How to change the value of a double variable in a .mat file

Hi! In have a mat file which has 3 main variables a1, a2, and a3. At the time, each of the 3 has 10 variables in it from b1 to b10

example:

a1.b1, a1.b2, ..... , a1.b10

a2.b1, a2.b2, ..... , a2.b10

a3.b1, a3.b2, ..... , a3.b10

I want to change the value of a1.b1 from 0 to 1

but the problem is that I can't use the code

save('sample.mat','a1.b1','-append')

I can't use save('sample.mat','a1','-append') because the variables a1.b2 to a1.b10 will be lost. Is there a code which can make me do this kind of thing?

Answer 1

[Azzi Abdelmalek]

If your file is not big, just load your data, modify them, then save again your data

 %Look at this example
a.b1=1;
a.b2=2;
c.b1=1;
c.b2=20;
save fic a c
clear
data=load('fic','a')
a=data.a;
a.b1=0;
save('fic','a','-append')

[ericson]

The problem is that my mat file is too big that it takes almost a minute just to load it in my gui.

[ericson]

I tried this but it, based on your code. a.b2 will be lost after doing that

[Azzi Abdelmalek]

No, if you load the variable a, a.b2 will not be lost

Answer 2

[Joseph Cheng]

From the documentation '-append' adds a new variable to the mat file, so I do not think it will append to an existing struct.

Have you tried save('sample.mat','a1','-append') and rewrite all a1? by doing something like this?

load('sample.mat','a1'); %load in just a1 variable
a1.b1=1;
 save('sample.mat','a1','-append')

[Joseph Cheng]

or even more fun you can go into the file http://www.mathworks.com/help/pdf_doc/matlab/matfile_format.pdf and find the variable and modify it.

[ericson]

When I experimented on it, when there is an existing variable which is the same as the one that I'm going to append, it will overwrite it.

I tried that and it deleted the other variables stored in a1

[Joseph Cheng]

Yes so i suggest loading just a1, swap a1.b1 and save by appending a1. You will not be able to adjust a1.b1 without loading in a1.