How do I access MATLAB structure fields within a loop?

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

The getfield approach is okay (although I don't have MATLAB available right now and it's not clear to me why the above wouldn't work).

For an alternative data structure, you may also want to look into MATLAB cell arrays. They would also allow you to store and index vectors of varying length.

Answer 2

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 3

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