Retime Yearly Maximum - Finding Corresponding Dates from Timetable

I'm using the retime function to find yearly maximum values for a timetable (e.g., x=timetable(Datetime, Var); YearlyMax = retime(x,'yearly','max')).

I need an efficient means of finding the corresponding date/time from the timetable for each maximum value rather than the 01-Jan-YYYY resulting timetable. Any ideas on the best way of finding the dates for each yearly maximum value with the timetable?

Answer 1

[Star Strider]

Try something like this —

Date = datetime(2020,01,01)+caldays(0:3.9*365.25).';
Temp = 80*rand(size(Date))-40;
Humd = randi([1 100],size(Date));
Wx = timetable(Date,Temp,Humd)                              % Original 'timetable'

Wx = 1425×2 timetable
       Date         Temp      Humd
    ___________    _______    ____

    01-Jan-2020     22.507    100 
    02-Jan-2020    -5.5977     28 
    03-Jan-2020     29.767     24 
    04-Jan-2020    -30.072     81 
    05-Jan-2020     22.233     36 
    06-Jan-2020     2.1534     53 
    07-Jan-2020    -6.4102     32 
    08-Jan-2020     -20.92     17 
    09-Jan-2020    -11.942     45 
    10-Jan-2020     20.802     51 
    11-Jan-2020     31.433     81 
    12-Jan-2020    -16.857     81 
    13-Jan-2020    -12.293    100 
    14-Jan-2020    -8.2595     32 
    15-Jan-2020    -27.176     93 
    16-Jan-2020     12.763     34 

WxYr = retime(Wx, 'yearly','max')                           % Yearly Maxima

WxYr = 4×2 timetable
       Date         Temp     Humd
    ___________    ______    ____

    01-Jan-2020    39.961    100 
    01-Jan-2021    39.978    100 
    01-Jan-2022    39.891    100 
    01-Jan-2023    39.987    100 

Yrs = year(WxYr.Date);
for k = 1:numel(Yrs)
    Lv1 = year(Wx.Date) == Yrs(k);                          % Logical Vector Selecting Days For Selected Year
    WxCurYr = Wx(Lv1,:);                                    % Current Year 'timetable'
    Lv2 = ismember(WxCurYr.Temp, WxYr(k,:).Temp);           % Select Variable To Compare (Here: Temperature) & Return Logical Vector Of Matches
    ResultTT{k,:} = WxCurYr(Lv2,:);                         % Save 'timetable' Arrays
    Result(k,:) = timetable2table(WxCurYr(Lv2,:));          % Save Rows In The 'Result' Table
end

Result                                                      % Table Of Days Matching Yearly Maximum Temperature

Result = 4×3 table
       Date         Temp     Humd
    ___________    ______    ____

    15-Nov-2020    39.961     65 
    12-Oct-2021    39.978     85 
    08-Jan-2022    39.891     85 
    06-May-2023    39.987     34 

If there is more than one matching value, this becomes slightly more complicated. In that instance, it will be necessary to either return all of the matching days (this usually requires that ‘result’ becomes a cell array, causing further complications later in the code), or use find with ‘Lv2’ and use an element of that vector to return only one of them (the approach I have characteristically taken in these situations, simply because it’s easier).

.

Answer 2

[Eric Sofen]

This example gives another way to get the time when the max occurred. See the findMax function and how it's used in rowfun. The difference is you're grouping by year, rather than by site.