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how to append variables in a specific .mat file in a specific path

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greeting
how to append variables in a specific .mat file in a specific path
[FileName,PathName] = uigetfile('*.mat','Select the Rotor DATA file')
FileName =
Data.mat
PathName =
C:\Users\PC\Desktop\
i need to add variabls to Data.mat using UI
plz help

Accepted Answer

Walter Roberson
Walter Roberson on 3 Jun 2017
matfile = fullfile(PathName, FileName);
save(matfile, 'NameOfVariableToAppend', '-append');
This will typically leave the other variables stored in the file in the same positions they already are in. If the variable to append already exists in the file then it would typically overwrite the existing block for that variable if the storage space required is no larger than it was before, but if the variable to append already exists in the file and the new version is larger then the existing storage block in the file would typically be marked as unused and the new version would be appended to the file. It is not documented as to whether the entire file might ever be rewritten to fill in "holes" if the file gets fragmented from these kinds of updates.
Note that if the variable already exists in the file, then the new data will replace the old data, rather than being added on to the end of it.

More Answers (1)

Wonsang You
Wonsang You on 3 Jun 2017
Assume that you want to save Var into 'data.mat'. Run the following code.
Var = 3;
A = load('data.mat');
save('data.mat','A','Var');
  1 Comment
Walter Roberson
Walter Roberson on 3 Jun 2017
That does not append.
When you have a .mat and you load() and assign that to a variable, then the resulting variable will be a scalar struct in which there was one field for every existing variable in the .mat file.
Your save('data.mat','A','Var') would overwrite the file, but the new contents would be exactly two variables, one named 'Var' and the other a structure named 'A'.
For example suppose the file originally contained variables 'A', 'B', and 'C'. Then
A = load('data.mat');
would result in a scalar struct A with fields 'A', 'B', and 'C'. So A.A would refer to what was previously the variable A in the file, A.B would refer to what was previously the variable B in the file, and A.C would refer to what was previously the variable C in the file.
When you then save('data.mat','A','Var') then the result in the .mat file would be 'Var' and this structure 'A'. You would have to know what had happened to the file so that you could know that now to refer to the old B you would have to load variable A from data.mat and refer to A.B . Whereas what you would want is that instead after 'Var' was appended, that the file would contain 'A' (the original), 'B', 'C', and 'Var'.
What you could do is
Var = 3;
A = load('data.mat');
A.Var = Var;
save('data.mat', '-struct', 'A');
The result of that would be to take the variable A and extract the fields from it and write each as a variable, thereby creating a file with 'A' (the original), 'B', 'C', and 'Var' stored in it. This approach would overwrite the entire file, rather than just updating the minimal part of it to write the new variable Var. Also, it requires you have enough free memory to be able to load the entire .mat . There is also the risk that you could end up changing .mat version -- for example if the file was a -v7.3 mat but your preference for the default save format is the old -v5.3 then the result of the save() would be a -v5.3 file because you would be overwriting all of the old file.

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