How to append a field to an existing structure in a mat file?

s.a = rand;
s.b = rand(2);
save('s.mat','s')
load s
s.c = 'yes' 
save('s.mat','s') % now the new field is added

Is there any other alternatives?, the '-append' flag is there not sure how to yield success though.

[dpb]

save(filename,variables,'-append') adds new variables to an existing file. 
If a variable already exists in a MAT-file, then save overwrites it with the 
value in the workspace.

A struct as the whole is the variable, not a field within the structure.

If the alternative includes that the new data be actually part of an existing struct variable, then I think the answer is "no".

If you would be content with just saving an additional variable but outside the already defined struct, then -append does the job.

If the variable were just an array, matfile would allow indexing including, I believe, extending the size but can't write the dot indexing expression to address a field of a struct

[madhan ravi]

Thanks a lot, dpb. Had the same thoughts :).

Answer 1

[Walter Roberson]

clear s
s.a = rand;
s.b = rand(2);
save('s.mat','-struct', 's', '-v7.3');

s = matfile('s.mat', 'writable', true);
s.c = 'yes';
clear s
whos -file s.mat

[madhan ravi]

Thank you , sir Walter. The role of -v7.3 is significant.

[Bruno Luong]

dpb is correct, I think Walter solution does not exactly question asked. This will save 'a' 'b' as variables and not inside the structure internally stored inside the matfile..

save('s.mat','-struct', 's', '-v7.3')

It just trick the load by MATFILE later inside a structure but there is no internal field add inside thet MATFILE.

It does not do more than

s.a = rand;
s.b = rand(2);
save('s.mat','-struct', 's')
c='yes'
save('s.mat','-append', 'c')

[Walter Roberson]

It does a bit more in that there is no need to explicitly save(): as soon as you create the reference, the new variable is written into the .mat file.

[madhan ravi]

Thanks, Bruno. I didn't notice it properly.