How do I access structure fields dynamically?

Question

I have a structure with many fields which are vectors of different lengths. I would like to access the fields within a loop, in order. I tried getfield as follows but MATLAB doesn't like that. How can I do this?

S = struct('A', [1 2], 'B',[3 4 5]); SNames = fieldnames(S); for loopIndex = 1:2   field = getfield(S, SNames(loopIndex));   %do stuff w/ field end ??? Index exceeds matrix dimensions 

I'm using structures in the first place because an array would have trouble with the different field lengths. Is there a better alternative to that?

Answer 1

Try dynamic field reference where you put a string in the parenthesis as seen on the line defining stuff.

S = struct('A', [1 2], 'B',[3 4 5]);  SNames = fieldnames(S);  for loopIndex = 1:numel(SNames)      stuff = S.(SNames{loopIndex}) end  

I concur with Steve and Adam. Use cells. This syntax is right for people in other situations though!

Answer 2

There are three points I'd like to make here:

• The reason you are getting an error in your above code is because of how you are indexing SNames. The function fieldnames returns a cell array of strings, so you have to use content indexing (i.e. curly braces) to access the string values. If you change the fourth line in your code to this:

  field = getfield(S, SNames{loopIndex}); 

  then your code should work without error.

• As suggested by MatlabDoug, you can use dynamic field names to avoid having to use getfield (which yields cleaner looking code, in my opinion).

• The suggestion from Adam to use a cell array instead of a structure is right on the mark. This is generally the best way to collect a series of arrays of different length into a single variable. Your code would end up looking something like this:

  S = {[1 2], [3 4 5]};        % Create the cell array for loopIndex = 1:numel(S)   % Loop over the number of cells   array = S{loopIndex};      % Access the contents of each cell   % Do stuff with array end