorized solutions are to be sought before resorting to brute force loops (although loops have become much less of a bottleneck with time and better optimizations than in the distant past where they got a bad reputation).

Let's see what's we got...I'm not positive I understand the intent but one other thing is the idea of building a zillion more tables as subsets is also generally not the proper approach; grouping variables and splitapply processing or rowfun or even retime with timetables can often do it all in a single or few lines of code...

load matlab2

whos

Name Size Bytes Class Attributes T3_filtered_final3 25000x22 4406135 table ans 1x36 72 char cmdout 1x33 66 char

tT=T3_filtered_final3; clear T3* ans cmd* % shorten table name for less typing

head(tT)

Datetime Latitude Longitude Altitude Aircraft North Velocity Aircraft East Velocity Aircraft Down Velocity Aircraft Roll Aircraft Pitch Heading Air Temperature Relative Humidity Reflected Shortwave Irradiance Emitted Longwave Irradiance Global Solar Irradiance Downwelling Longwave Irradiance Wind Component North Wind Component East wind down comp Surface Temperature Ground_speed Wind_speed _______________________ ________ _________ ________ _______________________ ______________________ ______________________ _____________ ______________ _______ _______________ _________________ ______________________________ ___________________________ _______________________ _______________________________ ____________________ ___________________ ______________ ___________________ ____________ __________ 2021-07-27T10:46:00.000 53.831 6.8643 122.7 66.7 -35.2 -0.2 -0.5 2.8 -33.5 18.4 85 33 411 600 354 3.9 5.9 0.1 18.6 75.418 7.0732 2021-07-27T10:46:00.010 53.831 6.8643 122.7 66.7 -35.2 -0.2 -0.5 2.8 -33.5 18.4 86 33 411 600 354 3.9 5.9 0 18.6 75.418 7.0725 2021-07-27T10:46:00.020 53.831 6.8643 122.7 66.7 -35.2 -0.1 -0.4 2.8 -33.5 18.4 85 33 411 600 354 3.9 5.9 0 18.6 75.418 7.0725 2021-07-27T10:46:00.030 53.831 6.8643 122.7 66.7 -35.2 -0.2 -0.4 2.8 -33.5 18.4 86 33 411 600 354 3.9 5.9 0.1 18.6 75.418 7.0732 2021-07-27T10:46:00.040 53.831 6.8642 122.7 66.7 -35.2 -0.2 -0.4 2.8 -33.5 18.4 85 33 411 599 354 3.9 5.9 0 18.6 75.418 7.0725 2021-07-27T10:46:00.050 53.831 6.8642 122.7 66.7 -35.2 -0.1 -0.4 2.8 -33.5 18.4 85 33 411 599 354 3.9 5.9 -0.1 18.6 75.418 7.0732 2021-07-27T10:46:00.060 53.831 6.8642 122.7 66.7 -35.2 -0.1 -0.4 2.8 -33.5 18.4 86 33 411 599 354 3.9 5.9 -0.1 18.6 75.418 7.0732 2021-07-27T10:46:00.070 53.831 6.8642 122.7 66.7 -35.2 -0.2 -0.4 2.8 -33.5 18.4 85 33 411 599 354 3.8 5.8 0 18.6 75.418 6.934

tT.Properties.VariableNames=strrep(tT.Properties.VariableNames,' ','_'); % get rid of blanks in variable names

plot(tT.Datetime,tT.Global_Solar_Irradiance), xlim([tT.Datetime(1) tT.Datetime(end)]) % see what time series looks like

xlabel('Date'), ylabel('Global Solar Irradiance')

Well, there's no data in the attached data set that meets your criteria, the minimum value is >450.

So, now what would you propose that you want to do???

dpb 2023-10-26

在 MATLAB Online 中打开

matlab2.mat

Other than the two limits, we still don't have a precise definition of the selection criterion with which to work -- given the new range of 500-650 with the above time trace, where would you choose from in the above noisy trace?

load matlab2

tT=T3_filtered_final3; clear T3* ans cmd* % shorten table name for less typing

tT.Properties.VariableNames=strrep(tT.Properties.VariableNames,' ','_'); % get rid of blanks in variable names

plot(tT.Datetime,tT.Global_Solar_Irradiance), xlim([tT.Datetime(1) tT.Datetime(end)]) % see what time series looks like

xlabel('Date'), ylabel('Global Solar Irradiance')

LO=500; HI=650;

yline(LO,'r-'), yline(HI,'r-')

Is the initial section from 10:46 until it drops below 500 in or out? What about the next double small peaks before drops below again? Or must there be an excursion out the other range, too, not just "within"?

Then, presuming it may be the excursions from LO to HI Is the interest just from the boundary crossings of the excursion between 10:47 and 10:48 or from the local minimum to the local maximum outside the boundaries? Similarly on the return from the peak...oh, so many options to choose...

dpb 2023-10-27

Why would 5-6 be included but not 6-7???

arjun luther 2023-10-27

5-6 has time gaps before and after, while annotating the graph i missed a point between 6-7, which would have started a little after 6. I opted for a little different approach for this whole scenario as i was not getting closer to the solution. I already had a program that would highlight me the dips. Usually between each dip there is a time gap of arbitrary length but more than 5 seconds. I made a logical indexing to find the indices of those time gaps and using these time gap indices, i created my new subsets ,i.e, using adjacent indices. So far the tables look fine to me with this aproach.

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Answer 1

SAI SRUJAN 2023-10-30

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2038586-how-to-do-logical-indexing-for-for-and-if-loops-to-create-a-new-dataset-every-time-the-condition-is#answer_1343421

在 MATLAB Online 中打开

Hi Arjun luther,

I understand that you have a time series dataset in MATLAB and you want to create tables based on a condition involving the "Global Solar Irradiance" variable. Specifically, you want to create new tables whenever the value of "Global Solar Irradiance" is between 180 and 400.

You can follow the given example to proceed further.

% Set the lower and upper thresholds
lower_threshold = 180;
upper_threshold = 400; 
% Initialize variables
subsetTables = {}; % Cell array to store subset tables
startIndex = 1; % Index to track the start of each subset - starting index
for i = 1:height(T3_filtered_final)
    if T3_filtered_final.("Global Solar Irradiance")(i) >= lower_threshold && T3_filtered_final.("Global Solar Irradiance")(i) <= upper_threshold
        % If the condition is met, update the start index
        if startIndex == 0
            startIndex = i;
        end
    else
        % If the condition is not met and a subset has started,   
        % create a new table and insert it.
        if startIndex ~= 0
            endIndex = i - 1;
            subsetTables{end+1} = T3_filtered_final(startIndex:endIndex, :);
            startIndex = 0; % Reset the start index
        end
    end
end
% Check if a subset is still in progress at the end of the loop
if startIndex ~= 0
    endIndex = height(T3_filtered_final);
    subsetTables{end+1} = T3_filtered_final(startIndex:endIndex, :);
end
% You will end up with a cell array of tables 
% which are created whenever conditon is met
% Access each subset table using subsetTables{index}

You can refer to the following documentation to understand more about "table" in MATLAB,

https://in.mathworks.com/help/matlab/ref/table.html

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