How to plot data from JSON Structure array

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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

Accepted Answer

Jon
Jon on 9 May 2022
Edited: Jon on 9 May 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 Comments
Jon
Jon on 16 May 2022
Thanks, good luck with your project

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More Answers (3)

Dharmesh Joshi
Dharmesh Joshi on 9 May 2022
Hi Jon
Yes, if i call the plot as
plot([downloaded_data.iot_battery])
it works. It seems that i had to enter [ ]. Is there a reason for this?
My data also includes a timestamp, which i belive it in a text formate as
'2022-04-08T10:41:33.3189578+00:00'
How do get this information on my x axis?
  12 Comments
Jon
Jon on 11 May 2022
The blank spots probably correspond to points where the temperature value is nan (not a number), so as you say missing data.

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Dharmesh Joshi
Dharmesh Joshi on 11 May 2022
Is there away to confirm if there is a nan, as my device are updating every 1 minute, if there is any issue , there would be not entry for that minute or a perticular field would be 0.
I noticed the following warning
Warning: Error updating Text.
String scalar or character vector must have valid interpreter syntax:
Temperature $^\circ \mathrm{C}$
> In defaulterrorcallback (line 12)
In matlab.graphics.axis.decorator.DatetimeRuler.get.TickLabelFormat
In matlab.graphics.axis.decorator/DatetimeRuler/format
In matlab.graphics.internal.makeNonNumeric
In matlab.graphics.internal.makeNonNumeric
In n02 (line 33)
  11 Comments
Dharmesh Joshi
Dharmesh Joshi on 12 May 2022
Edited: Dharmesh Joshi on 12 May 2022
Hi Jon
Yes, I noticed that small difference as well. I will incorporate the clean time Function. But with the previous method, is it really necessary to add the decimal point and value?

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Dharmesh Joshi
Dharmesh Joshi on 14 May 2022
Hi Jon
I got it working now.
Its ploting well.
At the moment, the data sample are per minute. how can i group all the data per hour and work out the mean or average for that hour?
  1 Comment
Jon
Jon on 16 May 2022
Edited: Jon on 16 May 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

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