How can I dynamically extract fields from a structure and create variables in the MATLAB workspace using the field names?

I have a created a structure containg fields a, b and c. Now I want to decompose my structure and save a, b and c as workspace variables with the filed name as the variable name.

Answer 1

[MathWorks Support Team]

One possibility to dynamically decompose your struct would be using FIELDNAMES command to determine the fields and create according variables using EVAL, like in the following example:

constants.a = 4;

constants.b = 8;

constants.c = 10;

names = fieldnames(constants);

for i=1:length(names)

eval([names{i} '=constants.' names{i} ]);

end

In case of nested structures, you can use nested FOR loops to access the fields.

constants.a.aa=4;

constants.a.ab = 5;

constants.b = 8;

constants.c = 10;

names = fieldnames(constants);

for i=1:length(names)

      if isstruct(eval(['constants.' names{i}]))

          names2 = fieldnames(eval(['constants.' names{i}]));

          for j=1:length(names2)

              eval([names{i} '_' names2{j} '=constants.' names{i} '.' names2{j}]);

          end

      else

          eval([names{i} '=constants.' names{i} ]);

      end

end

[Stephen23]

"What would you advise?"

Read these:

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

https://www.mathworks.com/matlabcentral/answers/235119-can-you-explain-this-weird-behavior#answer_190014

https://www.mathworks.com/matlabcentral/answers/230530-take-a-variable-with-unkown-array-length-consisting-of-only-numbers-and-assign-each-number-in-that

Just because something is possible does not make it a good way to write code.

"...then the variables assignments will be trapped in that function each time it gets called, so it would be useless."

Read this:

https://www.mathworks.com/help/matlab/matlab_prog/share-data-between-workspaces.html

and then write simple, neat, efficient code by using the method recommended in the MATLAB documentation and by all experienced MATLAB users: by simply passing variables as input/output arguments. How to pass input/output arguments is also explained in the introductory tutorials:

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

[Stephen23]

@Dan Goldwater: ask this as its own question, rather than hijacking a thread from January 2010. Then you are likely to get people to help you.

In your question please explain exactly what the problem is, e.g. "..I need to be able to change whether 'speed' is stored in 'x' or in 'y'" : currently neither of x or y contain the field speed, so it is not clear how this is relevant or how it can be a problem. At some point in your code you will have to know where the values come from and where they need to be allocated to.

[Steven Lord]

So you're trying to optimize a function of both length and time and depending on the specific optimization you want to fix length and optimize the best time or fix time and optimize the best length? Something like this might help (untested code):

function value = extractSpecifiedParameter(paramStruct, constStruct, paramName)
isparam = isfield(paramStruct, paramName);
isconst = isfield(constStruct, paramName);
if isparam && isconst
    error('%s cannot be both a parameter and a constant.', paramName);
elseif isparam
    % we know isconst is false or it would have been caught above
    value = paramStruct.(paramName);
elseif isconst
    % we know isparam is false or it would have been caught above
    value = constStruct.(paramName);
else
    % both isparam and isconst are false
    error('%s is neither a parameter nor a constant.', paramName);
end
end

Call this like:

function z = fun(x,y)
%Block in which I extract variables from x and y, such as
lengthParameter = extractSpecifiedParameter(x, y, 'length');
time = extractSpecifiedParameter(x, y, 'time');

I changed the name of your first parameter variable because length already has a meaning in MATLAB.

You could add a second output to the declaration and construct that output from isparam and isconst to specify whether the parameter was in the paramStruct or constStruct if that information is useful for your function.