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saving mat file changes my structures!

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Brad Rosenheim
Brad Rosenheim on 21 Jul 2015
Edited: Stephen on 28 Jun 2019
Hello -
I have a function that compiles data into a structure which consists of field: num which is a 1x24 cell. Thus, D.num = {1x24 cell} My function does this correctly, and allows me to interface with an existing function. However, when my function saves the structure, D, as a mat file, the mat file adds a field. I end up with: D.D.num = {1x24 cell]
I have probed the function and there is only one field when it runs. But the saving is what is changing it. I use the command: save('fname.mat','D') to save the matfile. When I do this from my workspace (and not within the program) it works fine. When the program (function) saves the file, it adds and extra field that is redundant. This is making further programming hard, and I don't see many other options to save this variable.
Thanks - Brad
  1 Comment
Cedric Wannaz
Cedric Wannaz on 21 Jul 2015
This is because SAVE offers the possibility to save multiple variables at once defined by their names (passed as strings):
>> a = 8 ; b = 9 ;
>> save( 'test.mat', 'a', 'b' ) ;
When we LOAD test.mat, in an ideal world where people would remember/know what was saved in their MAT-Files, MATLAB could assume that we would know that there were two variables, and build an output that we could use this way:
>> [a, b] = load( 'test.mat' ) ; % Wrong.
Yet, my guess is that MATLAB devs had to account for the fact that we usually don't know what we did, especially when someone else did it, so they preferred outputting a struct that contains the variables that were saved as fields:
>> data = load( 'test.mat' )
data =
a: 8
b: 9
What tricked you is that you named the output of LOAD D, so you thought that the struct had been kind of expanded like in Inception. If you name it differently though, like data, you'll see that it will contain D and the whole thing will make sense!
>> data = load( ... )
data =
D: [1x1 struct]
>> D = data.D % Extract field D which is the former variable D.
D =
num: {...}
Signed: someone who didn't understand why he got data.data after loading some MAT-File years ago.

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Answers (2)

Muthu Annamalai
Muthu Annamalai on 21 Jul 2015
Your problem maybe that you are loading wrong, whereas your save code seems OK.
Try this code for your load function,
function D = foo()
D = [];
load('fname.mat'); %this will load D in the local workspace.
return
end
which should overwrite the local copy D and return it to the caller.

Image Analyst
Image Analyst on 21 Jul 2015
You're probably doing
D = load('fname.mat');
Doing it this way will create a structure called D with all the variables you saved as fields off of that structure. So that's how you got a D.D. If you had also saved a myVar variable, you would get a D.myVar field.
Either read into a structure and pick off your variable from the structure:
storedStruct = load('fname.mat');
D = storedStructure.D;
or "poof" the variables into existence:
load('fname.mat');
I think the first method is the preferred one since people usually don't like to poof variables into workspaces.
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