Create new table wit differentiate or diff from existing timetable

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Daniel Ubbink
Daniel Ubbink 2019-8-28
评论： Olavo 2023-8-22
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

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回答（4 个）

Vinod
Vinod 2019-8-28
编辑：Vinod 2019-8-28
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 2019-8-28
Use a timetable() object and use retime()
Kristina Collins
Kristina Collins 2020-8-31
What if my timetable is missing entries, or the entries are unevenly timed?
Olavo
Olavo 2023-8-21
编辑：Walter Roberson 2023-8-21
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
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Steven Lord
Steven Lord 2023-8-21
Ran in:
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 2023-8-22
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;
Matches as few characters of any type - MATLAB wildcardPattern - MathWorks Nordic

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