Saving variables within a parfor loop

I wish to save individual variables in my script that are defined in a parfor loop. I defined a function called saveVariables.m that would save a specified variable to a .mat file as follows.

function l = saveVariables(local_filename, variable, index)
save(['simulation_' num2str(index) '\' local_filename], variable)
l = 1;
end

The issue that I have is that MATLAB produces the error

Error using saveVariables (line 4)
Argument must contain a string.

I use the backlash '\' since I am running Matlab on Windows. local_filename is meant to be the string of the name of the file that I wish to save. How can I work around this string error?

Answer 1

[Mohammad Abouali]

Two things:

(1) When using save() the way that you are using the second input must be also an string. So if you want to save a variable called myVariable in a file called myOutputFile='D:\myDataFolder\somename.mat' then you have to issue the command as follows:

save(myOutputFile,'myVariable')

Note the quote around myVariable.

This is different when you use the non-function form of the save() as follow:

save 'D:\myDataFolder\somename.mat' myVariable

(2) save() command does not work in SPMD or PARFOR section. Click here for another post about the same topic;

You need to first define a function as follows:

function savetofile(data,fullfilename)
    save(fullfilename,'data');
end

and then whenever you are in SPMD and PARFOR and willing to save something instead of using the regular save() command use the above function as follows:

savetofile(myVariable,myOutputFile)

Answer 2

[Walter Roberson]

But what are you passing in for "variable" ? It must be the name of a variable.

You should consider using fullfile() . And you should consider using sprintf() to construct the name.

save( fullfile(sprintf('simulation_%d',index), local_filename), variable)

[Sean Pierre]

Thanks for the tip regarding fullfile(). The only issue I am having now is that if I call the saveVariables() function as follow

saveVariables('RD_1pk.mat','RD', j);

in my main script, I receive the error

Error using saveVariables (line 3)
Variable 'RD' not found.

Should I make these variables global?

[Walter Roberson]

function l = saveVariables(local_filename, variable, index)
save(['simulation_' num2str(index) '\' local_filename], inputname(2))
l = 1;
end

and call with

saveVariables('RD_1pk.mat',RD, j);

[Sean Pierre]

I am still receiving the same error of

Error using saveVariables (line 3)
Variable 'RD' not found.

. I changed my saveVariables script to

function l = saveVariables(local_filename, variable, index)
save( fullfile(sprintf('simulation_%d',index), local_filename), inputname(2));
l = 1;
end

and I call it with

saveVariables(   'RD_1pk.mat',   RD, j);

[Walter Roberson]

function l = saveVariables(local_filename, variable, index)
  varname = inputname(2);
  savestruct.(varname) = variable;
  save( fullfile(sprintf('simulation_%d',index), local_filename), '-struct', 'savestruct')
  l = 1;
end