How to calculate the quantiles/percentiles values for each hour?

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Kofial 2021-1-21

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评论： dpb 2022-8-18

I calculate with this code the quantile points of all the data.

Data=[1.5 2.3 2.5 4 6.2 7.1 8];
First_data = quantile(Data, [0 0.25 0.50 0.75 1])';
QuantilePoints = {'Minimum', '25th percentile', '50th percentile', '75th percentile', 'Maximum'}';
T = table(QuantilePoints, First_data);
disp(T)

How can I calculate the quantile points for every hour? The hour is in the datetime format like below.

Time_datenum={'20-Apr-2020 11:06:00','20-Apr-2020 11:20:10','20-Apr-2020 11:45:30','20-Apr-2020 12:07:00','20-Apr-2020 12:35:40','20-Apr-2020 12:40:50','20-Apr-2020 13:07:00'};
Time_datetime = datetime(Time_One,'InputFormat','dd-MM-yyyy HH:mm:ss');

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Answer 1

dpb 2021-1-21

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编辑：dpb 2021-1-21

Use grouping variables and splitapply or put into a table or timetable and then rowfun and/or retime (timetable) are also options.

Earlier today I showed an example on another variable, but principles still apply <Splitting-data-array-into-sub-arrays#>

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dpb 2022-8-18

编辑：dpb 2022-8-18

在 MATLAB Online 中打开

Let's see your code and the returned error -- I'm not buying it being it impossible (yet, anyways... :) )

You will need to use an anonymous function as the functional input to retime that supplies the wanted percentile(s), yes, you can't just use @retime by itself, no. (I had thought that obvious so hadn't mentioned it explicitly...I suppose owing to too much familiarity myself and presuming too much for newcomers.)

ADDENDUM:

OK, I just tried it with one of the example datasets for timetable(after I made the initial time span a couple days longer) and

>> ttP=retime(TT2,'daily',@(x)prctile(x,5))
ttP =
  3×3 timetable
    MeasurementTime     Temp     Pressure    WindSpeed
    _______________    ______    ________    _________
    18-Dec-2015        37.523     29.997      6.5199  
    19-Dec-2015        40.037     29.936      6.7343  
    20-Dec-2015        41.541     29.898      7.1103  
>> ttP=retime(TT2,'daily',@(x)prctile(x,[5 95]))
ttP =
  3×3 timetable
    MeasurementTime          Temp              Pressure           WindSpeed    
    _______________    ________________    ________________    ________________
    18-Dec-2015        37.523    39.909    29.997    30.091    6.5199    12.544
    19-Dec-2015        40.037     41.42    29.936    29.992    6.7343    7.0799
    20-Dec-2015        41.541    42.348    29.898    29.931    7.1103    7.3119
>> 

worked just fine -- I converted the hourly data to find percentiles by day so there would be a consolidation operation since the base timetable had been retimed to the hourly basis to begin with. For a timetable by finer resolution than an hour, then 'hourly' will work same way for it.

And, it handles both the one- or multiple- percentiles automagically as well; just creates an array for each variable of as many elements as requested.

Maria Jacob 2022-8-18

编辑：Maria Jacob 2022-8-18

Thanks for checking. I literally started using timetables just a few weeks ago, and due to other commitments I haven't been able to dedicate it a lot of hours.

So yes, what I was saying is doing retime(data,'monthly',@prctile) is not allowed, because I need to specified what % I want. Sorry for not being more clear.

With respect to creating the custom function, what I tried is using rowfun, in the following way (which is a copy of an example in the Matlab documentation):

5prc = @(x) prctile(x,5);

B = rowfun(5prc,data,'GroupingVariable','month(data.Time)','OutputVariableName','5th prc');

And I get a parsing error at ) for the definition of 5prc, it literally says: Parse error at ')': usage might be invalid MATLAB syntax.

Regardless, I will try your way, since you have shown it works. I wish I had more time to tried myself before asking, sorry...

EDIT:

OMG, I'm an IDIOT!! It's the 5...