MATLAB Answers

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

26 views (last 30 days)
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:'Mathworks';
fid = fopen('testfile.txt','w+');

Accepted Answer

MathWorks Support Team
MathWorks Support Team on 27 Jun 2009
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'Mathworks';
NEWLINE = sprintf('\n');
for i=1:length(fields)
classes{i}=eval(['class(a.' fields{i} ')']);
fid = fopen('testfile.txt','w+');
for j = 1: length(fields)
if ~strcmp(classes(j),'char')
str = num2str(eval(['a.' fields{j} ]));
str = eval(['a.' fields{j} ]);
fwrite(fid, str, 'uchar');
fwrite(fid, NEWLINE, 'char');
%%%Call this loop if you have multiple structures too.
%%%Close the file.
Vandana Ravichandran
Vandana Ravichandran on 22 Feb 2017
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:
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.

Sign in to comment.

More Answers (0)


No tags entered yet.


Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!