using eval with save function

I'm using eval function to save a .mat file and I'm getting error:

I appreciate any hints!

try_num = 1;
pathdatasave = (['E:\abc\try' num2str(try_num) '\']); 

eval(['save([pathdatasave loc_vel_thresh' num2str(thresh) '_pln' num2str(pln_num) '_try' num2str(try_num) ...
     '_between two planes x1=' num2str(Xl) '-x2=' num2str(Xr)...
     '_myfile.mat],x_end_pln'  num2str(pln_num) '_try' num2str(try_num) ');' ])


Error using vertcat
Dimensions of arrays being concatenated are not consistent.

[Stephen23]

"I appreciate any hints!"

Don't use EVAL to call SAVE.

Don't force meta-data into variable names.

The variable name that you are trying to create

x_end_pln'  num2str(pln_num) '_try' num2str(try_num)

tells us that you made the mistake of forcing meta-data into your variable names (i.e. pln_num and try_num), which when you try to access dynamically forces you into writing slow, complex, inefficient, obfuscated, insecure, buggy code that is hard to debug:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

In short, bad data design leads to bad code, which is what you are finding out now.

Answer 1

[the cyclist]

It can be tricky to construct the string needed, from a combination of text and numerics. I would recommend a few things here.

The most important one is to not use eval. Instead, use sprintf to create the required strings for the filename and/or any variables you want to save, and then put those strings into the save command itself.

Second, my advice would be to always display the string itself to the command line, so that you can see what it looks like. In your case, it looks like this:

% I defined a bunch of these variables arbitrarily, because you had not
% defined them in the code you posted
try_num = 1;
thresh = 2;
pln_num = 3;
Xl = 4;
Xr = 5;

pathdatasave = (['E:\abc\try' num2str(try_num) '\']); 

savestring = ['save([pathdatasave loc_vel_thresh' num2str(thresh) '_pln' num2str(pln_num) '_try' num2str(try_num) ...
     '_between two planes x1=' num2str(Xl) '-x2=' num2str(Xr)...
     '_myfile.mat],x_end_pln'  num2str(pln_num) '_try' num2str(try_num) ');' ]

savestring = 'save([pathdatasave loc_vel_thresh2_pln3_try1_between two planes x1=4-x2=5_myfile.mat],x_end_pln3_try1);'

Notice how the displayed string is using the literal string 'pathdatasave' instead of the actual string you wanted to swap in? I think that is the main technical issue.

So, using a simpler example case, I would do something like

try_num = 1;
pln_num = 2;

filename = sprintf('E:\\abc\\try%d\\',try_num)

filename = 'E:\abc\try1\'

varname  = sprintf('blah_pln%d',pln_num)

varname = 'blah_pln2'

and then just save directly without using eval:

save(filename,varname)

[Ham Man]

after running SAVE, I get this error:

'blah_pln2' is not a valid variable name.

Isn't this due to the single quotation?

[Stephen23]

"Isn't this due to the single quotation?"

No, it is due to the fact that you are attempting to generate an invalid variable name.

The single quotes are not part of the character vector, they are just an artifact of how it is displayed.

Once we actually create the very badly-named variable blah_pln2 the code works correctly:

varname = 'blah_pln2';
blah_pln2 = 3;
save('test.mat',varname)
whos -file test.mat

  Name           Size            Bytes  Class     Attributes

  blah_pln2      1x1                 8  double              

But clearly, having numbered variable names is a sign that you are doing something wrong.

[the cyclist]

What @Stephen23 says is accurate. Probably the first thing I should have posted is this tutorial about why dynamic variable names are usually a terrible idea.

Answer 2

[Image Analyst]

Don't use eval. See the FAQ:

https://matlab.fandom.com/wiki/FAQ#Why_is_it_advised_to_avoid_using_the_%22eval%22_function?

It's simply not necessary.

try_num = 1;
% Construct the folder name.
folder = fullfile('E:\abc\try', num2str(try_num)); 
if ~isfolder(folder)
    mkdir(folder);
end
% Construct the base file name.
baseFileName = sprintf('loc_vel_thresh%d_pln%d_try%d_between two planes x1=%d-x2=%d_myfile.mat',...
    thresh, pln_num, Xl, Xr, pln_num)
% Construct the full name.
fullFileName = fullfile(folder, baseFileName)
% Save the mat file.
save(fullFilename, 'x_end_pln', 'pln_num', 'try_num');

[Ham Man]

How the variables in save command can be dynamic? like: x_end_pln num2str(pln_number)

[Image Analyst]

Not sure what you mean. Which variables do you actually want to save? Just list them by name in quotes inside the call to save.

Please give an example of what an actual filename would look like with all the numbers encoded into it.

[Ham Man]

Thank you so much every one

[Image Analyst]

So did this sprintf & save solution work for you? If so, could you click the "Accept this answer" link? Thanks in advance. 🙂 Otherwise tell us what's not working yet.