Table won't sort?

Hello all,

I have a table that is the product of this function: Day =varfun(@sum,Table,'InputVariables','Rain_mm','GroupingVariables',{'Month' 'Day' 'Year'});

The output is a table where the date is split into three columns (Day, Month, Year) and one column is the summed rain per day (sum_Rain_mm)

The problem I am having is that the output table 'Day' looks like this:

                  Day    Month    Year    GroupCount    sum_Rain_mm
                 ___    _____    ____    __________    ___________

    1_1_2016     1       1       2016    48            NaN        
    1_7_2015     1       7       2015    48              0        
    1_8_2015     1       8       2015    48              0        
    1_9_2015     1       9       2015    48            0.2        
    1_10_2015    1      10       2015    48            NaN        
    1_11_2015    1      11       2015    48            NaN        
    1_12_2015    1      12       2015    48            NaN        
    2_1_2016     2       1       2016    48            NaN        
    2_7_2015     2       7       2015    48              0        
    2_8_2015     2       8       2015    48              0        
    2_9_2015     2       9       2015    48              0        
    2_10_2015    2      10       2015    48            NaN        
    2_11_2015    2      11       2015    48            NaN        
    2_12_2015    2      12       2015    48            NaN        
    3_1_2016     3       1       2016    48            NaN        
    3_7_2015     3       7       2015    48              0

So the datestamps and totaled rain values are not in chronological order. When I try to make a plot of the rain data then against the time span (in days), the values are all out of order.

I cannot seem to get this output table to sort such that the data is chronological. I have tried:

 Day = sortrows(Day, {'Day' ,'Month', 'Year'})

But it doesn't work. Any suggestions?

Thanks

[Stephen23]

The simplest solution is to use ISO 8601 date format timestamps, because all ISO 8601 dates sort into chronological order without any extra work, just a standard sort:

yyyy-mm-dd

No splitting, converting to numeric, sorting of columns, or any mucking around with date functions is required to get the correct chronological order. Simply pick the only reliable date format in the world and your problem would be resolved:

https://xkcd.com/1179/

http://www.mathworks.com/matlabcentral/fileexchange/34095-date-vector-number-to-iso-8601-date-string

Answer 1

[Guillaume]

1) The columns of your table appear to be mislabeled since the Year appear to be months (or is it days), the Month column appears to be days and the Year column appears to be everything. That's down to the way you created the original table in the first place.

2) The name of the table is not very good. DailyRain may be better

3) What is the type of the Year column? To find out see the output of

summary(Day)

Is it a cell array of strings, or a datetime array, or something else?

[Rachel McLaughlin]

Sorry, the alignment of the table headings was off when I copied it over. The Day/Month/Year headings do match up to the correct columns. The Year is a 213 x 1 double, as are the Day and Month.

[Guillaume]

Well then, it should be

Day = sortrows(Day, {'Year' ,'Month', 'Day'})

The order of the variable matters for sortrows.

Answer 2

[Star Strider]

This is a bit less efficient than I’d like, but it has the virtue of working. I’m including the table creation as well so others won’t have to re-create that part of the code:

T = {'Day'    'Month'    'Year'    'Group' 'Count'    'sum_Rain_mm'
    '1_1_2016'     1       1       2016    48            NaN        
    '1_7_2015'     1       7       2015    48              0      
    '1_8_2015'     1       8       2015    48              0        
    '1_9_2015'     1       9       2015    48            0.2        
    '1_10_2015'    1      10       2015    48            NaN        
    '1_11_2015'    1      11       2015    48            NaN        
    '1_12_2015'    1      12       2015    48            NaN        
    '2_1_2016'     2       1       2016    48            NaN        
    '2_7_2015'     2       7       2015    48              0        
    '2_8_2015'     2       8       2015    48              0        
    '2_9_2015'     2       9       2015    48              0        
    '2_10_2015'    2      10       2015    48            NaN        
    '2_11_2015'    2      11       2015    48            NaN        
    '2_12_2015'    2      12       2015    48            NaN        
    '3_1_2016'     3       1       2016    48            NaN        
    '3_7_2015'     3       7       2015    48              0};

Tbl1 = cell2table(T(2:end,:));
Tbl1.Properties.VariableNames = T(1,:);

Tbl_Dates = Tbl1.Day;
new_dates = regexp(Tbl_Dates, '\d*','match');
Dmtxc = cellfun(@(x)sprintf('%02s%02s%4s',x{:}), new_dates, 'Uni',0);
Tbl_DN = datenum(Dmtxc, 'ddmmyyyy');
[~,idx] = sort(Tbl_DN);
Tbl2 = Tbl1(idx,:)

Tbl2 = 

          Day        Month    Year    Group    Count    sum_Rain_mm
      ___________    _____    ____    _____    _____    ___________

      '1_7_2015'     1         7      2015     48         0        
      '2_7_2015'     2         7      2015     48         0        
      '3_7_2015'     3         7      2015     48         0        
      '1_8_2015'     1         8      2015     48         0        
      '2_8_2015'     2         8      2015     48         0        
      '1_9_2015'     1         9      2015     48       0.2        
      '2_9_2015'     2         9      2015     48         0        
      '1_10_2015'    1        10      2015     48       NaN        
      '2_10_2015'    2        10      2015     48       NaN        
      '1_11_2015'    1        11      2015     48       NaN        
      '2_11_2015'    2        11      2015     48       NaN        
      '1_12_2015'    1        12      2015     48       NaN        
      '2_12_2015'    2        12      2015     48       NaN        
      '1_1_2016'     1         1      2016     48       NaN        
      '2_1_2016'     2         1      2016     48       NaN        
      '3_1_2016'     3         1      2016     48       NaN

Answer 3

[J. Webster]

One way to do this is to add another column in your table made up of datenums generated by your dates. Then sort your table based on that column.

myDatenums = datenum(Day{:,1})  %generate datenums from your first column
myNewTable = table(myDatenums); %make myDatenums into  a table so you can concatenate it.
Day = [Day myNewTable]          %append new column
Day = sortrows(Day,7);          %sort table based on new column
Day(:,7) = [];                  %delete the new column

I learned a while ago to save dates as yyyymmdd to avoid problems like this.

[Guillaume]

If you're using tables then you should be using datetime not the outdated datenum

[J. Webster]

whatever

[Guillaume]

It's not 'whatever'. If you try to do anything complex with dates, you'll quickly run into issues with datenum that you won't have with datetime.

If you're giving advice, give advice that leads to good practice.