MATLAB: Define variable inside functions

Question

Dear all:

I parse a structure into a function, which build the system matrix for a model. The code looks something like this:

function [m] = test(param)
%%SETUP GLOBAL PARAMETERS
F = fieldnames(param);
for i = 1:length(F)
  field = F{i};
  assignin('caller', field, param.(field));
end
%%SET UP MATRICES FOR SOLVING THE MODEL
 m = [kappa, alpha];
end

The structure param looks like the following:

param.kappa = 0.8;
param.alpha = 1;

but when I run the function test, matlab returns the following complain:

??? Undefined function or variable 'kappa'.

I would imagine 'assignin' assigns the value into the kappa, so kappa should be automatically defined, but this is not the case. If I take away kappa and just leave alpha in the matrix, matlab complains about alpha is not defined. Any ideas why this happens?

PS: The reason I do this is not obvious in this simplified function. My model is very complex and involves changing parameters in simulations, so I want to remain the variable name as their name rather than some vector/matrix index to minimise errors.

Thanks alot in advance!

Cheers Ben

Answer 1

The documentation of ASSIGNIN reveals, that the variable is created in the caller. Therefore the program would run as:

function m = MainProgram(param)
test(param);  % Call the subfunction to create the variables
% SET UP MATRICES FOR SOLVING THE MODEL

m = [kappa, alpha];
end
function test(param)  % The subfunction
F = fieldnames(param);
for i = 1:length(F)
  field = F{i};
  assignin('caller', field, param.(field));
end

As you can see, the confusion level of automagically created variables is very high. I strongly recommend to avoid such tricks, if you are not an advanced programmer and know exactly what you are doing. If you are a beginner, such Voodoo techniques will waste your time. Faster, nicer, easier and stable:

function m = MainProgram(param)
% SET UP MATRICES FOR SOLVING THE MODEL
m = [param.kappa, param.alpha];
end