How to plot data from JSON Structure array
16 visualizaciones (últimos 30 días)
Mostrar comentarios más antiguos
Dharmesh Joshi
el 9 de Mayo de 2022
Comentada: Jon
el 16 de Mayo de 2022
Hi
I have used HTTP to download a large amount structued data from an API.
This is my code
function n02()
downloaded_data = device_data("864475047548549","2022-04-08T10:31:00Z","2022-05-08T11:31:00Z");
%struct2table(downloaded_data)
plot(downloaded_data.Timestamp,downloaded_data.iot_battery);
end
This is the output i am getting
Error using plot
Invalid first data argument.
Error in n02 (line 6)
plot(x,y);
What am i doing wrong
0 comentarios
Respuesta aceptada
Jon
el 9 de Mayo de 2022
Editada: Jon
el 9 de Mayo de 2022
I don't understand what you are expecting to happen in your example code. Your variable downloaded_data is assigned to an array of 3 strings. The variable downloaded_data doesn't have any fields and in particular not the ones you are trying to plot.(Timetstamp or iot_battery).
Here's a small example of how you should be working with the json data and plotting it. Hopefully you can follow from this and adapt your code accordingly
If you have some json text, txt, you can turn it into a structure using jsondecode(txt)
So for example
txt = '{"t":[1, 2, 3, 4, 5],"x":[1, 4, 9, 16, 25]}'
mydat = jsondecode(txt)
plot(mydat.t,mydat.x)
5 comentarios
Más respuestas (3)
Dharmesh Joshi
el 9 de Mayo de 2022
12 comentarios
Jon
el 11 de Mayo de 2022
The blank spots probably correspond to points where the temperature value is nan (not a number), so as you say missing data.
Dharmesh Joshi
el 11 de Mayo de 2022
11 comentarios
Jon
el 12 de Mayo de 2022
Editada: Jon
el 12 de Mayo de 2022
Great job isolating the problem!
It looks like, for whatever reason, the raw date strings sometimes have less decimal places. So, the + character appears sooner. Cutting of at character 27 can therefore sometimes include a + in the string which leads to the NaT.
You could just try using one less decimal place and modify to
TimeStamp = cellfun(@(x)[x(1:10),' ',x(12:26)],...
TimeStampRaw,'uniformOutput',false);
You could also take a more robust approach and make a little function to clean up the strings like this:
...
% end, use cellfun to perform operation on every element in the cell array
TimeStamp = cellfun(@(x)cleanTime(x),...
TimeStampRaw,'uniformOutput',false);
% convert TimeStamp strings to datetime array
t = datetime(TimeStamp)
% plot result
plot(t,iot_battery)
% put function to clean the time stamps at end of script, or nested inside of your main function
function tstmp = cleanTime(str)
% clean up time stamps, get rid of letter T in middle and +00:00 at
% end
parts = strsplit(str,{'T','+'});
tstmp = [parts{1},' ',parts{2}];
end
Dharmesh Joshi
el 14 de Mayo de 2022
1 comentario
Jon
el 16 de Mayo de 2022
Editada: Jon
el 16 de Mayo de 2022
That's great to hear.
I think to keep this thread clear it would be good to accept my answer rather than accepting your own final note (which should probably just be another comment) as an answer. I think it is possible to modify which answer you accept.
Rather than starting up a new thread here I will anwer your question regarding the grouping of data as a further comment under my original answer
Ver también
Categorías
Más información sobre Scope Variables and Generate Names en Help Center y File Exchange.
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!