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How can I load a field from a saved structure?

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K E on 5 May 2016
Commented: K E on 6 May 2016
For my work, it is convenient to store measurement data in structures with many fields. The structure is useful for keeping all the measurements together, passing into functions that plot everything, etc. However the files get large and bulky to handle. I'd like to be able to load a specified field from the *.mat file containing the structure. I thought this was possible using this approach (saving with -struct flag), but only separate variables containing the fields are saved and the original structure is lost. Is there a way to retrieve a field out of an existing mat file containing a structure, as is shown below, perhaps using matfile ?
% Make the structure
a.Temperature.value = rand(1,10);
a.Temperature.units = 'C';
a.Humidity.value = rand(1,10);
a.Humidity.units = '%';
save('c:\temp\junk.mat', 'a'); % Save the structure
load('c:\temp\junk.mat', 'a.Temperature'); % Load a specified field (doesn't work)


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Accepted Answer

Adam on 5 May 2016
Edited: Adam on 5 May 2016
save('c:\temp\junk.mat', '-struct', 'a');
load('c:\temp\junk.mat', 'Temperature');
should work I think. Or
loadedVars = load('c:\temp\junk.mat', 'Temperature');;
Temperature = loadedVars.Temperature;
if you prefer.


K E on 5 May 2016
I was grumpy about the fact that the -struct file results in a mat file containing variables, i.e. I lose the original structure, until I realized I could regain it as shown below. However we have tons of already-saved *.mat files containing large structures which I would love to pull fields out of but sounds like that is not an option.
save('c:\temp\junk.mat', '-struct', 'a');
a = load('c:\temp\junk.mat');
% Has fields a.Temperature and a.Humidity
Adam on 6 May 2016
Yes, it seems even using the matfile function this cannot be done (as also indicated on its help page):
>> mFile = matfile( 'c:\temp\junk.mat' );
>> mFile.a.Temperature
Cannot index into 'a' because MatFile objects only support '()' indexing.
K E on 6 May 2016
I have put in a request to add the ability to load fields in future releases.

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