Name variable based on string MATLAB

Question

I have a variable that is created by a loop. The variable is large enough and in a complicated enough form that I want to save the variable each time it comes out of the loop with a different name.

PM25 is my variable. But I want to save it as PM25_year in which the year changes based on `str = fname(13:end)'

PM25 = permute(reshape(E',[c,r/nlay,nlay]),[2,1,3]); % Reshape and permute to achieve the right shape. Each face of the 3D should be one day
str = fname(13:end); % The year

% Third dimension is organized so that the data for each site is on a face
    save('PM25_str', 'PM25_Daily_US.mat', '-append') 

The str would be a year, like 2008. So the variable saved would be PM25_2008, then PM25_2009, etc. as it is created.

Answer 1

Defining new variables based on data isn't considered best practice, but you can store your data more efficiently using a cell array. You can store even a large, complicated variable like your PM25 variable within a single cell. Here's how you could go about doing it:

Place your PM25 data for each year into the cell array C using your loop:

for i = 1:numberOfYears
    C{i} = PM25;
end

Resulting in something like this:

C = { PM25_2005, PM25_2006, PM25_2007 };

Now let's say you want to obtain your variable for the year 2006. This is easy (assuming you aren't skipping years). The first year of your data will correspond to position 1, the second year to position 2, etc. So to find the index of the year you want:

minYear = 2005;
yearDesired = 2006;
index = yearDesired - minYear + 1;
PM25_2006 = C{index};

Answer 2

You can do this using eval, but note that it's often not considered good practice. eval may be a security risk, as it allows user input to be executed as code. A better way to do this may be to use a cell array or an array of objects.

That said, I think this will do what you want:

for year = 2008:2014
    eval(sprintf('PM25_%d = permute(reshape(E',[c,r/nlay,nlay]),[2,1,3]);',year));
    save('PM25_Daily_US.mat',sprintf('PM25_%d',year),'-append');
end

Answer 3

I do not recommend to set variables like this since there is no way to track these variables and completely prevents all kind of error checking that MATLAB does beforehand. This kind of code is handled completely in runtime.

Anyway in case you have a really good reason for doing this I recommend that you use the function assignin for this.

assignin('caller', ['myvar',num2str(1)], 63);