Why do I receive "Cannot write value: unsupported class struct error" when writing a structure using the FWRITE function?

If I try to write a structure to a file using the FWRITE function, i receive the following error:

 ??? Error using ==> fwrite

 Cannot write value: unsupported class struct

For example:

a.name='Mathworks';

a.number=10;

fid = fopen('testfile.txt','w+');

fwrite(fid,a)

Answer 1

[MathWorks Support Team]

The ability to write a structure to a file using FWRITE is not available in MATLAB.

To work around this issue, write each field of the structure sequentially to the file. This can be done either by calling the FWRITE function from the command line for every field, or by writing a program similar to the one shown below to loop through each variable and write it to the file:

%%%Open the file in the write mode, fid is the id of the file.

%%%Assume the structure is the variable ‘a’

%%%As a sample, let ‘a’ have two fields: name and number

a.name='Mathworks';

a.number=10;

NEWLINE = sprintf('\n');

fields=fieldnames(a);

for i=1:length(fields)

      fields(i);

      classes{i}=eval(['class(a.' fields{i} ')']);

end

fid = fopen('testfile.txt','w+');

for j = 1: length(fields)

      if ~strcmp(classes(j),'char')

          str = num2str(eval(['a.' fields{j} ]));

      else        

          str = eval(['a.' fields{j} ]);

      end

      fwrite(fid, str, 'uchar');    

end

fwrite(fid, NEWLINE, 'char');

%%%Call this loop if you have multiple structures too.

%%%Close the file.

[Vandana Ravichandran]

MATLAB converts .NET object to native MATLAB data types. In your case, System.Int64 should be converted to int64 scalar. Refer the following page from the documentation for more information: https://www.mathworks.com/help/matlab/matlab_external/handling-net-data-in-matlab.html

If you just want to save the workspace structure variables for future use inside MATLAB, you can use the "save" function. If you want to save the structure to a text file, you can convert the structure array to a table using "struct2table" function. Subsequently, you can use "writetable" function to write the table to a comma-delimited text file.