Saving a structure element as a .mat file

I have a set of structures 1.mat , 2.mat and so on.

Each structure has a field as below example:

1.mat.data has a 110 X 116 array

I want to copy 1.mat.data into a separate mat file

The same for the rest of the files.

How can I do it ?

[Rik]

Your question is a bit confusing. mat files contain variables and can be loaded to a struct. Is that what you mean? So you want to do this?

S=load('1.mat');
data=S.data;
save('new_1.mat',data)

Is that what you mean?

What have you tried already?

[Matt J]

I have a set of structures 1.mat , 2.mat and so on.

Not possible. Those are illegal names for structures, e.g.,

1.mat.data=5

Invalid expression. Check for missing multiplication operator, missing or unbalanced delimiters, or other syntax error. To construct matrices, use brackets instead of parentheses.

Error in connector.internal.fevalMatlab

Error in connector.internal.fevalJSON

Answer 1

[Steven Lord]

If you want to extract some but not all of the variables stored in a MAT-file into a separate file, I'd use load to read just the variables you want to copy over into a struct array then call save with the -struct option. For this example let's work in a temporary directory.

cd(tempdir)

The patients.mat file has a number of variables.

whos -file patients.mat

  Name                            Size            Bytes  Class      Attributes

  Age                           100x1               800  double               
  Diastolic                     100x1               800  double               
  Gender                        100x1             11412  cell                 
  Height                        100x1               800  double               
  LastName                      100x1             11616  cell                 
  Location                      100x1             14208  cell                 
  SelfAssessedHealthStatus      100x1             11540  cell                 
  Smoker                        100x1               100  logical              
  Systolic                      100x1               800  double               
  Weight                        100x1               800  double               

Let's make a new MAT-file named bloodPressure.mat (I'll add suffixes 1 and 2 to that name to distinguish two approaches) that contains only the Diastolic and Systolic variables from patients.mat. First load those two variables into a struct.

data = load('patients.mat', 'Diastolic', 'Systolic')

data = struct with fields:
    Diastolic: [100×1 double]
     Systolic: [100×1 double]

Now call save to save that data to a new MAT-file. We could save the data struct array or we could use the -struct option to save each field as a separate variable in the MAT-file.

save('bloodPressure1.mat', 'data') % Storing the struct
whos -file bloodPressure1.mat

  Name      Size            Bytes  Class     Attributes

  data      1x1              1936  struct              

save('bloodPressure2.mat', '-struct', 'data') % Storing each field separately
whos -file bloodPressure2.mat

  Name             Size            Bytes  Class     Attributes

  Diastolic      100x1               800  double              
  Systolic       100x1               800  double              

Note that in the case of bloodPressure2.mat the name data doesn't appear in the file anywhere. So if you wanted to do this to a series of files in a for loop you wouldn't have to use a different variable for each loop iteration. Just load the data into the temporary variable which will overwrite the contents from the previous loop iteration.

[sai prasanna sai prasanna m s]

Thanks for your elaborate answer.

This is what I had tried :

files = dir('*.mat');
for i=1:length(files)
    data=load(files(i).name,'data');
    save(erase(files(i).name,".mat"), '-struct','data');
end

Yet, name data appears in the new copied file. Could you please help.

[Rik]

Your load call results in a struct named data with a field named data. Then your save call write that struct to a mat file, treating each field as a variable. That is why you will have an array named data in your resulting mat file, exactly as expected.

Which part is not what you intended?

[sai prasanna sai prasanna m s]

Okay,

Originalfile.mat contains a struct called data. Data contains an array.

I want to copy just the array into newfile.mat.

As you rightly said, the above code is resulting in newfile.mat containing a struct called data, with the array.

But, I do not want a struct element. Instead, I want the newfile.mat to directly contain the array.

[Rik]

You misunderstand me.

Your load call resulted in a struct.

Your save call only resulted in a single variable in your mat file, not a struct (unless your original data was nested a layer deeper than you told us).

[sai prasanna sai prasanna m s]

originalfile =

struct with fields:

data: [196×111 double]

This is exactly what I have. I have multiple such files. I am running the exact same code as above. But it is resulting in files of this format, and thats my issue:

newfile =

struct with fields:

data: [196×111 double]

I want it to be newfile=[196×111 double]

[Stephen23]

S = load(..)
newfile = S.data