How to use the output of a nested function in parent function?

I trying to make a long function readable. I was planning to write nested function inside the parent function. How can I use the output of a nested function in parent function workspace? I am getting "variable must be explicitly defined before first use" error.

%parent function
function [time_in_feeder_zone, out] = rodent_trial2(rodent, trial_no) 
fn_all_data;
    %nested function
    function [X, Y, all_data] = fn_all_data(~) 
        % code
    end
% 4 quadtrants based on signs
Q1 = all_data(all_data.X>=0 & all_data.Y>=0,:);

Answer 1

[Stephen23]

If you define a function with output arguments and you then want to get those outputs then of course you will also need to call the function with those output arguments. This is true for every kind of function, including nested functions.

function [..] = rodent_trial2(rodent, trial_no) 
[~,~,out] = fn_all_data(); % you need to call the function with the output arguments!!!
idx = out.X>=0 & out.Y>=0;
Q1  = out(idx,:);
..
    %nested function
    function [X, Y, all_data] = fn_all_data() % if you defined output arguments here.
        % code
        all_data = .. whatever
    end
end

But I suspect that you were actually attempting to share the data via the parent workspace, perhaps something like this.

function [..] = rodent_trial2(rodent, trial_no)
all_data = []; % must be defined in the parent workspace before calling nested function!
fn_all_data() % no output arguments!
idx = all_data.X>=0 & all_data.Y>=0;
Q1  = all_data(idx,:);
..
    %nested function
    function fn_all_data() % no output arguments!
        % code
        all_data = .. whatever
    end
end

This is explained here: https://www.mathworks.com/help/matlab/matlab_prog/nested-functions.html#f4-73993

[Struggling in MATLAB]

I think I need all X, Y, and all_data in my parent function workspace. SInce I am looping through multiple files to get all X, Y data inside the nested function.

        X = cat(1, X, xx); % store XCenter of all files
        Y = cat(1, Y, yy);
        RT = cat(1, RT, rt);
        all_data = table(X, Y, RT);

So, it seems I needed the first part of your answer for now. If I understand correctly I could use the second part if I had X and Y in the parent function workspace.

Thank you very much for the detailed explanation!