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Check if field exists in Matlab struct without loading it

Answer 1

+3 A:

As far as I know, you have to load the file in order to be able to check if a saved structure contains a specific field.

However, if you save the .mat file with the '-struct'-option, it splits the fields into separate variables in the .mat file. You can recreate the structure by calling

myStructure = load('test.mat');

Saving this way also allows you to test for whether a field (variable) exists by using @Amro's approach (which is a lot cleaner than what I suggested before).

Jonas 2010-10-26 18:25:12

Answer 2

+7 A:

To check the contents of a MAT file without loading it, use:

vars = whos('-file','test.mat')
ismember('fieldname', {vars.name})

Amro 2010-10-26 18:32:11

@Amro: Does that work for fields of structures inside `test.mat`?

Jonas 2010-10-26 18:43:52

@Jonas: The above code only checks for variable names. So like you mentioned in your answer, the user should use the `-struct` option of SAVE to split the structure fields into separate variables when saving to a MAT-file.

Amro 2010-10-26 18:48:37

@Jonas: otherwise, and if the user is really concerned about memory use, we can simply write the fieldnames to a separate text file along with the actual MAT-file, then load and check the text file as needed before loading the actual structure data

Amro 2010-10-26 18:55:46

@Amro: Good point.

Jonas 2010-10-26 19:00:56

You could also save the list of field names (or other metadata) in a separate variable inside the same .mat file as the original struct, (e.g. with "fnames=fieldnames(s);save('myfile.mat','fnames','s')") and then selectively read it with the "load(filename, varname)" form. Could simplify file management by keeping the data and index in a single file. Amro's method of splitting into separate variables will also let you selectively load parts of the struct; this one won't. And if you have big cellstr arrays in there, convert them to char before saving if possible. Can save both space and time.

Andrew Janke 2010-10-26 20:26:53

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Check if field exists in Matlab struct without loading it

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