Adding rows to matrix conditionally

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HOl 2020-3-31

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/514282-adding-rows-to-matrix-conditionally

评论： HOl 2020-4-1

采纳的回答： Guillaume

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I have long datafiles that I need to alter and cannot figure out how to do this.

The simplified version of what I'm trying to do is:

x =  1 20
     3 50
     6 10

Column 1 is time (t) (sec 1, 3, 6) and col 2 the values.

I want to create a new matrix with time axis with 1 sec interval and that the values (col 2) are repeated until next (t) ( in col1)

The resulting matrix should be:

Sorry about the elementery question, I'm a newby

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Star Strider 2020-3-31

HOl data.csv

I will defer to Guillaume’s Answer, since he posted the solution first. The data file is attached.

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Answer 1

Guillaume 2020-3-31

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We don't have enough details to give you a complete answer but what you want to do should be easily done by

reading your file as a timetable. Could be as easy as:

data = readtimetable(yourfile);

depending on the actual file

retime the timetable to 1 second interval, which is simply:

resampled = retime(data, 'secondly', 'previous');  %retime in one second interval using the previously known value for the missing times

save into a new file

writetimetable(resampled, 'newfile.csv');

for example

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Guillaume 2020-3-31

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Now that we have example data, it's possible to give more details. The gist of my answer still stand: import the data as timetable, then one-liner retime to resample the data with whichever time interval you want.

The difficulty with your file is that the actual time/date is split over several variables which is not ideal. In addition, the time column being in 24 hours format but still having a PM indicator for afternon but no AM indicator for morning really complicates things a bit.

opts = detectImportOptions('data.csv');  %have to use import options because of the wonky format of the file
opts = opts.setvaropts(3, 'Type', 'char');  %prevent decoding as duration which would fail because of the AM/PM
data = readtable('data.csv', opts);  %import as table
data.(3) = duration(strrep(data.(3), ' PM', ''));  %get rid of the useless PM, then convert to duration
data.DateTimeUnified = data.(2) + data.(3) + seconds(data.(4) / 1e6);  %convert microseconds to seconds and add the whole lot
data.DateTimeUnified.Format = 'dd MMM yyyy HH:mm:ss.SSSSSS';  %use whichever display format you prefer
data(end, :) = []; %get rid of the last row which is nonsense
data(:, 2:4) = [];  %optionally remove original date/time columns
tdata = table2timetable(data, 'RowTimes', 'DateTimeUnified');  %convert to timetable

and then as said, retime to get the data in interval of 30 seconds:

resampled = retime(tdata, 'regular', 'previous', 'TimeStep', seconds(30));

HOl 2020-4-1

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Thank you very much for the help. Clearly this would have been above my skills to figure out!

When I try, the first line passes ok but the second yields an error:

>> opts = detectImportOptions('data.csv');  %have to use import options because of the wonky format of the file
>> opts = opts.setvaropts(3, 'Type', 'char');
Error using matlab.io.internal.functions.SetVarOpts/setType (line 140)
The 'Type' property cannot be set with SETVAROPTS. Use the following instead:
  opts = setvartype(opts,...,type)
Error in matlab.io.internal.shared.VarOptsInputs/set.Type (line 64)
            obj.Type_ = setType(obj,val);
Error in matlab.io.internal.functions.ExecutableFunction/validateArguments (line 82)
            func.(name) = paramStruct.(name); % validate by object setter
Error in matlab.io.internal.functions.ExecutableFunction/validate (line 103)
        [func, supplied, additionalArgs, results] = func.validateArguments(varargin{:});
Error in matlab.io.internal.functions.SetVarOpts/validate (line 105)
        [func, supplied, additionalArgs] = validate@matlab.io.internal.functions.ExecutableFunction(func,varargin{:});
Error in matlab.io.internal.functions.SetVarOpts/validateAndExecute (line 73)
        [func, supplied] = validate(func,varargin{:});
Error in matlab.io.ImportOptions/setvaropts (line 402)
            opts = func.validateAndExecute(opts,varargin{:});

Guillaume 2020-4-1

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You can construct a new time vector starting at the time of your choosing and pass that to retime instead of letting it construct it:

newtimes = (tdata.DateTimeUnified(1):seconds(30):tdata.DateTimeUnified(end)+seconds(30))';
resampled = retime(tdata, newtimes, 'previous');

HOl 2020-4-1

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Excellent, that works. Thank you very much!!

For any other newby's out there with wonky data here is the final code:

>> opts = detectImportOptions('data.csv');  %have to use import options because of the wonky format of the file
>> opts = opts.setvartype(3, 'char');
>> data = readtable('data.csv', opts);  %import as table
>> data.(3) = duration(strrep(data.(3), ' PM', ''));  %get rid of the useless PM, then convert to duration
>> data.DateTimeUnified = data.(2) + data.(3) + seconds(data.(4) / 1e6);  %convert microseconds to seconds and add the whole lot
>> data.DateTimeUnified.Format = 'dd MMM yyyy HH:mm:ss.SSSSSS';
>> data(end, :) = []; %get rid of the last row which is nonsense
>> tdata = table2timetable(data, 'RowTimes', 'DateTimeUnified');  %convert to timetable
>> newtimes = (tdata.DateTimeUnified(1):seconds(30):tdata.DateTimeUnified(end)+seconds(30))';
>> resampled = retime(tdata, newtimes, 'previous');