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Create new table wit differentiate or diff from existing timetable

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I've got below timetable with daily measurements from my gas meter at home:
Timestamps GasM3
____________________ _____________
14-Aug-2019 00:00:00 331.15
25-Aug-2019 00:00:00 332.37
26-Aug-2019 00:00:00 332.74
27-Aug-2019 00:00:00 333.97
I would like to know the daily consumption, so I tried playing with "differentiate" & "diff" to get these values in a new table.
I've tried
Test1 = differentiate(DailyGasDataTimeTable)
which gave the following error:
Error using Visualization: Differential test</a> (line 17</a>)
'differentiate' requires Curve Fitting Toolbox
and
Test2 = diff(DailyGasDataTimeTable)
which gave the following error:
Undefined function 'diff' for input arguments of type 'table'.
Error in Visualization: Differential test (line 17)
Unfortunately I'm stuck, please advise.
Kind regards,
Daniel

Answers (4)

Vinod
Vinod on 28 Aug 2019
Edited: Vinod on 28 Aug 2019
You can extract the specific column, GasM3, from the timetable and use the diff function. Something like this:
data = thingSpeakRead(YourChannelNum,'OutputFormat','TimeTable');
deltaM3 = diff(data.GasM3);
newTable = timetable(data.Timestamps(2:end), deltaM3)

Walter Roberson
Walter Roberson on 28 Aug 2019
Use a timetable() object and use retime()

Kristina Collins
Kristina Collins on 31 Aug 2020
What if my timetable is missing entries, or the entries are unevenly timed?

Olavo
Olavo on 21 Aug 2023
Edited: Walter Roberson on 21 Aug 2023
Hello, Thanks for your answer it's very helpful!
What if I the variable I what to differentiate is not a fixed name and will be changing along the code? for example:
for i=1:10;
deltaM(i) = diff(data.GasM3{i}); % -->> does not work with this notation
end
  3 Comments
Steven Lord
Steven Lord on 21 Aug 2023
Or you could use a numeric index.
T = array2table(magic(4))
T = 4×4 table
Var1 Var2 Var3 Var4 ____ ____ ____ ____ 16 2 3 13 5 11 10 8 9 7 6 12 4 14 15 1
A = T{:, 3} % Equivalent to T.Var3
A = 4×1
3 10 6 15
If you're release R2023a or later, you may be able to compute directly on the table or timetable, as described on this documentation page.
T2 = diff(T)
T2 = 3×4 table
Var1 Var2 Var3 Var4 ____ ____ ____ ____ -11 9 7 -5 4 -4 -4 4 -5 7 9 -11
Olavo
Olavo on 22 Aug 2023
Thanks Walter and Steven, very useful information.
Just to add some extra information, I found the function wildcardPattern very useful to get similar names from table. would be something like:
varname = "GasM3" + wildcardPattern;

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